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Remote Serial Console HOWTO
Glen Turner
Australian Academic and Research Network
<glen.turner+howto@aarnet.edu.au>
Mark F. Komarinski
<mkomarinskiATwayga.org>
v2.6 2003-03-31
Revision History
Revision 2.6 2003-03-31 Revised by: gdt
Correct opposing CTS/RTS explanations. Use <quote> in markup. TLDP PDF is now
good, so remove instructions for rendering PostScript to PDF. Typo in GRUB
configuration.
Revision 2.5 2003-01-20 Revised by: gdt
Only one console per technology type. Setting timezone. Use off parameter
rather than comments in inittab. Cable lengths.
Revision 2.4 2002-10-03 Revised by: gdt
Kernel flow control bug, more cabling, Debian, Livingston Portmaster, typos
(especially those found during translation to Japanese).
Revision 2.3 2002-07-11 Revised by: gdt
Updates for Red Hat Linux 7.3, corrections to serial port speeds and UARTs,
ioctlsave.
Revision 2.2 2002-05-22 Revised by: gdt
Minor changes
Revision 2.1 2002-05-16 Revised by: gdt
Corrections to kernel console syntax. Addition of USB and devfs.
Revision 2.0 2002-02-02 Revised by: gdt
Second edition.
Revision ??1.0 2001-03-20 Revised by: mfk
First edition.
An RS-232 serial console allows Linux to be controlled from a terminal or
modem attached to an asynchronous serial port. The monitor, mouse and
keyboard are no longer required for system administration. Serial consoles
are useful where Linux systems are deployed at remote sites or are deployed
in high-density racks.
This HOWTO describes how to configure Linux to attach a serial console.
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
Dedication
Glen Turner would like to thank his family for allowing him to work on this
project for the surprisingly large number of evenings which it took to write
this HOWTO. Thank you Karen, Kayla and Ella.
Table of Contents
1. Introduction
1.1. What is a console?
1.2. Why use a serial console?
1.3. Alternative meanings of "console"
1.4. Configuration overview
2. Preparation
2.1. Create fallback position
2.2. Select a serial port
2.3. Select a serial speed and parameters
2.4. Configure the modem or the null-modem cable
2.5. Configure the terminal or the terminal emulator
3. Optionally configure the BIOS
4. Configure the boot loader
4.1. Configure the LILO boot loader
4.2. Configure the GRUB boot loader
4.3. Configure the SYSLINUX boot loader
5. Configure Linux kernel
5.1. Configure Linux kernel using LILO
5.2. Configure Linux kernel using GRUB
5.3. Configure Linux kernel using SYSLINUX
6. Configure getty
6.1. init system
6.2. Traditional getty
6.3. agetty
6.4. mgetty
6.5. mingetty
6.6. No getty
7. Configure incidentals
7.1. Allow root to login from serial console
7.2. Change init level to textual
7.3. Remove saved console settings
7.4. Serial console is not /dev/modem
7.5. Alter target of /dev/systty
7.6. Configure Pluggable Authentication Modules
7.7. Configure Red Hat Linux
8. Reboot and test
8.1. Verify console operation
8.2. Re-create saved console settings
8.3. Test the console
8.4. Where to next from here?
9. Security
9.1. Use good passwords
9.2. Obey Data Terminal Ready and Data Carrier Detect
9.3. Use or configure a dumb modem
9.4. Restrict console messages
9.5. Modem features to restrict usage
9.6. BIOS features
9.7. Use a boot loader password
9.8. Non-interactive boot sequence
9.9. Magic SysRq key
9.10. Adjust behaviour of Ctrl-Alt-Delete
9.11. Log attempted access
9.12. Countering interception of telephony links
10. Configuring a kernel to support serial console
10.1. Linux kernel version 2.5
10.2. Linux kernel version 2.4
10.3. Linux kernel version 2.2
11. Serial cabling
11.1. Jargon
11.2. Cable from console port to modem
11.3. Cable from console port to terminal (or another PC)
11.4. Lengths of serial cables
11.5. Making serial cables
12. Modem configuration
12.1. Using Minicom to give commands to a modem
12.2. Configure dumb modem
12.3. Configure modem with AT commands
12.4. Internal modems
12.5. WinModems
A. Bugs and annoyances
A.1. Flow control in Linux kernel
A.2. Red Hat Linux 7.1 and SysVinit
A.3. BIOSs, keyboards and video cards
A.4. Modem hangs up upon reboot
A.5. init and syslog output does not display on secondary consoles
A.6. The console is unresponsive after connecting
A.7. Modem hangs up during initialization
A.8. Boot loader has no flow control
A.9. Boot loaders are vulnerable to line noise
A.10. Advanced Power Management
A.11. Modems and overseas telecommunications requirements
B. Uploading files from a serial console
B.1. Disable logging to console
B.2. ASCII upload and cat
B.3. Xmodem, Ymodem and Zmodem
B.4. Kermit
C. Upgrading Red Hat Linux from a serial console
C.1. Select boot disk
C.2. Configure the BIOS to use the serial port
C.3. Configure modem to ignore DTR and assert DCD
C.4. Prepare a network install floppy diskette
C.5. Prepare HTTP server
C.6. Record network configuration
C.7. Record LILO configuration
C.8. Upgrade Red Hat distribution
C.9. Create boot disk for serial console
C.10. Further references
D. Upgrading Debian GNU/Linux from a serial console
E. Terminal server configuration
E.1. Considerations when buying second-hand terminal servers
E.2. Cisco 2511
E.3. Xyplex/iTouch MAXserver 1600
E.4. Xylogics/Bay/Nortel Annex
E.5. Livingston/Lucent Portmaster
F. Gratuitous advice for developers
F.1. Advice for boot loader authors
F.2. Advice for BIOS authors
G. About this HOWTO
G.1. Copyright
G.2. Disclaimer
G.3. Acknowledgments
G.4. Comments and corrections
Colophon
List of Tables
1-1. Different ways of referring to the "console"
2-1. Many names for the same serial port
2-2. Interrupts used for IBM PC/AT RS-232 ports
4-1. SYSLINUX flow control bitmap
10-1. IBM-PC/AT serial port bit rates and their bit-clock divisors
11-1. Data rates and the maximum distances recommended in RS-232
List of Figures
2-1. Using the setserial command in /etc/rc.serialto disable the serial port
/dev/ttyS2
2-2. Syntax for serial bits per second rate, in extended Backus-Naur form
2-3. Syntax for serial parity, in extended Backus-Naur form
2-4. Syntax for serial data bits, in extended Backus-Naur form
2-5. Syntax for serial stop bits, in extended Backus-Naur form
2-6. Syntax for serial flow control, in extended Backus-Naur form
2-7. Syntax for kernel serial parameters, in extended Backus-Naur form
4-1. Syntax of LILO serial command, in EBNF
4-2. LILO serial EBNF variables
4-3. LILO boot loader sample configuration
4-4. Using md5crypt to create a hashed password for GRUB
4-5. GRUB configuration to require a password
4-6. GRUB configuration for serial console
4-7. GRUB configuration for serial console and attached monitor and keybaord
console
4-8. GRUB output to default device when configured for serial and attached
monior output
4-9. GRUB configuration for command line interface for terminals other than
VT100
4-10. Adding a single user mode option to the GRUB menu
4-11. Syntax of SYSLINUX serial command, in EBNF
4-12. SYSLINUX serial EBNF variables
5-1. Kernel console syntax, in EBNF
5-2. Recommended kernel parameters, PCs with video card
5-3. Recommended kernel parameters, PCs without video card
5-4. Recommended kernel parameters, LILO configuration
5-5. Recommened kernel parameters, GRUB configuration
5-6. Recommended kernel parameters, SYSLINUX configuration
6-1. Interactively altering the connecting terminal's make and model
6-2. Interactively altering the connecting terminal's time zone
6-3. getty is started by init, based upon an entry in /etc/inittab
6-4. Define CON9600 in gettydefs
6-5. Syntax of entries in /etc/gettydefs, in EBNF
6-6. /etc/inittab entry for agetty
6-7. /etc/inittab entry for mgetty
6-8. mgetty configuration file mgetty.config
6-9. Fewer virtual terminals. Removing mingetty entries from /etc/inittab
6-10. Fewer virtual terminals. Deallocating unused virtual terminals and
removing their device files.
6-11. Contents of /etc/rc.serial to lock console serial port when no getty
used
7-1. Alter securetty to allow root to log in from the serial console
7-2. Xservers from Red Hat Linux 7.2
7-3. [servers] section of gdm.conf from Red Hat Linux 7.2
7-4. Removal of ioctl.save containing the saved console parameters
7-5. Remove /dev/modem if it points to the serial console's port
7-6. Default value of /dev/systty in /etc/makedev.d/linux-2.4.x
7-7. Alter value of /dev/systty in MAKEDEV configuration file
7-8. Installing new value of /dev/systty
7-9. Default <console> in console.perms refers to attached keyboard and
screen
7-10. Default device listing in console.perms
7-11. Devices in console.perms required for attached keyboard and screen
7-12. Add <sconsole> in console.perms to refer to serial console
7-13. Remaining devices in console.perms altered to refer to serial console
7-14. Alterations to /etc/sysconfig/init for Red Hat Linux
7-15. Alterations to /etc/sysconfig/kudzu for Red Hat Linux
8-1. Using ioctlsave to create /etc/ioctl.save without entering single user
mode
9-1. Extract from Crackers favour war dialling and weak passwords
9-2. /etc/syslog.conf modified to copy log messages to a log server
9-3. Allowing remote log messages by setting options in /etc/sysconfig/syslog
9-4. Restrict syslog messages to remote.example.edu.au
9-5. Using nscd to cache reverse DNS lookups
9-6. Restrict sending of messages to console user
9-7. Restrict sending of messages to console user, /etc/profile.d/mesg.sh
9-8. Restrict sending of messages to console user, /etc/profile.d/mesg.csh
9-9. Install files into /etc/profile.d
9-10. Using sysctl to defeat the magic SysRq key
9-11. Configuring /etc/sysctl.conf to defeat the magic SysRq key
9-12. Kernel make menuconfig showing disabled SysRq key
9-13. Kernel .config showing disabled SysRq key
9-14. Default handling of Ctrl-Alt-Delete in /etc/inittab
9-15. Ignoring Ctrl-Alt-Delete in /etc/inittab
9-16. Shut down cleanly upon Ctrl-Alt-Delete in /etc/inittab
10-1. Kernel configuration for serial console using make menuconfig
10-2. Kernel configuration for serial console using .config
10-3. Kernel configuration for USB dongle serial console using make
menuconfig
10-4. Kernel configuration for USB dongle serial console using .config
10-5. Kernel configuration for serial console using make menuconfig
10-6. Kernel configuration for serial console using .config
11-1. Null modem cable with full status and handshaking
11-2. Variation on null modem cable with full status and handshaking
11-3. Null modem cable with falsified status and handshaking
11-4. Null modem cable with no status or handshaking
11-5. One-way null modem cable with no status or handshaking
12-1. Front panel of a dumb modem
12-2. Testing the modem's port speed
12-3. Configure modem using AT commands
12-4. Resetting a Hayes AT-style modem
A-1. A kernel console parameter with CTS/RTS flow control
A-2. Kernel source code for console CTS/RTS flow control
A-3. setserial causes a modem to hang up as the machine initializes
B-1. Supressing kernel messages to the console in Red Hat Linux
C-1. Configuring BIOS to use serial link
C-2. Configuring BIOS to boot from hard disk
C-3. Extract from Red Hat Linux 7.2 mkbootdisk which creates SYSLINUX.CFG
C-4. Altered extract from mkbootdisk, which creates a SYSLINUX.CFG that uses
a serial console
E-1. Basic configuration for Cisco 2511 terminal server to Linux PC
E-2. Portmaster unit configuration
E-3. Portmaster port configuration
F-1. Configuring /dev/nvram to access the CMOS configuration
F-2. Getting the CMOS configuration
F-3. Setting the CMOS configuration
List of Examples
4-1. Using kernel parameters to avoid access permissions
5-1. Complete LILO configuration, as installed by vendor
5-2. Complete LILO configuration, modified for serial console
5-3. Complete GRUB configuration, as installed by vendor
5-4. Complete GRUB configuration, modified for serial console
8-1. Dialing into a serial console
C-1. Displaying the Internet Protocol configuration
C-2. Displaying the LILO configuration
-----------------------------------------------------------------------------
Chapter 1. Introduction
<EFBFBD> console n. [From latin consolatio(n)
"comfort, spiritual solace."] A device
for displaying or printing condolances
or obituaries for the operator.
Stan Kelly-Bootle, The Computer
Contradictionary.
-----------------------------------------------------------------------------
1.1. What is a console?
The console is the text output device for system administration messages.
These messages come from the kernel, from the init system and from the system
logger.
On modern small computers the console is usually the computer's attached
monitor and keyboard.
On many older computers the console is an RS-232 link to a terminal such as a
DEC VT100. This terminal is in a locked room and is continually observed by
the minicomputer's operators. Large systems from Sun, Hewlett-Packard and IBM
still use serial consoles.
It is usually possible to login from the console. A login session from the
console is treated by many parts of the operating system as being more
trustworthy than a login session from other sources. Logging in as the root
super-user from the console is the Command Line of Last Resort when faced
with a misbehaving system.
-----------------------------------------------------------------------------
1.2. Why use a serial console?
For the average user a serial console has no advantage over a console offered
by a directly attached keyboard and screen. Serial consoles are much slower,
taking up to a second to fill a 80 column by 24 line screen. Serial consoles
generally only support non-proportional ASCII text, with limited support for
languages other than English. A new terminal can be more expensive than an
old PC.
There are some scenarios where serial consoles are useful. These are:
Systems administration of remote computers
Linux is a good operating system for deployment at unstaffed sites. Linux
is also good for hosting critical network infrastructure such as DNS and
DHCP services. These services are generally installed at every site of an
organisation including sites which may be too small or too remote to have
information technology staff.
System administration of these remote computers is usually done using SSH
, but there are times when access to the console is the only way to
diagnose and correct software failures. Major upgrades to the installed
distribution may also require console access.
In these cases the serial console is attached to a modem. Access to the
console is gained from a remote computer by dialing into the modem. This
allows the console to be reached from any telephone socket.
High density racks of computers
Clusters of personal computers can outperform mainframe computers and
form competitive supercomputers for some applications. See the
Cluster-HOWTO for more information on clustering.
These clusters are typically assembled into 19 inch telecommunications
equipment racks and the system unit of each computer is typically one
rack unit (or 1.75 inches) tall. It is not desirable to put a keyboard
and monitor on each computer, as a small cathode ray tube monitor would
consume the space used by sixteen rack units.
A first glance it seems that a monitor and keyboard switch is the best
solution. However the VGA signal to the monitor is small, so even with
the switch the monitor cannot be placed very far away from the rack of
computers.
It is desirable to allow the consoles to be monitored in the operators'
room of the computer center, rather than in the very expensive space of
the machine room. Although monitor switches with remote control and fiber
optical extensions are available, this solution can be expensive.
A standard RS-232 cable can be 15 meters in length. Longer distances are
easily possible. The cabling is cheap. Terminal servers can be used to
allow one terminal to access up to 90 serial consoles.
Recording console messages
This is useful in two very different cases.
Kernel programmers are often faced with a kernel error message that is
displayed a split second before the computer reboots. A serial console
can be used to record that message. Another Linux machine can be used as
the serial terminal.
Some secure installations require all security events to be unalterably
logged. One way to meet this requirement is to print all console
messages. Connecting the serial console to a serial printer can achieve
this.[1]
Embedded software development
Linux is increasingly being used as an operating system for embedded
applications. These computers do not have keyboards or screens.
A serial port is a cheap way to allow software developers to directly
access the embedded computer. This is invaluable for debugging. Most chip
sets designed for embedded computers have a serial port precisely for
this purpose.
The shipping product need not present the RS-232 port on an external
connector. Alternatively the RS-232 port is often used for downloading
software updates.
Craft terminal for telecommunications equipment
Linux is increasingly being used as the operating system inside
telecommunications equipment. The Carrier Grade Linux consortia hopes to
accelerate and coordinate this trend.
Most telecommunications equipment is remotely managed from a distant
computer. However, site technicans (called craft personnel in
telco-speak) need to access the equipment to test installation changes,
check the status of reported faults, and so on. The terminal used by the
craft personnel is called the craft terminal. The craft terminal plugs
into the craft interface on the equipment. The serial console makes an
ideal craft interface.
Unlike minicomputer systems, the IBM PC was not designed to use a serial
console. This has two consequences.
Firstly, Power On Self-Test messages and Basic Input/Output System (BIOS)
messages are sent to the screen and received from the keyboard. This makes it
difficult to use the serial port to reconfigure the BIOS and impossible to
see Power On Self-Test errors.
An increasing number of manufacturers of rackable server equipment are
altering their BIOSs to optionally use the RS-232 port for BIOS configuration
and test messages. If you are buying a machine specifically for use with
serial console you should seek this feature. If you have an existing machine
that definitely requires access to the BIOS from the serial port then there
are hardware solutions such as PC Weasel 2000.
Secondly, the RS-232 port on the IBM PC is designed for connecting to a
modem. Thus a null modem cable is needed when connecting the PC's serial port
to a terminal.
-----------------------------------------------------------------------------
1.3. Alternative meanings of "console"
Some authors use the word "console" to refer to the keyboard and monitor that
are attached to the system unit. This is described as a "physical console" by
some Linux documentation. The console where system messages appear is
described as the "logical console" by that documentation.
As an illustration of the difference, X Windows should start on the physical
console but system messages issued by failures when starting X Windows should
be written to the logical console.
To avoid confusion this HOWTO uses the word "console" to describe the place
where system messages are printed. This HOWTO uses the phrase "attached
monitor and keyboard" rather than the confusing words "physical console".
These distinctions are also made in the naming of devices. The device /dev/
console is used to send messages to the console. The symbolic link /dev/
systty points to the device which is used by the attached monitor and
keyboard, often /dev/tty0.
Table 1-1. Different ways of referring to the "console"
+------------------------+-----------------+--------------------------------+
|Document |<7C> |<7C> |
+------------------------+-----------------+--------------------------------+
|This HOWTO |"Console" |"Attached monitor and keyboard" |
+------------------------+-----------------+--------------------------------+
|Some Linux documentation|"Logical console"|"Physical console" |
+------------------------+-----------------+--------------------------------+
|Device names |/dev/console |/dev/systty |
+------------------------+-----------------+--------------------------------+
-----------------------------------------------------------------------------
1.4. Configuration overview
There are five major steps to configuring a serial console.
1. Optionally, the BIOS may be configured to use the serial port.
2. If needed, the boot loader may be configured to use the serial port.
3. The Linux kernel must be configured to use the serial port as its
console. This is done by passing the kernel the console parameter when
the kernel is started by the boot loader.
4. The init system should keep a process running to monitor the serial
console for logins. The monitoring process is traditionally called getty.
5. A number of system utilities need to be configured to make them aware of
the console, or configured to prevent them from disrupting the console.
Examples in this HOWTO are from Red Hat Linux versions 7.1 through to 7.3
(released 2001 through to 2002). The maintainer would appreciate updates when
new versions of Red Hat Linux appear. The maintainer would very much
appreciate examples for Linux distributions that are dissimilar to Red Hat
Linux; particularly Debian GNU/Linux and Slackware Linux. All contributors
are acknowledged in Section G.3.
-----------------------------------------------------------------------------
Chapter 2. Preparation
This chapter ensures that access the existing console can be restored should
the serial console fail to start.
This chapter then discusses the selection of the RS-232 port and its
parameters.
-----------------------------------------------------------------------------
2.1. Create fallback position
Good system administrators always have a viable fallback plan to cope with
failures. A mistake configuring the serial console can make both the serial
console and the attached monitor and keyboard unusable. A fallback plan is
needed to retrieve console access.
Many Linux distributions allow boot diskettes to be created. Writing a boot
diskette before altering the console configuration results in a boot diskette
that passes good parameters to the kernel rather than parameters that may
contain an error.
Under Red Hat Linux a boot diskette is created by determining the running
kernel version
+---------------------------------------------------------------------------+
|bash$ uname -r |
|2.4.2-2 |
+---------------------------------------------------------------------------+
and then using that version to create the boot diskette
+---------------------------------------------------------------------------+
|bash# mkbootdisk --device /dev/fd0 2.4.2-2 |
+---------------------------------------------------------------------------+
Under Debian GNU/Linux the boot diskette is created by determining the
version of the running kernel and then using that version to write the boot
diskette
+---------------------------------------------------------------------------+
|bash# mkboot /boot/vmlinuz-2.4.2-2 |
+---------------------------------------------------------------------------+
An alternative fallback position is have a rescue diskette with the machine.
A common choice is [http://www.toms.net/rb/] Tom's root boot.
-----------------------------------------------------------------------------
2.2. Select a serial port
2.2.1. Serial port names
Linux names its serial ports in the UNIX tradition. The first serial port has
the file name /dev/ttyS0, the second serial port has the file name /dev/
ttyS1, and so on.
This differs from the IBM PC tradition. The first serial port is named COM1:,
the second serial port is named COM2:, and so on. Up to four serial ports can
be present on a IBM PC/AT computer and its successors.
Most boot loaders have yet another naming scheme. The first serial port is
numbered 0, the second serial port is numbered 1, and so on.
If your distribution of Linux uses the devfs device manager then the serial
ports have yet another name. The first serial port is /dev/tts/0, the second
serial port is /dev/tts/1, and so on.
The result is that the first serial port is labeled COM1: on the chassis of
the IBM PC; is known as /dev/ttyS0 to Linux; is known as /dev/tts/0 to Linux
when configured with devfs; and is known as port 0 to many boot loaders.
The examples in this HOWTO use this first serial port, as that is the serial
port which most readers will wish to use.
Table 2-1. Many names for the same serial port
+------+-------------+------------------------+------------------+
|IBM PC|Linux kernel |Linux kernel with devfs |Most boot loaders |
+------+-------------+------------------------+------------------+
|COM1: | /dev/ttyS0 | /dev/tts/0 | 0 |
+------+-------------+------------------------+------------------+
|COM2: | /dev/ttyS1 | /dev/tts/1 | 1 |
+------+-------------+------------------------+------------------+
|COM3: | /dev/ttyS2 | /dev/tts/2 | 2 |
+------+-------------+------------------------+------------------+
|COM4: | /dev/ttyS3 | /dev/tts/3 | 3 |
+------+-------------+------------------------+------------------+
-----------------------------------------------------------------------------
2.2.2. Cannot share interrupt used for console's serial port
When used for a console the serial port cannot share an interrupt with
another device. The IBM PC devices are usually installed as shown in Table
2-2. If you use the serial port /dev/ttyS0 for the console then you should
avoid sharing interrupt 4 by not installing a serial port /dev/ttyS2 in your
PC. If /dev/ttyS2 cannot be physically removed then disable it using the
setserial command, as shown in Figure 2-1.
Table 2-2. Interrupts used for IBM PC/AT RS-232 ports
+-----------+----------+-----+
| Device |Interrupt |Port |
+-----------+----------+-----+
|/dev/ttyS0 | 4 |0x3f8|
+-----------+----------+-----+
|/dev/ttyS1 | 3 |0x2f8|
+-----------+----------+-----+
|/dev/ttyS2 | 4 |0x3e8|
+-----------+----------+-----+
|/dev/ttyS3 | 3 |0x2e8|
+-----------+----------+-----+
Figure 2-1. Using the setserial command in /etc/rc.serialto disable the
serial port /dev/ttyS2
# Disable /dev/ttyS2 so interrupt 4 is not shared,
# then /dev/ttyS0 can be used as a serial console.
setserial /dev/ttyS2 uart none port 0x0 irq 0
Reading the source code suggests that the interrupt-sharing constraint
applies to all computer architectures, not just Intel Architecture-32.
-----------------------------------------------------------------------------
2.3. Select a serial speed and parameters
This HOWTO does not discuss the RS-232 standard, which is formally known as
ANSI/TIA/EIA-232-F-1997 Interface Between Data Terminal Equipment and Data
Circuit-Terminating Equipment Employing Serial Data Interchange. For an
explanation of "bits per second", "start bits", "data bits", "parity", "stop
bits" and "flow control" refer to the Serial-HOWTO and the Modem-HOWTO.
The description of the command syntax for setting the serial parameters in
the kernel, boot loaders and login applications uses the following variables
which describe RS-232 parameters.
<speed>
The speed of the serial link in bits per second.
The Linux kernel on a modern PC supports a serial console speeds of 1200,
2400, 4800, 9600, 19200, 38400, 57600 and 115200 bits per second.
The kernel supports a much wider range of serial bit rates when the
serial interface is not being used as a serial console.[2]
Very recent Linux kernels can also offer a serial console using a USB
serial dongle at speeds of 1200, 2400, 4800, 9600, 19200, 38400, 57600
and 115200 bits per second.
Most boot loaders only support a different range of speeds than are
supported by the kernel. LILO 21.7.5 supports 110, 150, 300, 600, 1200,
2400, 4800, 9600, 19200, 38400, 56000, 57600 and 115200 bits per second.
SYSLINUX 1.67 supports 75 to 56000 bits per second. GRUB 0.90 supports
2400, 4800, 9600, 19200, 38400, 57600 and 115200 bits per second.
You must chose the same speed for both the boot loader and for the Linux
kernel. An operating system may use more than one boot loader. For
example, Red Hat Linux uses SYSLINUX to install or upgrade the operating
system; LILO as the boot loader for Red Hat Linux 7.1 and earlier; and
GRUB as the boot loader for Red Hat Linux 7.2 and later.
If you are using a serial terminal or if you are using a dumb modem then
the bit rate of the terminal or dumb modem must also match the bit rate
selected in the boot loader and kernel.
If the serial console is connected to a Hayes-style modem slower than
9600bps then configure the serial console with the same speed as the
modem. Modems faster than 9600bps will generally automatically
synchronize to the speed of the serial port.
The selected bit rate must also be supported by the serial port's UART
semiconductor chip. Early UARTs without on-chip receive buffers could
only reliably receive at up to 14400bps, this includes models 8250A,
82510, 16450 and 16550 (with no A). Recent UARTs with receive buffers
will work at all serial console bit rates, this includes models 16550A,
16552, 16650, 16654, 16750, 16850 and 16950.
Unless you have good reason, use the popular bit rate of 9600 bits per
second. This is the default bit rate of a great many devices.
The speeds that are supported by the kernel, the three common boot
loaders, and all IBM PCs capable of running Linux are: 2400, 4800, 9600
and 19200 bits per second. This is a depressingly small selection: not
slow enough to support a call over an international phone circuit and not
fast enough to upload large files. You may need to choose a speed that
will result in a less robust software configuration.
Figure 2-2. Syntax for serial bits per second rate, in extended
Backus-Naur form
<speed><3E>::=<3D><><digits>
<digits><3E>::=<3D><digit><3E>|<7C><digit><digits>
<digit><3E>::=<3D>0<EFBFBD>|<7C>1<EFBFBD>|<7C>??<3F>|<7C>9
<parity>
Number of parity bits and the interpretation of a parity bit if one is
present.
Allowed values are n for no parity bit, e for one bit of even parity and
o for one bit of odd parity.
Using no parity bit and eight data bits is recommended.
If parity is used then even parity is the common choice.
Parity is a simple form of error detection. Modern modems have much
better error detection and correction. As a result the parity bit guards
only the data on the cable between the modem and the serial port. If this
cable has a low error rate, and it should, then the parity bit is not
required.
Figure 2-3. Syntax for serial parity, in extended Backus-Naur form
<parity><3E>::=<3D>n<EFBFBD>|<7C>e<EFBFBD>|<7C>o
<data>
The number of data bits per character.
Allowed values are 7 bits or 8 bits, as Linux uses the ASCII character
set which requires at least seven bits.
Eight data bits are recommended. This allows the link to easily be used
for file transfers and allows non-English text to be presented.
Figure 2-4. Syntax for serial data bits, in extended Backus-Naur form
<data><3E>::=<3D>7<EFBFBD>|<7C>8
<stop>
The number of stop bit-times.[3]
Allowed values are 1 or 2.
One stop bit-time is recommended.
If the RS-232 cable is very long then two stop bit-times may be needed.
You may occassionally see 1.5 stop bit-times. The intent is to gain 4%
more data throughput when a link is too long for one stop bit-time but is
too short to require two stop bit-times. 1.5 stop bit-times is now rare
enough to be a hazard to use.
Figure 2-5. Syntax for serial stop bits, in extended Backus-Naur form
<stop><3E>::=<3D>1<EFBFBD>|<7C>2
<flow_control>
The type of flow control to use.
The Linux kernel allows no flow control and CTS/RTS flow control.
No flow control is the default, this is indicated by omitting <
flow_control>.
CTS/RTS flow control is recommended, especially if login access is also
provided to the serial port. This is indicated by a <flow_control> of r.
CTS/RTS flow control regulates the flow of chatacters. The computer does
not send characters until Clear To Send is asserted by the modem. If the
computer is has enough buffering to recieve characters from the modem the
computer asserts Ready to Send. Thus neither the computer nor the modem's
buffers are filled to overflowing.
Caution The kernel's CTS/RTS flow control is currently buggy.
Machines can take a significant time to write console
messages if flow control is enabled but CTS will never be
asserted (as occurs when there is no call present on a
modem or no session on a null modem cable or cable to a
terminal server). As a result of the large number of
kernel messages when the kernel is started a machine
configured with the kernel's CTS/RTS flow control can
take many minutes to reboot.
The kernel's CTS/RTS flow control cannot be recommended
at this time. The HOWTO's author has a kernel patch
available which he is seeking to have included in the
mainstream kernel source code.
The CTS/RTS flow control in user-space applications does
not share the kernel's bugs and CTS/RTS flow control is
still recommended for getty.
Figure 2-6. Syntax for serial flow control, in extended Backus-Naur form
<flow_control><3E>::=<3D><nil><3E>|<7C>r
At present the RS-232 status lines are ignored by the kernel. A kernel
message will be printed even if Data Carrier Detect and Data Set Ready are
not asserted. This leads to the kernel messages being sent to a modem which
is idle and in command mode.
The console's slack interpretation of CTS, DSR and DCD makes it impossible to
connect a serial console to an RS-232 multi-drop circuit. Multi-drop circuits
have more than two computers on the circuit; they are traditionally
four-wire, satelite or wireless services.
The Linux kernel uses the syntax in Figure 2-7 to describe the serial
parameters. Many boot loaders use a variation of the syntax used by the Linux
kernel.
Figure 2-7. Syntax for kernel serial parameters, in extended Backus-Naur form
<mode><3E>::=<3D><speed><parity><data><flow_control>
Note that <mode> does not include <stop>. The kernel assumes the number of
stop bits to be one. This shortcoming needs to be considered when deploying
long RS-232 cables.
Most boot loaders default to 9600n8. A common default found on older
terminals is 9600e7.
Use 9600n8 if possible, as this is the default for most Linux software and
modern devices.
This HOWTO always configures the serial speed and parameters, even where not
strictly necessary. This is so that people configuring parameters other than
the recommended and common default value 9600n8 will know what to alter.
-----------------------------------------------------------------------------
2.4. Configure the modem or the null-modem cable
If a modem is used, configure it to be a dumb modem at the port speed
selected in Section 2.3. If the modem accepts Hayes AT commands see Chapter
12 to dumb-down the modem.
Alternatively if a terminal and a null-modem cable are used see Section 11.3,
which discusses the pinout of the null modem cable.
-----------------------------------------------------------------------------
2.5. Configure the terminal or the terminal emulator
Configure the terminal to match the serial parameters. The data bits, parity
bits and stop bits must match. If a modern "smart" modem is used then the bit
speeds need not match. If a dumb modem or a null modem cable is used then the
bit speeds must match.
Set CTS/RTS handshaking on, DTR/DSR handshaking off and XON/XOFF handshaking
off. Your equipment may call CTS/RTS handshaking or DTR/DSR handshaking
"hardware handshaking" and may call XON/XOFF handshaking "software
handshaking".
Set automatic line wrapping on. This allows all of a long console message to
be read.
Set the received end of line characters to CR LF (carriage return then line
feed). Set the transmitted end of line characters to just CR (carriage
return).
If you are using a terminal emulator then it is best to choose to emulate the
popular DEC VT100 or VT102 terminal. Later terminals in the DEC VT range are
compatible with the VT100. If this terminal is not available then try to
emulate another terminal that implements ANSI X3.64-1979 Additional Controls
for Use with American National Standard Code for Information Interchange (or
its successor ISO/IEC 6429:1992 ISO Information technology ?? Control
functions for coded character sets). For example, many emulators have a
terminal called ANSI BBS which uses the IBM PC character set, the 16 IBM PC
colors, a 80 column by 25 line screen and a selection of X3.64-1979 control
sequences.
See the Text-Terminal-HOWTO for much more information on configuring
terminals.
-----------------------------------------------------------------------------
Chapter 3. Optionally configure the BIOS
Some BIOSs provide support for serial consoles. If your computer's BIOS is
one of these you should investigate the extent of the support provided.
Depending upon the extent of serial console support you may not need to
explicitly configure the boot loader to use the serial port.
The contributors to this HOWTO have encountered the following styles of BIOS
support for serial consoles.
Redirection of textual VGA output to the serial port
The BIOS takes the interrupt 0x10 "video" requests used to write to the
screen and sends the characters that would have appeared on the screen to
the serial port. Characters recieved from the serial port are used to
supply characters to BIOS interrupt 0x16 "read key" requests.
Any 16-bit application which uses the BIOS functions for outputing text
to the screen and reading from the keyboard is redirected to the serial
port. This includes the BIOS itself, the boot loader, and 16-bit
operating systems (such as MS-DOS).
When a 32-bit operating system (such as Linux, BSD or Windows NT/2000/XP)
loads the 16-bit BIOS is no longer accessible and the BIOS can no longer
be used for input and output. The 32-bit operating system loads its own
device drivers for this purpose. These device drivers then need to
provide the redirection of console I/O to the serial port.
If your BIOS uses this technique then you should:
1. Configure the BIOS to redirect keyboard input and video output to the
serial port.
2. Do not configure the boot loader, as the BIOS will redirect this
16-bit application's input and output to the serial port.
3. Configure Linux to use the serial port as a console, as Linux is a
32-bit operating system.
BIOS configuration and power on self-test uses the serial port
These BIOSs use the serial port for configuration and the power-on
self-test, but do not redirect the interrupt 0x10 "video" requests
interrupt 0x16 "read key" requests to the serial port.
Some BIOSs which usually redirect all keyboard and video output to the
serial port can be configured in only to redirect BIOS input and output.
Look for a BIOS configuration option similar to Cease redirection after
boot.
If your BIOS uses this technique or you choose to set Cease redirection
after boot then you should:
1. Configure the BIOS to send its output to the serial port.
2. Configure the boot loader to use the serial port.
3. Configure Linux to use the serila port as the console, as Linux is a
32-bit operating system.
Redirection of graphical VGA output to the serial port
Some graphical 32-bit operating systems do not provide their own
facilities to send console output to the serial port. Some BIOSs attempt
to overcome this shortcoming, using a propietary serial protocol to send
graphical output to a remote serial client.
As these machines cannot be connected to from a standard terminal
emulator this facility is best left unconfigured when using the Linux
operating system.
1. Configure the BIOS not to send output to the serial port.
2. Configure the boot loader to use the serial port.
3. Configure Linux to use the serial port as the console.
No serial port facilities
The BIOS cannot be accessed from the serial port, so power-on self-test
messages cannot be seen.
Note that BIOS may still be able to be configured remotely using the /dev
/nvram device. This takes some care.
1. Configure the boot loader to use the serial port.
2. Configure Linux to use the serial port as the console.
If you need to configure the boot loader to use the serial port then continue
to Chapter 4. Otherwise go directly to Chapter 5 to configure the kernel;
this is done by configuring the boot loader to pass boot parameters to the
Linux kernel.
-----------------------------------------------------------------------------
Chapter 4. Configure the boot loader
When a PC boots the CPU it runs code from Read-Only Memory. This code is the
Basic Input/Output System, or BIOS. The BIOS then loads a boot loader from
the Master Boot Record of the first hard disk.[4] In turn, the boot loader
reads the operating system into memory and then runs it.[5]
Neither the BIOS nor the boot loader are strictly necessary. For example,
there are [http://www.acl.lanl.gov/linuxbios/] versions of Linux that run
directly from the flash memory which usually contains the BIOS. Linux was
originally designed to run without an interactive boot loader, by placing the
kernel at particular sectors of the disk.
The benefits of using a boot loader are:
<EFBFBD><EFBFBD>*<2A>Multiple operating systems can be booted. See the Linux + Windows HOWTO
for more information.
<EFBFBD><EFBFBD>*<2A>Parameters can be passed to the kernel interactively. This is useful for
solving hardware problems; for example, some interrupt lines can be
disabled, direct memory access to some drives can be disabled, and so on.
See the Linux BootPrompt-HOWTO for a list of kernel parameters.
<EFBFBD><EFBFBD>*<2A>Differing kernels can be interactively loaded. This is useful when
deploying a new kernel, as it provides simple fallback to a proven
kernel.
For these reasons systems administrators want to be able to interactively
control the boot loader from the serial console.
LILO, GRUB and SYSLINUX are popular boot loaders for IBM PCs. Find which of
these boot loaders your Linux installation uses and then follow the
instructions for your boot loader in the following section.
-----------------------------------------------------------------------------
4.1. Configure the LILO boot loader
LILO is the Linux Boot Loader used on Intel machines. Other boot loaders for
Intel machines exist, common alternatives are GRUB and SYSLINUX. Equivalents
to LILO exist for other processor architectures, their names are usually some
play upon "LILO".
LILO is documented in the lilo(8) and lilo.conf(5) manual pages; the LILO
Generic boot loader for Linux ?? User's Guide found in the file /usr/share/
doc/lilo??/doc/User_Guide.ps; and the LILO mini-HOWTO.
The LILO configuration is kept in the file /etc/lilo.conf. The first part of
the file applies to all images. The following parts are image descriptions
for each kernel.
Set LILO to use the serial port. The syntax of the serial line parameters
follows that used by the kernel.
Figure 4-1. Syntax of LILO serial command, in EBNF
serial=<serial_port>[,<speed>[<parity>[<data>]]]
Where the variables are the same as used by the kernel (shown in Figure 2-7)
and:
Figure 4-2. LILO serial EBNF variables
<serial_port><3E>::=<3D>0<EFBFBD>|<7C>1|<7C>??<3F>|<7C>3
Our examples use /dev/ttyS0, which LILO knows as port 0.
Figure 4-3. LILO boot loader sample configuration
serial=0,9600n8
timeout=100
restricted
password=PASSWORD
The parameters restricted and password are used to avoid someone dialing in,
booting the machine, and stepping around the Linux access permissions by
typing:
Example 4-1. Using kernel parameters to avoid access permissions
+---------------------------------------------------------------------------+
|LILO: linux init=/sbin/sash |
+---------------------------------------------------------------------------+
The password should be good, as it can be used to gain root access. The LILO
password is stored in plain text in the configuration file, so it should
never be the same as any other password. The permissions on the configuration
file should be set so that only root can read /etc/lilo.conf.
+---------------------------------------------------------------------------+
|bash# chmod u=rw,go= /etc/lilo.conf |
+---------------------------------------------------------------------------+
LILO has an option to display a boot message. This does not work with serial
consoles. Remove any lines like:
message=/boot/message
LILO is now configured to use the serial console. The kernels booted from
LILO are yet to be configured to use the serial console.
-----------------------------------------------------------------------------
4.2. Configure the GRUB boot loader
GRUB is a boot loader designed to boot a wide range of operating systems from
a wide range of filesystems. GRUB is becoming popular due to the increasing
number of possible root filesystems that can Linux can reside upon.
GRUB is documented in a GNU info file. Type info grub to view the
documentation.
The GRUB configuration file is /boot/grub/menu.lst. Some distributions use
another configuration file; for example, Red Hat Linux uses the file /boot/
grub/grub.conf.
GRUB configuration files are interpreted. Syntax errors will not be detected
until the machine is rebooted, so take care not to make typing errors.
Edit the GRUB configuration file and remove any splashimage entries. If these
entries are not removed GRUB 0.90 behaves very oddly, transferring control
between the serial console and the attached monitor and keyboard.
If there is not already a password command in the GRUB configuration file
then create a hashed password, see Figure 4-4. The password should be good,
as it can be used to gain root access.
Figure 4-4. Using md5crypt to create a hashed password for GRUB
+---------------------------------------------------------------------------+
|grub> md5crypt |
|Password: ********** |
|Encrypted: $1$U$JK7xFegdxWH6VuppCUSIb. |
+---------------------------------------------------------------------------+
Use that hashed password in the GRUB configuration file, this is shown in
Figure 4-5.
Figure 4-5. GRUB configuration to require a password
password --md5 $1$U$JK7xFegdxWH6VuppCUSIb.
Define the serial port and configure GRUB to use the serial port, as shown in
Figure 4-6.
Figure 4-6. GRUB configuration for serial console
serial --unit=0 --speed=9600 --word=8 --parity=no --stop=1
terminal serial
--unit is the number of the serial port, counting from zero, unit 0 being
COM1.
Note that the values of --parity are spelt out in full: no, even and odd. The
common abbreviations n, e and o are not accepted.
If there is mysteriously no output on the serial port then suspect a syntax
error in the serial or terminal commands.
If you also want to use and attached monitor and keyboard as well as the
serial port to control the GRUB boot loader then use the alternative
configuration in Figure 4-7.
Figure 4-7. GRUB configuration for serial console and attached monitor and
keybaord console
password --md5 $1$U$JK7xFegdxWH6VuppCUSIb.
serial --unit=0 --speed=9600 --word=8 --parity=no --stop=1
terminal --timeout=10 serial console
When both the serial port and the attached monitor and keyboard are
configured they will both ask for a key to be pressed until the timeout
expires. If a key is pressed then the boot menu is displayed to that device.
Disconcertingly, the other device sees nothing.
If no key is pressed then the boot menu is displayed on the whichever of
serial or console is listed first in the terminal command. After the timeout
set by the timeout the default option set by default is booted.
Figure 4-8. GRUB output to default device when configured for serial and
attached monior output
+-------------------------------------------------------------------------------+
|Press any key to continue. |
|Press any key to continue. |
|Press any key to continue. |
|Press any key to continue. |
|Press any key to continue. |
|Press any key to continue. |
|Press any key to continue. |
|Press any key to continue. |
|Press any key to continue. |
|Press any key to continue. |
| |
| GRUB version 0.90 (639K lower / 162752K upper memory) |
| |
| +-------------------------------------------------------------------------+ |
| | [ Red Hat Linux (2.4.9-21) ] | |
| | | |
| | | |
| +-------------------------------------------------------------------------+ |
| Use the ^ and v keys to select which entry is highlighted. |
| Press enter to boot the selected OS or 'p' to enter a |
| password to unlock the next set of features. |
| |
| The highlighted entry will be booted automatically in 10 seconds. |
+-------------------------------------------------------------------------------+
If you are not using a VT100 terminal then the cursor keys may not work to
select a GRUB menu item. The instructions shown in Figure 4-8 are literally
correct: Use the ^ and v keys means that the caret key (Shift-6) moves the
cursor up and letter vee key (V) moves the cursor down.
Note when configuring GRUB that there are two timeouts involved. Press any
key to continue is printed for terminal --timeout=10 seconds, waiting for
someone on the keyboard or terminal to press a key to get the input focus.
Then the menu is displayed for timeout 10 seconds before the default boot
option is taken.
If the terminal attached to the serial port is not a real or emulated VT100,
then force GRUB to use it's command line interface. This interface is much
more difficult to use than GRUB's menu interface; however, the command line
interface does not assume the VT100's terminal language.
Figure 4-9. GRUB configuration for command line interface for terminals other
than VT100
terminal --timeout=10 --dumb serial console
This HOWTO does not discuss the use of GRUB's command line. It is far too
complex and error-prone to recommend for use on production machines. Wizards
will know to consult GRUB's info manual for the commands required to boot the
kernel.
GRUB's menu's can be edited interactively after P is pressed and the password
supplied. A better approach is to add menu items to boot the machine into
alternative run levels. A sample configuration showing a menu entry for the
default run level and an alternative menu entry for single user mode (run
level s) is shown in Figure 4-10. Remember to use the lock command to require
a password for single user mode, as single user mode does not ask for a Linux
password.
Figure 4-10. Adding a single user mode option to the GRUB menu
password --md5 $1$U$JK7xFegdxWH6VuppCUSIb.
default 0
title Red Hat Linux (2.4.9-21)
root (hd0,0)
kernel /vmlinuz-2.4.9-21 ro root=/dev/hda6
initrd /initrd-2.4.9-21.img
title Red Hat Linux (2.4.9-21) single user mode
lock
root (hd0,0)
kernel /vmlinuz-2.4.9-21 ro root=/dev/hda6 s
initrd /initrd-2.4.9-21.img
File names in the kernel and initrd commands are relative to the GRUB
installation directory, which is usually /boot/grub. So /vmlinuz-2.4.9-21 is
actually the file /boot/grub/vmlinuz-2.4.9-21.
GRUB is now configured to use the serial console. The kernels booted from
GRUB are yet to be configured to use the serial console.
-----------------------------------------------------------------------------
4.3. Configure the SYSLINUX boot loader
SYSLINUX is a boot loader that is installed on a MS-DOS floppy disk. As
directed by it's configuration file \SYSLINUX.CFG it will load one of the
files from the floppy disk as a Linux kernel.
SYSLINUX presents a simple text interface that can be used to select between
canned configurations defined in the configuration file and can be used to
add parameters to the kernel.
ISOLINUX and PXELINUX are variants of SYSLINUX for CD-ROMs and Intel's
Preboot Execution Environment.
SYSLINUX supports a variety of serial port speeds, but it only supports eight
data bits, no parity and one stop bit. SYSLINUX supports the serial ports
COM1: through to COM4:, as with most boot loaders these are written as port 0
through to port 3.
For SYSLINUX to support a serial console add a new first line to \
SYSLINUX.CFG:
Figure 4-11. Syntax of SYSLINUX serial command, in EBNF
serial<EFBFBD><space><3E><serial_port><3E>[<5B><space><3E><speed><3E>[<5B><space><3E><
syslinux_flow_control><3E>]<5D>]
The variables are the same as used by syntax descriptions in Figure 2-7 and
Figure 4-2 plus those in Figure 4-12.
Figure 4-12. SYSLINUX serial EBNF variables
<space><3E>::=<3D>?? ??
<syslinux_flow_control><3E>::=<3D><hex_digits>
<hex_digits><3E>::=<3D>0x<hex_digit><hex_digit><hex_digit>
<hex_digit><3E>::=<3D>0<EFBFBD>|<7C>1<EFBFBD>|<7C>??<3F>|<7C>9<EFBFBD>|<7C>a<EFBFBD>|<7C>b<EFBFBD>|<7C>??<3F>|<7C>f
The <syslinux_flow_control> variable controlling the RS-232 status and flow
control signals is optional. If your null-modem cable does not present any
status or handshaking signals then do not use it. The value of <
syslinux_flow_control> is calculated by adding the hexadecimal values for the
desired flow control behaviours listed in Table 4-1.
The behaviours for a correctly-wired null-modem cable or a correctly
configured modem are marked "Required for full RS-232 compliance" in the
table. The sum of these values is 0xab3.
Table 4-1. SYSLINUX flow control bitmap
+------------------------------+---------+----------------------------------+
| Flow control behaviour |Hex value| Required for full RS-232 |
| | | compliance? |
+------------------------------+---------+----------------------------------+
|Assert DTR | 0x001 | Yes |
+------------------------------+---------+----------------------------------+
|Assert RTS | 0x002 | Yes |
+------------------------------+---------+----------------------------------+
|Wait for CTS assertion | 0x010 | Yes |
+------------------------------+---------+----------------------------------+
|Wait for DSR assertion | 0x020 | Yes |
+------------------------------+---------+----------------------------------+
|Wait for RI assertion | 0x040 | No |
+------------------------------+---------+----------------------------------+
|Wait for DCD assertion | 0x080 | Yes |
+------------------------------+---------+----------------------------------+
|Ignore input unless CTS | 0x100 | No |
|asserted | | |
+------------------------------+---------+----------------------------------+
|Ignore input unless DSR | 0x200 | Yes |
|asserted | | |
+------------------------------+---------+----------------------------------+
|Ignore input unless RI | 0x400 | No |
|asserted | | |
+------------------------------+---------+----------------------------------+
|Ignore input unless DCD | 0x800 | Yes |
|asserted | | |
+------------------------------+---------+----------------------------------+
Our preferred configuration of 9600bps, port 0, full RS-232 status signals
and CTS/RTS flow control is written as:
serial 0 9600 0xab3
Tip When using this configuration SYSLINUX will not display anything and will
not accept any typed character until the RS-232 status signals show a
connected modem call (or a connected terminal if you are using a
null-modem cable).
If you have a null modem cable with no RS-232 status signals and no flow
control then use:
serial 0 9600
Remember that the serial command must be the first line in \SYSLINUX.CFG.
-----------------------------------------------------------------------------
Chapter 5. Configure Linux kernel
The Linux kernel is configured to select the console by passing it the
console parameter. The console parameter can be given repeatedly, but the
parameter can only be given once for each console technology. So console=tty0
console=lp0 console=ttyS0 is acceptable but console=ttyS0 console=ttyS1 will
not work.
When multiple consoles are listed output is sent to all consoles and input is
taken from the last listed console. The last console is the one Linux uses as
the /dev/console device.
The syntax of the console parameter is given in Figure 5-1.
Figure 5-1. Kernel console syntax, in EBNF
console=ttyS<serial_port>[,<mode>]
console=tty<virtual_terminal>
console=lp<parallel_port>
console=ttyUSB[<usb_port>[,<mode>]
<serial_port> is the number of the serial port. This is defined in Figure 4-2
and discussed in Section 2.2. The examples in this HOWTO use the first serial
port, giving <serial_port> the value 0, which in turn gives kernel parameter
console=ttyS0.
If you are using the devfs device filesystem with your Linux installation the
kernel parameter for the first serial port is still ttyS0, even though the
first serial device is no longer known as /dev/ttyS0 but as /dev/ttys/0.
<mode> is defined in Figure 2-7 and is discussed in Section 2.3. The examples
in this HOWTO use 9600 bits per second, one start bit, eight data bits, no
parity, one stop bit, and no CTS/RTS flow control giving <mode> the value of
9600n8. When the current kernel flow control bugs are corrected this HOWTO
will once again recommend the value 9600n8r.
<usb_port> can specify the address of a USB dongle containing a serial port
to be used as a serial console.[6] For example, the serial port console=
ttyS0,9600n8 when moved to a USB serial dongle would be written as console=
ttyUSB0,9600n8. The USB subsystem is started rather late in the boot process,
console messages printed during boot before the USB subsystem is loaded will
be lost.
With no console parameter the kernel will use the first virtual terminal,
which is /dev/tty0. A user at the keyboard uses this virtual terminal by
pressing Ctrl-Alt-F1.
If your computer contains a video card then we suggest that you also
configure it as a console. This is done with the kernel parameter console=
tty0.
For computers with both a video card and a serial console in the port marked
"COM1:" this HOWTO suggests the kernel parameters:
Figure 5-2. Recommended kernel parameters, PCs with video card
console=tty0 console=ttyS0,9600n8
Kernel messages will appear on both the first virtual terminal and the serial
port. Messages from the init system and the system logger will appear only on
the first serial port. This can be slightly confusing when looking at the
attached monitor: the machine will appear to boot and then hang. Don't panic,
the init system has started but is now printing messages to the serial port
but is printing nothing to the screen. If a getty has been configured then a
login: prompt will eventually appear on the attached monitor.
For PCs without a video card, this HOWTO suggests the kernel parameters:
Figure 5-3. Recommended kernel parameters, PCs without video card
console=ttyS0,9600n8
These parameters are passed to the booting kernel by the boot loader. Next we
will configure the boot loader used by your Linux installation to pass the
console parameters to the kernel.
-----------------------------------------------------------------------------
5.1. Configure Linux kernel using LILO
For each image entry in /etc/lilo.conf add the line:
Figure 5-4. Recommended kernel parameters, LILO configuration
append="console=tty0 console=ttyS0,9600n8"
Sometimes the append line will already exist. For example
append="mem=1024M"
In this case, the existing append line is modified to pass all the
parameters. The result is:
append="mem=1024M console=tty0 console=ttyS0,9600n8"
As a complete example, a typical /etc/lilo.conf configuration from Red Hat
Linux 7.1 is:
Example 5-1. Complete LILO configuration, as installed by vendor
boot=/dev/hda
map=/boot/map
install=/boot/boot.b
prompt
timeout=50
message=/boot/message
default=linux
image=/boot/vmlinuz-2.4.2-2
label=linux
read-only
root=/dev/hda6
initrd=/boot/initrd-2.4.2-2.img
This is modified to
Example 5-2. Complete LILO configuration, modified for serial console
boot=/dev/hda
map=/boot/map
install=/boot/boot.b
prompt
default=linux
# Changes for serial console on COM1: in global section
# Deleted: message=/boot/message
serial=0,9600n8
timeout=100
restricted
password=de7mGPe3i8
image=/boot/vmlinuz-2.4.2-2
label=linux
read-only
root=/dev/hda6
initrd=/boot/initrd-2.4.2-2.img
# Changes for serial console on COM1: in each image section
append="console=tty0 console=ttyS0,9600n8"
Now that we have finished configuring LILO, use the lilo command to install
the new boot record onto the disk:
+---------------------------------------------------------------------------+
|bash# chown root:root /etc/lilo.conf |
|bash# chmod u=rw,g=,o= /etc/lilo.conf |
|bash# lilo |
|Added linux * |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
5.2. Configure Linux kernel using GRUB
Find each title entry in the GRUB configuration file. It will be followed by
a kernel line. For example
title Red Hat Linux (2.4.9-21)
root (hd0,0)
kernel /vmlinuz-2.4.9-21 ro root=/dev/hda6
initrd /initrd-2.4.9-21.img
Modify each of the kernel lines to append the parameters that inform the
kernel to use a serial console.
Figure 5-5. Recommened kernel parameters, GRUB configuration
title Red Hat Linux (2.4.9-21)
root (hd0,0)
kernel /vmlinuz-2.4.9-21 ro root=/dev/hda6 console=tty0 console=ttyS0,9600n8
initrd /initrd-2.4.9-21.img
As a complete example, Example 5-3 is a typical GRUB configuration from Red
Hat Linux 7.2.
Example 5-3. Complete GRUB configuration, as installed by vendor
default=0
timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
password --md5 $1$wwmIq64O$2vofKBDL9vZKeJyaKwIeT.
title Red Hat Linux (2.4.9-21)
root (hd0,0)
kernel /vmlinuz-2.4.9-21 ro root=/dev/hda6
initrd /initrd-2.4.9-21.img
The modified configuration file is shown in Example 5-4.
Example 5-4. Complete GRUB configuration, modified for serial console
default=0
timeout=10
password --md5 $1$wwmIq64O$2vofKBDL9vZKeJyaKwIeT.
serial --unit=0 --speed=9600 --word=8 --parity=no --stop=1
terminal --timeout=10 serial console
title Red Hat Linux (2.4.9-21)
root (hd0,0)
kernel /vmlinuz-2.4.9-21 ro root=/dev/hda6 console=tty0 console=ttyS0,9600n8
initrd /initrd-2.4.9-21.img
title Red Hat Linux (2.4.9-21) single user mode
lock
root (hd0,0)
kernel /vmlinuz-2.4.9-21 ro root=/dev/hda6 console=tty0 console=ttyS0,9600n8 s
initrd /initrd-2.4.9-21.img
-----------------------------------------------------------------------------
5.3. Configure Linux kernel using SYSLINUX
Edit each LABEL entry to add an APPEND line containing the serial console
parameter to pass to the Linux kernel. Like LILO, if a kernel already has
parameters, then add our parameters to the list after APPEND.
For example:
Figure 5-6. Recommended kernel parameters, SYSLINUX configuration
APPEND console=tty0 console=ttyS0,9600n8
There are some traps for beginners in the differences between LILO and
SYSLINUX. LILO uses append=, whereas SYSLINUX uses just append. lilo needs to
be run after each change to /etc/lilo.conf, whereas syslinux does not need to
be run after changing \SYSLINUX.CFG.
-----------------------------------------------------------------------------
Chapter 6. Configure getty
getty monitors serial lines, waiting for a connection. It then configures the
serial link, sends the contents of /etc/issue, and asks the person connecting
for their login name. getty then starts login and login asks the person for
their password. If the user does nothing, getty or login hang up and getty
goes back to waiting.
The getty command has been re-implemented numerous times. There is a wide
selection of getty clones, each with slight differences in behavior and
syntax. We will describe the traditional getty, and then some popular
alternatives.
One of the jobs of a getty is to set the TERM environment variable to
indicate the make and model of the terminal which is connecting. In this
HOWTO we set the terminal to the commonly emulated DEC VT100. If you
occassionally connect using a different terminal emulation then you can
interactively change your choice of terminal by setting TERM to the
appropiate terminal listed in /etc/termcap.
Figure 6-1. Interactively altering the connecting terminal's make and model
+---------------------------------------------------------------------------+
|bash$ TERM=kermit |
|bash$ tset -r |
+---------------------------------------------------------------------------+
A getty is also responsible for setting the time zone when a
permanently-connected remote terminal is located beyond the machine's default
time zone. The getty overrides the default timezone by setting the TZ
environment variable. As with the TERM environment variable, a user
connecting from a modem can interactively override the default time zone.
Figure 6-2. Interactively altering the connecting terminal's time zone
+---------------------------------------------------------------------------+
|bash$ TZ=Australia/Adelaide |
|bash$ export TZ |
+---------------------------------------------------------------------------+
If you do not know your time zone name, run the tzselect utility to generate
the appropiate contents for TZ.
But first, let's see how getty gets started in the first place.
-----------------------------------------------------------------------------
6.1. init system
The file /etc/inittab contains the background programs that used to keep the
system running. One of these programs is one getty process per serial port.
Figure 6-3. getty is started by init, based upon an entry in /etc/inittab
+---------------------------------------------------------------------------+
|co:2345:respawn:/sbin/getty ttyS0 CON9600 vt102 |
+---------------------------------------------------------------------------+
Each field in inittab is separated by a colon and contains:
co
Arbitrary entry for inittab. As long as this entry doesn't appear
anywhere else in inittab, you're okay. We named this entry co because
it's for the console.
Red Hat Linux 7.3 has a program called kudzu which configures the system
when it is booted. kudzu treats an inittab entry of co specially, setting
it for the attached monitor and keyboard or the serial console.
Hardcoding the value of co prevents this behaviour.
2345
Run levels where this entry gets started. Run levels 2, 3, 4 and 5 can be
used for an operational system, getty should not be used in other run
levels. The serial console still works in run level 1 (or single user
mode) even without a getty.
respawn
Re-run the program if it dies. We want this to happen so that a new login
prompt will appear when you log out of the console.
/sbin/getty ttyS0 CON9600 vt102
The command to run. In this case, we're telling getty to connect to /dev/
ttyS0 using the settings for CON9600 which exists in /etc/gettydefs. This
entry represents a terminal running at 9600bps. Initially assume that the
terminal is a later-model VT100.
After changing /etc/inittab restart init with
+---------------------------------------------------------------------------+
|telinit q |
+---------------------------------------------------------------------------+
An alternative is to send the hangup signal to init with the command kill
-HUP 1. This is not recommended: if you make a typing mistake and actually
kill init then your system will suddenly halt.
Note Comments in inittab and Red Hat's kudzu
<EFBFBD> kudzu uses the # line comment to activate and deactivate the gettys for
the attached monitor and keyboard and for the serial port. To prevent a
genuine comment from becoming confused with a line saved by kudzu use ##
at the start of a line of genuine comments.
-----------------------------------------------------------------------------
6.2. Traditional getty
Traditional getty implementations include uugetty and getty_ps.
The traditional getty is listed in /etc/inittab with the name of a section in
/etc/gettydefs to use for its configuration. Our example in Figure 6-3 used
the section CON9600.
There is no CON9600 in the standard gettydefs. This is deliberate, as serial
consoles sometimes require slight tweaking. Copy the DT9600 entry and use it
as your model.
Figure 6-4. Define CON9600 in gettydefs
# Serial console 9600, 8, N, 1, CTS/RTS flow control
CON9600# B9600 CS8 -PARENB -ISTRIP CRTSCTS HUPCL # B9600 SANE CS8 -PARENB -ISTRIP CRTSCTS HUPCL #@S @L login: #CON9600
Separate each line with a blank line.
Each configuration line has the syntax:
Figure 6-5. Syntax of entries in /etc/gettydefs, in EBNF
<label>#<23><initial_flags><3E>#<23><final_flags><3E>#<login_prompt>#<next_label>
The <label> is referred to on the getty command line.
The <next_label> is the definition used if a RS-232 Break is sent. As the
console is always 9600bps, this points back to the original label. See
Section 9.9 if you ever intend to have more one line for CON9600 in
gettydefs.
<initial_flags> are the serial line parameters used by getty. These are
modeled on the stty(1) and termios(3) options and the full list varies
depending upon your getty variant. The parameters in Figure 6-4 ensure that a
line at 9600bps with eight data bits and no parity is configured.
<final_flags> are the serial line parameters set by getty before it calls
login. You will usually want to set a 9600bps line, SANE terminal handling,
eight data bits, no parity and to hang up the modem when the login session is
finished.
The <login_prompt> for serial lines is traditionally the name of the machine,
followed by the serial port, followed by login: and a space. The macro that
inserts the name of the machine and the serial port varies, see the
documentation for your getty.
-----------------------------------------------------------------------------
6.3. agetty
agetty is an "alternative getty". It takes all of its parameters on the
command line, with no use of /etc/gettydefs or any other configuration file.
agetty is documented in the manual page agetty(8).
Figure 6-6 shows how to invoke agetty for use with a serial console.
Figure 6-6. /etc/inittab entry for agetty
co:2345:respawn:/sbin/agetty -h -t 60 ttyS0 9600 vt102
ttyS0 refers to the serial device /dev/ttyS0.
9600 is the bits per second of the serial link. agetty will support multiple
values, using the modem's CONNECT message or the RS-232 Break signal to
select between them. Only use one value, as serial consoles only have only
one data rate.
vt102 sets the TERM environment variable to indicate that a VT100 terminal is
connecting.
-h activates CTS/RTS handshaking.
-t 60 allows 60 seconds for someone to attempt to log in before the modem is
hung up. You should test this feature to ensure that init is not restarting
agetty every 60 seconds when the link is idle. Look for a continually
changing process identifier for agetty.
agetty uses escape sequences in /etc/issue to insert information. For
example, \n.\o \l will appear as remote.example.edu.au ttyS0.
When you log out agetty does not appear to lower the Data Terminal Ready
signal to force the modme to hang up. If having people automatically
disconnected at the end of their login session matters to you then you might
consider mgetty instead.
-----------------------------------------------------------------------------
6.4. mgetty
mgetty is a modem-aware getty. It supports modems with the Hayes AT command
set and is especially designed for supporting modems that are used to send
faxes and to dial out as well as dial in. These features are not required for
a serial console.
mgetty does not require the traditional /etc/gettydefs file. As a result
mgetty is invoked from /etc/inittab without supplying an entry in /etc/
gettydefs.
Figure 6-7. /etc/inittab entry for mgetty
co:2345:respawn:/sbin/mgetty ttyS0
mgetty is configured using the file /etc/mgetty+sendfax/mgetty.config. It
should contain an entry for the port used by the serial console.
Figure 6-8. mgetty configuration file mgetty.config
port ttyS0
speed 9600
direct yes
data-only yes
toggle-dtr yes
need-dsr yes
port-owner root
port-group root
port-mode 600
login-prompt @ \P login:\040
login-time 60
term vt102
All the options are documented in the PostScript file /usr/share/doc/mgetty??
/mgetty.ps.
We set direct, data-only, need-dsr and toggle-dtr so that the RS-232 control
lines are used correctly for a dumb modem.
port-owner, port-group and port-mode set the serial device to be accessible
only by the root user. Modem applications, which normally use the uucp group,
cannot now accidentally use the serial console.
login-prompt shows the machine (@) and serial port (\P) being used. The text
\040 is simply the octal code for a space after login:.
term vt102 gives the make and model of the terminal most likely to dial in.
This sets the TERM environment variable, which you can change if you are
dialling in from another terminal type.
The remaining configuration files, /etc/mgetty+sendfax/dialin.config and /etc
/mgetty+sendfax/login.config, do not need to be altered.
If you wish to alter the suggested configuration then note that mgetty's
blocking and toggle-dtr parameters do not co-exist well.
If you have difficulties, activate debugging by adding debug 8 to
mgetty.config. mgetty's actions are then visible in the file /var/log/
mgetty.log.ttyS0.
-----------------------------------------------------------------------------
6.5. mingetty
mingetty is designed to be a minimal getty for the virtual terminals on the
the workstation's monitor and keyboard. It has no support for serial lines.
You must not use mingetty for the serial line in /etc/inittab, but the
current mingetty entries for the virtual terminals can remain.
Each virtual terminal uses about 8KB of kernel memory. If this matters, it is
easy to allocate fewer virtual terminals. In the Linux 2.4 kernel virtual
terminals are created on demand, so not starting mingetty on the virtual
terminal will not create the virtual terminal. If the machine does not have a
video card then remove all the mingetty entries from /etc/inittab.
Figure 6-9. Fewer virtual terminals. Removing mingetty entries from /etc/
inittab
1:2345:respawn:/sbin/mingetty tty1
# Additional virtual terminals are not used
2:2345:off:/sbin/mingetty tty2
3:2345:off:/sbin/mingetty tty3
4:2345:off:/sbin/mingetty tty4
5:2345:off:/sbin/mingetty tty5
6:2345:off:/sbin/mingetty tty6
After restarting init it would be wise to remove the unused device files.
Figure 6-10. Fewer virtual terminals. Deallocating unused virtual terminals
and removing their device files.
+---------------------------------------------------------------------------+
|bash# telinit q |
|bash# deallocvt /dev/tty[2-9] /dev/tty[0-9][0-9] |
|bash# rm /dev/tty[2-9] /dev/tty[0-9][0-9] |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
6.6. No getty
If you are using serial console simply to print console messages then do not
run a getty process on the serial port.
getty follows a locking convention that prevents other serial port
applications from using the serial port. Since we do not want other processes
to use the serial port, but are not running getty, manually create the lock
file.
Create a file /var/lock/LCK..ttyS0 to contain the text 1. This lets other
potential serial port applications know that process 1 has the serial port in
use. Process 1 is always the init process, and init is always running, so the
serial port is always locked.
The file is created upon each system boot, as lock files are often cleared
when the system boots. A convenient place to create the lock file is from /
etc/rc.serial. It should contain:
Figure 6-11. Contents of /etc/rc.serial to lock console serial port when no
getty used
# Lock /dev/ttyS0 as it is used by an output-only console
(umask 022 && \
rm -f '/var/lock/LCK..ttyS0' && \
echo '1' > '/var/lock/LCK..ttyS0')
-----------------------------------------------------------------------------
Chapter 7. Configure incidentals
A surprising number of other configuration files need small modifications
before the serial console works well.
The configuration of many items depends upon your security requirements,
especially depending upon the level of trust and corresponding need for
security at the remote site. By assuming a high need for security at the
remote site this HOWTO can illustrate a large number of configuration items.
-----------------------------------------------------------------------------
7.1. Allow root to login from serial console
The file /etc/securetty controls the devices that the root user can log in
upon.
It is usually desirable to have root be able to log in from the console, so
add the basename of the serial console device to /etc/securetty.
Figure 7-1. Alter securetty to allow root to log in from the serial console
ttyS0
Almost anyone can now dial into the modem and attempt to guess the root
password. Normally we do not allow root to log in from a remote site, rather
we have a normal user log in and then use su or sudo to become root. This
gives some traceability.
Unfortunately, the root user needs to be able to log in from the console to
fix a full disk. Disk subsystems typically reserve 5% of their space for
root's exclusive use.[7] This is enough space for the root user to log in and
start deleting the files that filled the disk.
Note securetty and Red Hat's kudzu
<EFBFBD> kudzu automatically adds the device being used as the console to
securetty.
-----------------------------------------------------------------------------
7.2. Change init level to textual
There is little point in running the X Window System on a server with no
screen. Edit /etc/inittab finding the line containing initdefault, such as
id:5:initdefault:
Alter the default from run level 5 (multiuser with X Window System) to run
level 3 (multiuser).
id:3:initdefault:
The startx command can be used if an occassional X Windows session is
required upon an attached keyboard and monitor.
Note Run levels and Red Hat's kudzu
<EFBFBD> kudzu automatically updates the initdefault entry in inittab to use run
level 3 if a serial device is being used as a console.
-----------------------------------------------------------------------------
7.2.1. Continuing to run X
Sometimes a computer with a serial console and no attached monitor still
needs to run the X Window System. For example, the computer might host a
number of X terminals.
In this case the computer should remain in run level 5, but should not run a
X server for any attached monitors. Alter /etc/X11/xdm/Xservers and remove
any lines starting with a colon (which indicates an X server on the local
machine). Figure 7-2 shows an unaltered Xservers file.
Figure 7-2. Xservers from Red Hat Linux 7.2
:0 local /usr/X11R6/bin/X
If the operating system uses GNOME's gdm then alter its configuration file /
etc/X11/gdm/gdm.conf, removing any entries for local X servers from the
[servers] section. Figure 7-3shows an unaltered [servers] section.
Figure 7-3. [servers] section of gdm.conf from Red Hat Linux 7.2
[servers]
0=/usr/bin/X11/X
-----------------------------------------------------------------------------
7.3. Remove saved console settings
The file /etc/ioctl.save contains the serial and terminal parameters to use
in single user mode. The serial and terminal parameters are usually set by
getty ?? during single user mode no getty runs and the contents of /etc/
ioctl.save are used to set the serial and terminal parameters.
As we are changing consoles, the saved settings are no longer correct.
Figure 7-4. Removal of ioctl.save containing the saved console parameters
+---------------------------------------------------------------------------+
|bash# rm -f /etc/ioctl.save |
+---------------------------------------------------------------------------+
We re-create this file once we can log in from the serial console.
-----------------------------------------------------------------------------
7.4. Serial console is not /dev/modem
In many Linux distributions the file /dev/modem is a symbolic link to the
serial port containing a modem which is available for use.
Although the serial console is a serial port with a modem, we certainly don't
want it used to place an outgoing call.
Check that /dev/modem does not point to the serial port being used for the
console, say /dev/ttyS0. If it does, then remove the symbolic link.
Figure 7-5. Remove /dev/modem if it points to the serial console's port
+---------------------------------------------------------------------------+
|bash$ ls -l /dev/modem |
|lrwxrwxrwx 1 root root 10 Jan 01 00:00 /dev/modem -> /dev/ttyS0 |
|bash# rm /dev/modem |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
7.5. Alter target of /dev/systty
In many Linux distributions the file /dev/systty is a symbolic link to the
device which is used as the by the attached monitor and keyboard. See Section
1.3 for a fuller description.
If there is no attached keyboard and monitor or no wish to give the attached
keyboard and monitor greater capabilities then a text terminal, then alter /
dev/systty to point to the serial console.
Rather than directly altering this symbolic link, it is better to modify the
configuration file used by MAKEDEV, which is then run to recreate the
symbolic link. The configuration file is in the directory /etc/makedev.d. The
default configuration will point to the first virtual terminal, as shown in
Figure 7-6.
Figure 7-6. Default value of /dev/systty in /etc/makedev.d/linux-2.4.x
l systty tty0
Modify this to point to the serial port being used by the console, as shown
in Figure 7-7.
Figure 7-7. Alter value of /dev/systty in MAKEDEV configuration file
+---------------------------------------------------------------------------+
|bash# cd /etc/makedev.d |
|bash# fgrep systty * |
|linux-2.4.x:l systty tty0 |
|bash# vi linux-2.4.x |
+---------------------------------------------------------------------------+
l systty ttyS0
Now re-create /dev/systty using its new definition, as shown in Figure 7-8.
Figure 7-8. Installing new value of /dev/systty
+---------------------------------------------------------------------------+
|bash# cd /dev |
|bash# rm systty |
|bash# ./MAKEDEV systty |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
7.6. Configure Pluggable Authentication Modules
The Pluggable Authentication Module system can be used to give special
privileges to users that logged in through the console. It is used to make
devices like the floppy disk mountable by the console's user; usually they
would need to become the super-user to mount a disk.
The PAM configuration file /etc/security/console.perms contains the <console>
variable. For Red Hat Linux 7.1 <console> is the regular expression:
Figure 7-9. Default <console> in console.perms refers to attached keyboard
and screen
<console>=tty[0-9][0-9]* vc/[0-9][0-9]* :[0-9]\.[0-9] :[0-9]
Later in the file the <console> user is granted permission to use some
devices. This is done by altering the devices' permissions upon login and
logout.
Figure 7-10. Default device listing in console.perms
<console> 0660 <floppy> 0660 root.floppy
<console> 0600 <sound> 0600 root
<console> 0600 <cdrom> 0660 root.disk
<console> 0600 <pilot> 0660 root.uucp
<console> 0600 <jaz> 0660 root.disk
<console> 0600 <zip> 0660 root.disk
<console> 0600 <ls120> 0660 root.disk
<console> 0600 <scanner> 0600 root
<console> 0600 <camera> 0600 root
<console> 0600 <memstick> 0600 root
<console> 0600 <flash> 0600 root
<console> 0600 <fb> 0600 root
<console> 0600 <kbd> 0600 root
<console> 0600 <joystick> 0600 root
<console> 0600 <v4l> 0600 root
<console> 0700 <gpm> 0700 root
<console> 0600 <mainboard> 0600 root
<console> 0600 <rio500> 0600 root
There are two types of devices listed above: those devices required by
someone connecting from an attached keyboard and monitor and those devices
that allow convenient access to devices. The configuration file fails to make
the distionction between logical and physical console noted in Section 1.3.
The configuration file is modified to create that distinction.
Figure 7-11. Devices in console.perms required for attached keyboard and
screen
<console> 0600 <fb> 0600 root
<console> 0600 <kbd> 0600 root
<console> 0600 <joystick> 0600 root
<console> 0600 <v4l> 0600 root
<console> 0700 <gpm> 0700 root
The remaining devices should be altered to give control only to people
attaching from the serial console. For example, we don't want an unprivileged
user at a co-location site mounting a floppy disk. Define a new console type
for the serial console, say <sconsole>.
Figure 7-12. Add <sconsole> in console.perms to refer to serial console
<sconsole>=ttyS0
Now modify the remaining entries from <console> to <sconsole>.
Figure 7-13. Remaining devices in console.perms altered to refer to serial
console
<sconsole> 0660 <floppy> 0660 root.floppy
<sconsole> 0600 <sound> 0600 root
<sconsole> 0600 <cdrom> 0660 root.disk
<sconsole> 0600 <pilot> 0660 root.uucp
<sconsole> 0600 <jaz> 0660 root.disk
<sconsole> 0600 <zip> 0660 root.disk
<sconsole> 0600 <ls120> 0660 root.disk
<sconsole> 0600 <scanner> 0600 root
<sconsole> 0600 <camera> 0600 root
<sconsole> 0600 <memstick> 0600 root
<sconsole> 0600 <flash> 0600 root
<sconsole> 0600 <mainboard> 0600 root
<sconsole> 0600 <rio500> 0600 root
-----------------------------------------------------------------------------
7.7. Configure Red Hat Linux
Red Hat Linux stores parameters concerning system start up in the file /etc/
sysconfig/init.
Alter the parameter BOOTUP to use terminal-independent commands to write the
OK, PASSED and FAILED messages. These messages will no longer appear in
green, yellow or red. The comments in /etc/sysconfig/init suggest that any
value other than color will do, but it seems that BOOTUP must be set to
serial.
Alter the PROMPT parameter to disallow interactive start up. Allowing an
unauthenticated keystroke to stop system services is not robust against line
noise and allows anyone that dials in during system boot to deny services.
Figure 7-14. Alterations to /etc/sysconfig/init for Red Hat Linux
BOOTUP=serial
PROMPT=no
Red Hat Linux runs a hardware discoverer, named kudzu. When attempting to
identify a serial port Kudzu resets the serial port. This stops the serial
console. Kudzu is configured from the file /etc/sysconfig/kudzu.
Kudzu can be prevented from resetting hardware by setting the configuration
parameter SAFE to yes.
Figure 7-15. Alterations to /etc/sysconfig/kudzu for Red Hat Linux
SAFE=yes
-----------------------------------------------------------------------------
Chapter 8. Reboot and test
8.1. Verify console operation
If possible, plug an RS-232 breakout box into the serial port. During reboot
the Data Terminal Ready line should become active and then the Transmit Data
lights should flash as console messages appear.
Attach a modem, or a null modem cable and a terminal. Configure them to match
the serial parameters used by the serial console port. If using a modem, dial
in to it from a terminal emulator.
+---------------------------------------------------------------------------+
|+++ |
|AT Z |
|AT DT 1234-5678 |
|CONNECT 9600 |
+---------------------------------------------------------------------------+
Configure the terminal or terminal emulator to match the serial parameters
used by the serial console. If using a modern Hayes AT-style modem then the
speed need not match. If using a directly-attached terminal then the speed
must match.
Reboot the computer.
+---------------------------------------------------------------------------+
|bash# shutdown -r now |
+---------------------------------------------------------------------------+
During reboot the terminal should see the usual boot loader text, and then
the default kernel booting, then the init output, and finally the contents of
/etc/issue and getty asking you to login.
+--------------------------------------------------------------------------------------------------------------------------+
|LILO: |
| |
|Linux version ?? |
|Kernel command line: auto BOOT_IMAGE=linux ro root=306 BOOT_FILE=/boot/vmlinuz-2.4.3-12 console=tty0 console=ttyS0,9600n8 |
|?? |
|INIT version ?? |
|?? |
|/etc/issue says "All your base are belong to us". |
|remote.example.edu.au ttyS0 login: |
+--------------------------------------------------------------------------------------------------------------------------+
If you do not see the login: message then press Return or Enter.
-----------------------------------------------------------------------------
8.2. Re-create saved console settings
Log in as root from the serial console and send the console into single user
mode. The modem may hang up whilst doing this and you may need to re-connect.
Without a /etc/ioctl.save containing the saved terminal settings, init
assumes a directly attached terminal running at 9600bps with 8 data bits, no
parity, 1 stop bit and no flow control. Configure your terminal with these
settings.
+---------------------------------------------------------------------------+
|remote.example.edu.au ttyS0 login: root |
|Password: ?? |
|sh# rm -f /etc/ioctl.save |
|bash# telinit 1 |
|??Telling INIT to go to single user mode. |
|INIT: Going single user |
|INIT: Sending processes the TERM signal |
|sh# stty sane -parenb cs8 crtscts brkint -istrip -ixoff -ixon |
+---------------------------------------------------------------------------+
As you use stty to alter the Linux's terminal settings remember to also alter
the settings of the attached terminal.
Exiting from single user mode back to the default run level will save the
serial console termnial configuration into /etc/ioctl.save.
+---------------------------------------------------------------------------+
|sh# exit |
|?? |
|bash# ls -l /etc/ioctl.save |
|-rw------- 1 root root 60 Jan 1 00:00 /etc/ioctl.save |
+---------------------------------------------------------------------------+
The terminal settings saved in /etc/ioctl.save will be used if the machine
boots into single user mode for any reason.
If your attached terminal or modem cannot alter speed to 9600bps then the
above procedure cannot be followed. ioctlsave has been written for this
special case. It saves the current terminal settings to a file in the same
format as ioctl.save. The procedure is shown in Figure 8-1.
Figure 8-1. Using ioctlsave to create /etc/ioctl.save without entering single
user mode
+---------------------------------------------------------------------------+
|remote.example.edu.au ttyS0 login: root |
|Password: ?? |
|bash# rm -f /etc/ioctl.save |
|bash# ioctlsave -t /dev/ttyS0 /etc/ioctl.save |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
8.3. Test the console
Dial in from a machine, perhaps using Minicom.
Example 8-1. Dialing into a serial console
+---------------------------------------------------------------------------+
|localhost bash$ minicom |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|Initializing modem |
|Welcome to minicom 1.83.1 |
|Press ALT-Z for help on special keys |
|AT S7=45 S0=0 L1 V1 X4 &C1 E1 Q0 |
|OK |
|Alt-D remote.example.edu.au-ttyS0 |
|Dialing: remote.example.edu.au-ttyS0 At: 1234-5678 |
|Connected. Press any key to continue |
|Any |
|CONNECT 115200/V34/LAPM/V42BIS/33600:TX/33600:RX |
+---------------------------------------------------------------------------+
+--------------------------------------------------------------------------------+
|Enter |
|/etc/issue says "All your base are belong to us". |
|remote.example.edu.au ttyS0 login: user |
|Password: ******** |
|Message of the day is "be careful out there". |
|remote bash$ stty -a |
|speed 9600 baud; rows 0; columns 0; line = 0; |
|intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; |
|eol2 = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; |
|lnext = ^V; flush = ^O; min = 1; time = 0; |
|-parenb -parodd cs8 hupcl -cstopb cread -clocal crtscts |
|-ignbrk brkint ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl -ixon -ixoff |
|-iuclc -ixany -imaxbel |
|opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab3 bs0 vt0 ff0 |
|isig icanon -iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt |
|-echoctl -echoke |
|?? |
|remote bash$ logout |
+--------------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|NO CARRIER |
|Alt-X |
|Leave Minicom? Yes |
|Resetting modem |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|localhost bash$ |
+---------------------------------------------------------------------------+
Interestingly the stty -a command, used to display the terminal settings,
reports that the link from the modem to the serial console is 9600bps. The
CONNECT message reports that link between the two modems operates at 33600bps
. The constant speed modem-computer link is a very useful feature of the
Hayes AT-style modems: the calling computer need not know in advance the line
speed of the called serial console.
-----------------------------------------------------------------------------
8.4. Where to next from here?
The serial console is now configured. Check the security pointers given in
Chapter 9 to complete the job.
-----------------------------------------------------------------------------
Chapter 9. Security
Using a serial console with a modem gives anyone the opportunity to connect
to the console port. This connection is not mediated by firewalls or
intrusion detection sniffers. It is important to prevent the misuse of the
serial console by unauthorized people.
The resurgence of the BBS-era technique of "war dialling" is described in
@Stake's Wardialling Brief and reported upon by The Register, see an extract
in Figure 9-1.
Figure 9-1. Extract from Crackers favour war dialling and weak passwords
With all the talk about zero day exploits and sometimes esoteric
vulnerabilities its easy to lose sight of the role of older, less
sophisticated techniques as a mainstay of cracker activity.
During a hacking debate at InfoSecurity Europe yesterday [2002-04-25],
black hat hacker KP said that when he broke into a network he did so 90
per cent of the time through an unprotected modem, often through war
dialling.
War dialling involves systematically trying to locate the numbers
associated with corporate modems through testing each extension of a
corporate phone system in turn.
"Intrusion detection systems are no real deterrent for me because I get
in through the back door," he said. "Many networks are constructed like
Baked Alaska ?? crunchy on the outside and soft in the middle."
KP often takes advantage of weak or default passwords to break into
networks??
Crackers favour war dialling and weak passwords
John<EFBFBD>Leyden,<2C>The Register,<2C>2002-04-26.
-----------------------------------------------------------------------------
9.1. Use good passwords
Anyone that can guess the BIOS password, the boot loader password, or the
root password can get full control of the machine. These should be different,
unrelated, excellent passwords. Random text and digits are by far the best
choice. You should never use a password that you think would return a hit
from a search engine.[8]
Guessing a user's password is only slightly less severe, as a hacker can
obtain root access simply by waiting. The hacker waits for a "local exploit"
for a flaw in the operating system to appear and uses that exploit before the
machine is patched.
Severely limit the number of users on the machine. Ensure that only good
passwords are chosen by using a fascist password checker such as a cracklib
-based PAM module.
You should write down the BIOS password, the boot loader password and the
root password. Now you don't need to remember them, so there is no reason for
them not to be totally random, unrelated, excellent passwords. Fold the page,
put it in an envelope and seal it.
Now we have turned a computer security problem into a physical security
problem. We know how to solve those problems: locks, keys, alarms, safes,
guards, regular inspections. If your site has staffed security then a good
option is to leave the envelope in the care of the guard post with
instructions to treat the envelope with the same procedures used for the
site's master keys. Smaller sites can use a safe, a cash box or a locked
drawer. A thief forcing a locked drawer still leaves shows more apparent
signs of entry and more clues to their identity than is left by a hacker
behind a modem.
These three passwords are an important corporate asset. If the machine is
secure then forgetting the major passwords for the machine should result in a
machine whose configuration cannot be altered by actions short of
disassembly. You should have written procedures controlling the generation,
storage, lifetime and use of major passwords.
-----------------------------------------------------------------------------
9.2. Obey Data Terminal Ready and Data Carrier Detect
The RS-232 Data Terminal Ready signal is lowered when the computer wishes the
modem to hang up. The computer wishes to hang up when people have ended their
login session ends or when they fail to respond to the login: prompt.
Using a modem cable that has DTR wired and a modem that is configured to obey
DTR is essential to prevent denial of service attacks upon the access to the
console.
Without DTR a caller can simply hold the modem line open, denying system
administrators access to the console.
The RS-232 Data Carrier Detect signal is lowered when the user hangs up.
Using a modem cable that has DCD wired and a modem that is configured to
assert DCD is essential to prevent people dialling in after a user has hang
up and from carrying on their session.
Without DCD the session is not cleared when an accidental disconnection
occurs. This allows any subsequent caller to resume the previous session. The
machine is totally compromised if the previous user was root.
-----------------------------------------------------------------------------
9.3. Use or configure a dumb modem
Most modems use the Hayes AT command set. The modem's attention is gained by
sending +++ surrounded by some idle time. Commands are then sent prefixed by
AT.
Unfortunately, if the modem sees +++ during a call it may revert to command
mode. The modem can then be configured by the caller. For example, the modem
could be set to permit incoming calls only from the number "0", this would
deny the system administrators access to the modem.
The attention command can be removed using AT S2=255. Of course once that is
done no more AT commands can be given to the modem, so any other
configuration of the modem needs to be done prior to that command.
Unfortunately, when power to the modem is applied the modem starts in command
mode. So a carefully chosen console message could disable the modem.
The best solution is to select a modem that has a "dumb" or "select profile"
DIP switch or jumper. These switches disable command mode and load the
modem's saved configuration when they start.
-----------------------------------------------------------------------------
9.4. Restrict console messages
9.4.1. Restrict console messages from the system log
Generating a stready stream of console messages can easily overwhelm a 9600
bps link.
Although displaying all syslog messages on the console appears to be a good
idea, this actually provides the unprivileged user a simple method to deny
effective use of the remote console.
Configure system log messages to the console to the bare minimum. Look in /
etc/syslog.conf for lines ending with /dev/console.
Consider sending all log messages to another machine for recording and
analysis. Figure 9-2 shows the standard /etc/syslog.conf from Red Hat Linux
7.2 modified to record log messages to a log server. Each line of syslog.conf
has been repeated to send a copy of the message to the log server. The log
server has the DNS alias loghost.example.edu.au; using a DNS alias allows the
log server to be moved without updating the configuration of all the remote
machines. The local copy of the log message is no longer the only means of
determining the cause of a system failure, so we can gain some performance
advantage by disabling synchronous file writes, although this increases the
odds of an inconsistent filesystem (an issue with filesystems that do not do
journalling). Placing a - before the filename disables synchronous file
writes.
Figure 9-2. /etc/syslog.conf modified to copy log messages to a log server
# Log anything (except mail) of level info or higher.
# Don't log private authentication messages!
*.info;mail.none;authpriv.none;cron.none @loghost.example.edu.au
*.info;mail.none;authpriv.none;cron.none -/var/log/messages
# The authpriv file has restricted access.
authpriv.* @loghost.example.edu.au
authpriv.* /var/log/secure
# Log all the mail messages in one place.
mail.* @loghost.example.edu.au
mail.* -/var/log/maillog
# Log cron stuff
cron.* @loghost.example.edu.au
cron.* -/var/log/cron
# Everybody gets emergency messages
*.emerg @loghost.example.edu.au
*.emerg *
# Save news errors of level crit and higher in a special file.
uucp,news.crit @loghost.example.edu.au
uucp,news.crit -/var/log/spooler
# Save boot messages also to boot.log
local7.* @loghost.example.edu.au
local7.* -/var/log/boot.log
A log server is configured using the standard /etc/syslog.conf configured to
allow the reception of remote syslog messages. This configuration for Red Hat
Linux is shown in Figure 9-3. In addition to configuring the system log
daemon, also prevent denial of service attacks by configuring IP Tables to
restrict the sources of the syslog messages; and also improve performance by
checking that nscd is running to cache reverse DNS lookups.
Figure 9-3. Allowing remote log messages by setting options in /etc/sysconfig
/syslog
# Red Hat Linux default value, does not write timer mark messages
SYSLOGD_OPTIONS="-m 0"
# Add option to accept remote syslog messages
SYSLOGD_OPTIONS="${SYSLOGD_OPTIONS} -r"
Figure 9-4. Restrict syslog messages to remote.example.edu.au
+-----------------------------------------------------------------------------------------------------------------+
| bash# chkconfig iptables on |
| bash# /etc/init.d/iptables restart |
|# Allow all IP traffic from this machine |
| bash# iptables --append INPUT --source 127.0.0.0/8 --in-interface lo --jump ACCEPT |
|# Perhaps filter other traffic |
|?? |
|# Accept syslog messages from remote.example.edu.au |
| bash# iptables --append INPUT --source remote.example.edu.au --protocol udp --destination-port syslog -j ACCEPT |
|# Silently drop unexpected syslog messages |
| bash# iptables --append INPUT --protocol udp --destination-port syslog -j DROP |
|# Save the running configuration |
| bash# /etc/init.d/iptables save |
+-----------------------------------------------------------------------------------------------------------------+
Figure 9-5. Using nscd to cache reverse DNS lookups
+---------------------------------------------------------------------------+
|bash# chkconfig nscd on |
|bash# /etc/init.d/nscd restart |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
9.4.2. Restrict broadcast messages to the console
Users that are logged into the serial console should not accept broadcast
messages. Add new files to /etc/profile.d to do this. Figure 9-6 shows a file
for use by the Bourne shell.
Figure 9-6. Restrict sending of messages to console user
#
# Do we have files referred to?
if [ -x /usr/bin/mesg -a -x /usr/bin/tty ]
then
# Are we on serial console?
if [ `/usr/bin/tty` = /dev/ttyS0 ]
then
# Do not accept broadcast messages
/usr/bin/mesg n
fi
fi
As this file is run frequently, we use a faster but less readable version of
Figure 9-6, shown in Figure 9-7.
Figure 9-7. Restrict sending of messages to console user, /etc/profile.d/
mesg.sh
#
# /etc/profile.d/mesg.sh -- prevent people hassling the serial console user
[ -x /usr/bin/mesg -a -x /usr/bin/tty -a `/usr/bin/tty` = /dev/ttyS0 ] && /usr/bin/mesg n
We also need a C shell version, shown in Figure 9-8.
Figure 9-8. Restrict sending of messages to console user, /etc/profile.d/
mesg.csh
#
# /etc/profile.d/mesg.csh -- prevent people hassling the serial console user
if (-X mesg && -X tty && `tty` == /dev/ttyS0) then
mesg n
endif
Although mesg.sh and mesg.csh are included by the parent shell rather than
executed, the files need the execute permission set. The procedure in Figure
9-9 installs the files and sets the permissions.
Figure 9-9. Install files into /etc/profile.d
+---------------------------------------------------------------------------+
|bash# cp mesg.*sh /etc/profile.d/ |
|bash# chown root:root /etc/profile.d/mesg.*sh |
|bash# chmod u=rwx,g=rx,o=rx /etc/profile.d/mesg.*sh |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
9.5. Modem features to restrict usage
Most modems support the addition of a password. This is not particularly
useful as it has the same strengths and weaknesses of all other password
authentication schemes. We already have password authentication in the BIOS,
in the boot loader and in login.
Many modems support call-back. The modem is called and a few seconds after
hang-up it calls a pre-configured number. This limits the locations that can
gain access to the console.
Many modems support checking the calling line identification (CLI) against a
predefined list. If the calling number is not on the list then the call is
cleared. The phone line to the modem must be configured to send CLI, this may
incur an additional charge from the phone company. Not all calling phones can
send CLI and some valid callers may have asked their phone company to
suppress the sending of CLI.
Many modems can be configured to log the calling line identification. This is
useful when tracing misuse.
Many modems support encryption. Some modems allow multiple keys. This gives a
neat solution: only authorized modems can dial in, but they can do so from
any location. The modems usually need to be of the same make, and perhaps of
the same model.
Warning Encryption dual-use technology
<EFBFBD> Possessing, using, buying, selling, importing or exporting modems
with encryption features is a serious criminal offense in some
countries.
You must acquiant yourself with the laws in your jurisdiction and the
laws of jurisdictions you may travel through.
Many telephone services or PBX lines can be set to allow only incoming calls.
This is useful as it prevents misuse of the modem should the computer be
compromised. A "demon dialler" can call many numbers seeking an answering
modem and the cost of these calls can be significant.
-----------------------------------------------------------------------------
9.6. BIOS features
Most BIOSs can be configured with a "configuration password". This should set
and tested. Some motherboards will require a jumper to be set to allow the
password to take effect. Some BIOSs have well-known "master passwords", use a
search engine to ensure that your BIOS is not one of these. The password
should not be the same as the boot loader or root passwords.
The BIOS configuration will have a "boot order" setting. It should be set to
boot from the hard disk before any other media. This prevents someone
inserting a rescue diskette, booting the machine, and gaining access to the
filesystems as root.
-----------------------------------------------------------------------------
9.7. Use a boot loader password
Configure the boot loader to request a password when booting a non-default
image or when supplying parameters from the command line.
This prevents someone from dialing in during the boot sequence and booting
the kernel with options to take control of the machine, as in Example 4-1.
The password should not be the same as the BIOS or root passwords.
-----------------------------------------------------------------------------
9.8. Non-interactive boot sequence
Red Hat Linux has an "interactive boot" option that can be used to prevent
services from being started. This may not be pleasant if the purpose of the
machine is web serving and the HTTP daemon is interactively prevented from
starting by an unauthenticated person.
Edit /etc/sysconfig/init to contain the line
PROMPT=no
-----------------------------------------------------------------------------
9.9. Magic SysRq key
The "magic SysRq key" is a key sequence that allows some basic commands to be
passed directly to the kernel. Kernel software developers use this interface
to debug their software. Under most circumstances it can also be used to
uncleanly reboot the computer, something that is otherwise difficult or
expensive to do remotely.
For computers that are not used for kernel software development the magic
SysRq key makes an ideal denial of service device. A few unauthenticated
keystrokes and the computer is dead in the water. The console, serial or
otherwise, must be in an area with access limited to trusted people.
The serial console uses the RS-232 break function as the "magic SysRq key". A
"break" is a period of no transmission on the serial line, on traditional
terminals it is activated by pressing a key labeled Break.
Anyone can dial into a modem and send a break, so if the serial console is
attached to a modem we need to disable the magic SysRq key . If the serial
console is attached to a terminal server which asks for authentication, or is
attached directly to another terminal using a null modem cable then you may
decide to activate the magic SysRq key.
The magic SysRq key can be disabled by setting a kernel variable or by not
compiling support for the key.
Writing a 0 into /proc/sys/kernel/sysrq will disable the magic SysRq key. The
command sysctl can also be used:
Figure 9-10. Using sysctl to defeat the magic SysRq key
+---------------------------------------------------------------------------+
|bash# sysctl -w kernel.sysrq=0 |
+---------------------------------------------------------------------------+
Your Linux distribution may have a file /etc/sysctl.conf which is used to run
sysctl during the boot of the machine. Add the lines:
Figure 9-11. Configuring /etc/sysctl.conf to defeat the magic SysRq key
# Disables the magic SysRq key
kernel.sysrq = 0
Even when setting the magic SysRq key off in /etc/sysctl.conf there is a
period of vulnerability after the kernel boots but before contents of the
file are applied.
It is much better to compile your own kernel and set the following
configuration parameter:
Figure 9-12. Kernel make menuconfig showing disabled SysRq key
+---------------------------------------------------------------------------+
|Kernel hacking ---> |
| [ ] Magic SysRq key |
+---------------------------------------------------------------------------+
This should place the following configuration parameter in /usr/src/linux
/.config.
Figure 9-13. Kernel .config showing disabled SysRq key
# CONFIG_MAGIC_SYSRQ is not set
-----------------------------------------------------------------------------
9.10. Adjust behaviour of Ctrl-Alt-Delete
The IBM PC used Ctrl-Alt-Delete to launch a reboot of the computer. Linux
traps this key chord and makes it available to the init system. This is done
by sending the init process a SIGINT signal (although ctrlaltdel hard can
undo this trap and make the key chord reboot the comptuer immediately). The
init system uses /etc/inittab to determine how to handle the signal generated
by the Ctrl-Alt-Delete key chord.
Most distributions cleanly reboot the system, mimicing the behaviour that
most users expect. Figure 9-14 shows how this is done.
Figure 9-14. Default handling of Ctrl-Alt-Delete in /etc/inittab
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
Depending upon each individual site you may wish to disable Ctrl-Alt-Delete.
This is shown in Figure 9-15.
Figure 9-15. Ignoring Ctrl-Alt-Delete in /etc/inittab
# Trap CTRL-ALT-DELETE and do nothing
ca::ctrlaltdel:
Alternatively, you may wish to cleanly shut down the computer. This is very
easy to explain to operators and instructions can be displayed on the monitor
using /etc/issue or a Post-it Note. If the computer uses Advanced Power
Management (or APM) then shutting down the computer will also remove the
power.
Figure 9-16. Shut down cleanly upon Ctrl-Alt-Delete in /etc/inittab
# Trap CTRL-ALT-DELETE and shut down
ca::ctrlaltdel:/sbin/shutdown -t3 -h now
-----------------------------------------------------------------------------
9.11. Log attempted access
Look in the system logs for the output of getty. Add the monitoring of these
messages to your log-watching procedures.
-----------------------------------------------------------------------------
9.12. Countering interception of telephony links
Modems calls over telephones can be intercepted. This can be an issue if you
do not trust a telecommunications carrier in your call's path, or if you do
not trust the law enforcement agencies that may request interception
facilities from that carrier.
International calls are particularly exposed. Calls which are routed across
satellite or wireless links can be intercepted by readily-available radio
receivers. Calls routed across undersea links are much more expensive to
intercept, so this is probably limited to national governments, such as those
using the Echelon system.
If you do not pass sensitive data over the link, then the major exposure is
typing in your user name and password. Look into S/KEY or look into OPIE and
its related An OPIE for PAM.
These one-time password systems have flaws, a good summary of these is
Vulnerabilities in the S/KEY one time password system by Peiter ??mudge??
Zatko.
Warning Cryptographic key material
<EFBFBD> Possessing cryptographic key material, such as a one-time password
generator or list of one-time passwords, is a serious criminal
offense in some countries.
You must acquiant yourself with the laws in your jurisdiction and the
laws of jurisdictions you may travel through.
Warning Defeating telecommunications interception
<EFBFBD> Taking steps to defeat or avoid legislatively-approved
telecommunications interception is a serious criminal offense in some
countries.
You must acquiant yourself with the laws in your jurisdiction and the
laws of jurisdictions you may travel through.
-----------------------------------------------------------------------------
Chapter 10. Configuring a kernel to support serial console
Most Linux kernels shipped by distributors are configured to allow the serial
console to be enabled. However system administrators will almost certainly
encounter some problems best solved by recompiling a kernel. In these cases
configure the kernel to support the serial console. The usual virtual
terminal console is also configured, as we normally want console messages to
go a monitor as well as the serial port.
-----------------------------------------------------------------------------
10.1. Linux kernel version 2.5
Kernel version 2.5 is under active development, so this section may be out of
date. Version 2.5 includes support for the console to a serial port attached
to a USB dongle. The -dj patch to the version 2.5 kernel has a rewritten
console layer; it is not known if the rewritten layer effects the user-space
use of the serial console.
When configuring the kernel set the following configuration parameters:
Figure 10-1. Kernel configuration for serial console using make menuconfig
+---------------------------------------------------------------------------+
|Character devices ---> |
| [*] Virtual terminal |
| [*] Support for console on virtual terminal |
| <*> Standard/generic (8250/16550 and compatible UARTs) serial support |
| [*] Support for console on serial port |
+---------------------------------------------------------------------------+
This should set the following configuration parameters in /usr/src/linux
/.config.
Figure 10-2. Kernel configuration for serial console using .config
CONFIG_VT=y
CONFIG_VT_CONSOLE=y
CONFIG_SERIAL=y
CONFIG_SERIAL_CONSOLE=y
If you also want to use a serial port attached to a USB bus, then in addition
to the usual USB configuration, configure the kernel to load the USB console
driver and one of the USB serial dongles (our example uses the generic serial
dongle).
Figure 10-3. Kernel configuration for USB dongle serial console using make
menuconfig
+---------------------------------------------------------------------------+
|USB Serial Converter support ---> |
| <M> USB Serial Converter support |
| [M] USB Serial Console device support |
| [M] USB Generic Serial Driver |
+---------------------------------------------------------------------------+
This should set the following configuration parameters in /usr/src/linux
/.config
Figure 10-4. Kernel configuration for USB dongle serial console using .config
CONFIG_USB_SERIAL=m
CONFIG_USB_SERIAL_CONSOLE=m
CONFIG_USB_SERIAL_GENERIC=m
You should also configure the kernel without the magic SysRq key, as
described in Section 9.9.
-----------------------------------------------------------------------------
10.2. Linux kernel version 2.4
When configuring the kernel set the following configuration parameters:
Figure 10-5. Kernel configuration for serial console using make menuconfig
+---------------------------------------------------------------------------+
|Character devices ---> |
| [*] Virtual terminal |
| [*] Support for console on virtual terminal |
| <*> Standard/generic (8250/16550 and compatible UARTs) serial support |
| [*] Support for console on serial port |
+---------------------------------------------------------------------------+
This should set the following configuration parameters in /usr/src/linux
/.config.
Figure 10-6. Kernel configuration for serial console using .config
CONFIG_VT=y
CONFIG_VT_CONSOLE=y
CONFIG_SERIAL=y
CONFIG_SERIAL_CONSOLE=y
You should also configure the kernel without the magic SysRq key, as
described in Section 9.9.
-----------------------------------------------------------------------------
10.3. Linux kernel version 2.2
The later Linux 2.2 kernels use the same build parameters and parameter
syntax as the Linux version 2.4 kernels.
For earlier kernels see the [http://www.linuxjournal.com/article.php?sid=
2040] article by Francesco Conti in issue 36 of Linux Journal published in
April 1997.
This article included some patches for the kernel, which have been extended
in the notes below to use a broader range of serial port speeds.
Choose to use the serial console by adding a couple of #defines at the start
of /usr/src/linux/drivers/char/console.c:
#define CONFIG_SERIAL_ECHO
#define SERIAL_ECHO_PORT 0x3f8 /* COM1 port address */
Alternatively, to use ttyS1 use these lines:
#define CONFIG_SERIAL_ECHO
#define SERIAL_ECHO_PORT 0x2f8 /* COM2 port address */
The kernel assumes a serial link speed of 9600bps. If you are using a
differing bit rate then find these two lines:
serial_echo_outb(0x00, UART_DLM); /* 9600 baud */
serial_echo_outb(0x0c, UART_DLL);
and change 0x0c to one of the values in Table 10-1.
Table 10-1. IBM-PC/AT serial port bit rates and their bit-clock divisors
+----------+---------+
| Bit Rate | Divisor |
+----------+---------+
|115200bps | 0x01 |
+----------+---------+
| 57600bps | 0x02 |
+----------+---------+
| 38400bps | 0x03 |
+----------+---------+
| 19200bps | 0x06 |
+----------+---------+
| 9600bps | 0x0c |
+----------+---------+
| 4800bps | 0x18 |
+----------+---------+
| 2400bps | 0x30 |
+----------+---------+
| 1200bps | 0x60 |
+----------+---------+
-----------------------------------------------------------------------------
Chapter 11. Serial cabling
11.1. Jargon
RS-232 cables were originally intended to link terminals to modems. The
terminal is formally named a Data Terminal Equipment, abbreviated to DTE. The
modem is formally named a Data Communications Equipment, abbreviated to DCE.
A standard RS-232 cable has a 25-pin D-type socket, which connects to the DTE
, and a 25-pin D-type plug, which connects to the DCE. All 25 pins are
connected, with pin 1 on the plug wired to pin 1 on the socket, pin 2 on the
plug wired to pin 2 on the socket, and so on. The shielding of the cable is
attached to the metallic cover on the socket.
RS-232 signaling is much more robust than the signalling of many other
communications standards. Pins can be shorted, not connected or drive more
than one output.
Signals are named from the point of view of the Data Terminal Equipment. So
Transmit Data on the DTE is connected to Transmit Data on the DCE. The
Transmit Data pin on the DTE actually transmits data, whereas Transmit Data
pin on the DCE actually recieves data.
-----------------------------------------------------------------------------
11.2. Cable from console port to modem
The RS-232 standard defines the interconnection of computers and modems, so
there is little to go wrong here by simply purchasing a pre-assembled cable.
There are two types of cable: cables with connectors for a standard 25-pin D
connector on the computer; and cables with connectors for a proprietary 9-pin
D connector used on the IBM PC/AT and many other computers. The cables have
titles like RS-232 25-pin computer (DTE) to 25-pin modem (DCE) or RS-232
9-pin IBM PC/AT computer (DTE) to 25-pin modem (DCE). Most modems are
packaged with a suitable cable.
If you need to manufacture your own cables, see the Serial-HOWTO for the
RS-232 pinout for your computer. Connect Transmit Data on the computer to
Transmit Data on the modem, Receive Data on the computer to Receive Data on
the modem, and so on for Signal Ground, Clear to Send, Ready to Send, Data
Set Ready, Data Terminal Ready and Data Carrier Detect.
For professional computer room installations consider routing the serial
cable through an RJ-45 patch panel. There are two common pinouts on used on
the RJ-45 connector: [http://yost.com/Computers/RJ45-serial/] Yost and Cisco
2500-series.
If you create your own pinout for unshielded twisted pair cable then be sure
that your pinout twists a Signal Ground wire with the Transmit Data wire and
another Signal Ground wire with the Receive Data wire. Although the RS-232
signals are not balanced, this twist will result in the least amount of
signal degradation and noise pickup.
-----------------------------------------------------------------------------
11.3. Cable from console port to terminal (or another PC)
The RS-232 standard allows for, but does not specify, the interconnection of
two computers without intervening modems. A special cable is required, called
a "null modem" cable.
The wiring within the null modem cable depends upon the handshaking and
control signals that are needed. Differing manufacturers have differing views
on this topic, so don't buy a null modem cable that does not come with a
wiring diagram.
Linux needs all of the flow control and modem control signals to be correctly
wired. The correct wiring of a null modem cable is shown in Figure 11-1 with
an alternative shown in Figure 11-2.
Linux uses CTS and RTS to do handshaking, preventing the computer from
overrunning the terminal and preventing the terminal from overrunning the
computer. If you are connecting two computers together, then you will not get
reliable file transfers without CTS/RTS handshaking.
Linux uses DSR and DCD to sense that a terminal is connected. It will then
request a login. If a session is established and DCD falls then Linux will
log out the user.
Linux uses DTR to force the link to be cleared. It does this after a user
logs off to free up the communications channel.
Either of the null modem designs in Figure 11-1 or Figure 11-2 meets the
requirements of the Linux kernel. Figure 11-2 may be marginally better when
both computers are remotely located, as the differing states of DSR and DCD
can be used to determine which end of the null modem cable has become faulty.
All null modem designs have a common flaw. Computers interconnected with real
modems modem will drop Data Set Ready for some time after the local modem is
reset by the local computer dropping Data Terminal Ready. Most software is
designed to accomodate this slight difference between modem links and null
modem links.
Major security exposures and significant loss of reliability can occur with
incorrectly wired null modem cables, including the cables in Figure 11-3,
Figure 11-4 and Figure 11-5.
Figure 11-1. Null modem cable with full status and handshaking
Signal ground ---------------------- Signal ground
Receive data ---------------------- Transmit data
Transmit data ---------------------- Receive data
Ready to send ---------------------- Clear to send
Clear to send ---------------------- Ready to send
Data terminal ready -----------------+---- Data carrier detect
|
+---- Data set ready
Data carrier detect ----+----------------- Data terminal ready
|
Data set ready ----+
Ring indication -- not connected
not connected -- Ring indication
Figure 11-2. Variation on null modem cable with full status and handshaking
Signal ground ---------------------- Signal ground
Receive data ---------------------- Transmit data
Transmit data ---------------------- Receive data
Ready to send ---------------------- Clear to send
Clear to send ---------------------- Ready to send
Data terminal ready ----+----------------- Data carrier detect
|
Data set ready ----+
+---- Data set ready
|
Data carrier detect ----+------------+---- Data terminal ready
Ring indication -- not connected
not connected -- Ring indication
Unfortunately not all Linux boot loaders support the control signals required
by the Linux operating system. This odd state of affairs may force you to do
away with control signals and handshaking if you need to issue commands to
the boot loader.
There are two ways of defeating the RS-232 handshaking: software and
hardware.
If you have a modem then by far the best technique is to disable the control
signals and handshaking by using AT commands to configure the modem's
software. This allows the handshaking to be restored when the boot loader
authors correct their support for serial connections.
For a null modem cable the best approach is to disable handshaking in your
terminal emulation software.
In the worst case for a null modem you will need a cable that falsifies the
handshaking and control signals. Try not to use these cables in a production
environment.
Figure 11-3. Null modem cable with falsified status and handshaking
Signal ground ---------------------- Signal ground
Receive data ---------------------- Transmit data
Transmit data ---------------------- Receive data
Data terminal ready ---+ +--- Data terminal ready
| |
Clear to send ---+ +--- Clear to send
| |
Data carrier detect ---+ +--- Data terminal ready
| |
Data set ready ---+ +--- Data set ready
Ready to send -- not connected
not connected -- Ready to send
Ring indication -- not connected
not connected -- Ring indication
If you are happy with a quick hack, perhaps just to use a serial console to
grab a kernel oops message, then you can configure some getty programs to
ignore the RS-232 status signals. For example, mgetty has the direct option
in mgetty.conf. In this case only a three-wire or two-wire RS-232 null modem
cable is needed.
Figure 11-4. Null modem cable with no status or handshaking
Signal ground ---------------------- Signal ground
Receive data ---------------------- Transmit data
Transmit data ---------------------- Receive data
Figure 11-5. One-way null modem cable with no status or handshaking
Signal ground ---------------------- Signal ground
Transmit data ---------------------- Receive data
Don't use these cables in a production environment.
-----------------------------------------------------------------------------
11.4. Lengths of serial cables
The RS-232 standard 9600bps port will drive 15 metres of shielded cable. More
precisely, an RS-232 line driver will operate against a capacitance of up to
2500 picoFarad with low enough skew to allow a 9600bps signal to be
recovered.
If you select a cable with lower capacitance you can drive further distances.
For example, ANSI/TIA/EIA-568-A unshielded twisted pair category 5 cable has
a maximum capacitiance of 55pF per metre, so this popular "UTP cat 5" cable
can be safely driven up to 45m. Beyond that you should check the cable
manufacturers specifications for the actual "shunt capacitance" (a common
figure is 47.5 pF/m, giving a maximum cable length of about 50m). However
long runs of unshielded cable will pick up noise easily, as the RS-232
signals are not balanced. Some cable manufacturers offer shielded low
capacitance cables which can be driven up to 100m.
Similarly, if you select a lower data rate you can drive further distances.
Table 11-1 shows the maximum distances over standard shielded cable at
differing data rates.
Table 11-1. Data rates and the maximum distances recommended in RS-232
+---------------+---------------+
|Data rate (bps)| Distance (m) |
+---------------+---------------+
| 2400 | 60 |
+---------------+---------------+
| 4800 | 30 |
+---------------+---------------+
| 9600 | 15 |
+---------------+---------------+
| 19200 | 7.6 |
+---------------+---------------+
| 38400 | 3.7 |
+---------------+---------------+
| 56000 | 2.6 |
+---------------+---------------+
If you are comfortable in working beyond specifications then you might note
that the experience of enterprise network operators has been that structured
cabling layout in buildings is limited by the 100m distance limitation of
fast ethernet over category 5 cable, not by the practical distances achieved
by RS-232 asynchronous signals at 9600bps over category 5 cable.
For longer distances use an RS-232 line driver; these will typically drive up
to 2000 meters over category 3 UTP cable. For greater distances consider
using fiber optical modems, the global telephony system, the mobile telephony
system, satellite or radio.
-----------------------------------------------------------------------------
11.5. Making serial cables
If you use a serial console for densely-racked computers you will end up
making a lot of null-modem serial cables. This section has some hints on
making serial cables. If you are making more than ten cables and live in a
city you will probably find it economic to have the cables made by a
specialty cabling firm.
Attempt to minimise noise in your cabling design. Many BIOSs and boot loaders
will wait forever if they receive a single character of line noise. You might
choose to use shielded UTP cables (these require special RJ-45 plugs but use
standard RJ-45 sockets).
If the environment has a lot of radio frequency noise then use traditional
shielded cable and metal RS-232 connector shells. Connect the shield in the
cable to the computer at one end. This can be done by connecting the drain
wire of the shield it to the Protective Ground (if present) or by soldering
the drain wire to the shell of the connector. If there is a substantial
amount of noise also place a ferrite core over the shielded cable at both
ends of the cable. Follow the usual good practices of making the cable to the
correct length and screwing home the D connectors into the chassis.
If you are making one of these cables and have some soldering skill, you can
easily do the jumpering of the signal wires within the backshell of the DB9
or DB25 connector.
If you are making a large number of cables then crimping systems are much
faster than soldering. Again, pin jumpering can be done within the backshell.
No matter what system is adopted, use the Resistance setting of a multimeter
to check for dead and shorted pins. A minute here can save hours later.
For structured cabling systems, space is tight within DB9/RJ-45 backshells,
so the jumpering is better done behind the patch panel. The DB9/RJ-45
connectors present the IBM PC pinout at the DB9 connector and present the
Yost or Cisco pinout at the RJ-45 connector.
Caution Incompatible devices in structured cabling systems
<EFBFBD> Take care to connect only RS-232 devices to RS-232 devices when
patching structured cabling systems. Other cables may be carrying
ethernet, ISDN, telephony, alarm and DC power voltages. Connecting
incompatible voltages may destroy equipment.
-----------------------------------------------------------------------------
Chapter 12. Modem configuration
12.1. Using Minicom to give commands to a modem
Minicom is a full-screen serial terminal emulation package, very much like
the classic Telix terminal emulator for MS-DOS.
Firstly, start Minicom in configuration mode with the command:
+---------------------------------------------------------------------------+
|bash# minicom -o -s |
+---------------------------------------------------------------------------+
The following menu appears:
+---------------------------------------------------------------------------+
|Filenames and paths |
|File transfer protocols |
|Serial port setup |
|Modem and dialing |
|Screen and keyboard |
|Save setup as dfl |
|Save setup as.. |
|Exit |
|Exit from Minicom |
+---------------------------------------------------------------------------+
Select Serial port setup and set
+---------------------------------------------------------------------------+
|A - Serial Device: /dev/ttyS0 |
|B - Lockfile Location: /var/lock |
|C - Callin Program: |
|D - Callout Program: |
|E - Bps/Par/Bits: 9600 8N1 |
|F - Hardware Flow Control: Yes |
|G - Software Flow Control: No |
+---------------------------------------------------------------------------+
Now save the configuration
+---------------------------------------------------------------------------+
|Give name to save this configuration? |
|> console |
+---------------------------------------------------------------------------+
and exit Minicom.
To configure a modem use the command minicom -o console to start Minicom
without sending an initialization string to the modem. Now issue the AT
commands to configure the modem.
When finished use the Quit option to leave Minicom without sending a reset
string to the modem; this option is Alt-Q.
Sometimes Minicom will use Ctrl-A rather than Alt to access the menu system,
look for a hint in Minicom's start up message:
+---------------------------------------------------------------------------+
|Press ALT-Z for help on special keys |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|Press CTRL-A Z for help on special keys |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
12.2. Configure dumb modem
Linux, like most UNIX-like operating systems, expects a serial console to be
connected to a dumb modem. Dumb modems are not seen much these days, perhaps
only on exotic hardware such as ISDN terminal adapters or satellite ground
terminals.
A dumb modem is configured using hardware. Figure 12-1 shows the front panel
of a fanciful dumb modem. In reality the speed and mode settings are likely
to be done using jumpers or DIP switches.
Figure 12-1. Front panel of a dumb modem
+-----------------------------+
| |
| SPEED MODE |
| [ ] 300 [ ] Originate |
| [ ] 600 [X] Answer |
| [ ] 2400 |
| [X] 9600 |
| |
+-----------------------------+
The modem's speed is set to the desired bit rate, in our case 9600bps. The
modem's mode is set to Answer, that is, to wait for incoming calls and to
answer them.
If the RS-232 control line Data Terminal Ready is low, the modem will not
answer a call. The computer is off or the computer's serial interface is not
yet initialized. Once DTR is high the modem will answer incoming calls.
Once an incoming call is established the modem raises the Data Carrier Detect
control line. Only when DCD is high is received data valid (data receieved
from a dumb modem when DCD is not asserted is probably line noise). Only when
DCD is high is transmitted data passed through the link.
getty on the Linux computer has been waiting for DCD to come high, and getty
welcomes the user and requests them to log in.
Whilst the user is logged in and data is flowing, Clear to Send and Ready to
Send are used between the modem and the computer to prevent data being sent
too soon. The computer lowers Ready to Send when it is too busy to receive a
character. The modem lowers Clear to Send when it is too busy to receive a
character.
When the user hangs up, Data Carrier Detect falls and the hang up signal is
sent to all processes associated with the dial in session.
Alternatively, the user can log out. When the shell dies, the computer pulls
Data Terminal Ready low, causing the modem to hang up. When the getty brings
Data Terminal Ready high again, the modem will accept more incoming calls.
We have not yet described Data Set Ready. This line is low if the modem is
off or if the modem has not yet initialized. When DSR is low all other
signals from the modem are undefined. For example, if DSR is low but DCD
"floats" to the high voltage then software should behave as if DCD is not
asserted.
-----------------------------------------------------------------------------
12.3. Configure modem with AT commands
Most modems today are smart modems based upon the Hayes modems and their
command sets. But as discussed above, the Linux serial console is designed to
operate with a dumb modem.
Thus the smart modem is dumbed-down until it resembles a dumb modem. Some
expensive modems will have a DIP switch or board jumper to put them into dumb
mode.
It is essential to have a manual for the modem which describes that modem's
AT commands. Although most modems agree on the more popular AT commands, they
differ in the more technical commands.
-----------------------------------------------------------------------------
12.3.1. Configure port speed
Hayes AT-style modems can maintain a static speed between the computer and
the modem, no matter what speed the dialing modem uses.
For most modems this is set automatically based upon the speed of the first
characters sent after power-on.
Power cycle the modem and connect to it with the command minicom -o console.
Press Enter a few times. The modem should now be running at the same bit rate
used by Minicom, which we set to the speed of the serial console in Section
12.1.
You can check the port speed by asking the modem to generate some output.
Figure 12-2. Testing the modem's port speed
+---------------------------------------------------------------------------+
|bash# minicom -o console |
|Welcome to minicom |
|Press CTRL-A Z for help on special keys |
| |
|Enter Enter Enter |
| |
|ATI Enter |
|56k V.90 Series 3 External V2.20 |
| |
|Ctrl-A Q |
|Leave without reset? Yes |
+---------------------------------------------------------------------------+
Some modems have an AT command to re-establish the port speed, look in your
modem's manual for the AT&B1 command. Some modems have a command to
explicitly set the port speed, look in you modem's manual for the ATB
command.
-----------------------------------------------------------------------------
12.3.2. Configure answer mode
The modem will answer an incoming call on the second ring using the command
ATS0=2.
Don't answer the phone on the first ring as this may invalidate the
certification of the modem in some telephony jurisdictions.
-----------------------------------------------------------------------------
12.3.3. Configure CTS/RTS handshaking
CTS/RTS handshaking prevents lost characters.
The AT command is AT&K3.
-----------------------------------------------------------------------------
12.3.4. Configure Data Carrier Detect
Data Carrier Detect should follow the presence or absence of a calling modem.
The AT command is AT&C1.
-----------------------------------------------------------------------------
12.3.5. Configure Data Terminal Ready
Data Terminal Ready should control the modem. If DTR is high the modem is
ready to receive calls. If DTR is low the modem should not receive any more
calls and should hang up any existing call.
The AT command is AT&D2.
-----------------------------------------------------------------------------
12.3.6. Configure no CONNECT messages
A Hayes AT-style modem usually outputs a message when a call is received. For
example:
+---------------------------------------------------------------------------+
|CONNECT 9600 |
+---------------------------------------------------------------------------+
The modem has a "quiet mode" that disables these messages.
The AT command is ATQ1. There will be no OK printed in response to this
command.
-----------------------------------------------------------------------------
12.3.7. Configure no echo of commands
Echoing commands can confuse the console, so turn off command echoing.
The AT command is ATE0.
-----------------------------------------------------------------------------
12.3.8. Optionally, configure silent connection
Most modems have a speaker. By default this is connected whilst a modem is
connecting and negotiating a common protocol and speed. This is very useful
for a dialing modem, as it prevents a human being accidentally repeatedly
called. The speaker can be annoying on answering modems.
If a quieter computer room is desirable, use the ATM0 command to turn off the
speaker.
-----------------------------------------------------------------------------
12.3.9. Optionally, configure DTR delay
Data Terminal Ready drops when the semiconductor that supports the RS-232
link is reset. This then hangs up the modem. This can be annoying. If the
getty supports a parameter similar to mgetty's toggle-dtr-waittime then it is
possible to extend the time that the modem will ignore DTR. The time that
getty holds DTR low to force a hang up is extended beyond the modem's
setting. The result is that resetting the semiconductor does not hang up the
modem, but getty can still hang up the modem at the end of a login session.
Check your modem's documentation. Our example modem uses S-register 25 to
contain the threshold for noticing a change in DTR. The value is in
one-hundreds of a second. By setting the modem with ATS25=150 (1.5 seconds)
and setting mgetty with toggle-dtr-waittime 2000 (2 seconds) we ignore small
blips in DTR.
-----------------------------------------------------------------------------
12.3.10. Configure no attention sequence
Once the modem is correctly configured and works well, disable the +++
sequence that gives access to the modem's command mode.
The AT command is ATS2=255.
If this command is accidentally given see Section 12.3.12 to reset the modem
to its factory default parameters and start again.
-----------------------------------------------------------------------------
12.3.11. Configuration example
Figure 12-3. Configure modem using AT commands
+---------------------------------------------------------------------------+
|bash# minicom -o console |
|Welcome to minicom |
|Press CTRL-A Z for help on special keys |
| |
|AT &F Enter |
|OK |
| |
|AT Z Enter |
|OK |
| |
|AT &C1 &D2 &K3 S0=2 M0 Enter |
|OK |
| |
|AT E0 Q1 S2=255 &W Enter |
| |
|Alt-A Q |
|Leave without reset? Yes |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
12.3.12. Resetting the modem
If you need to issue more AT commands to the modem then power cycle the
modem. This should place the modem into command mode.
Now issue the following commands to restore the modem's factory
configuration.
Figure 12-4. Resetting a Hayes AT-style modem
+---------------------------------------------------------------------------+
|bash# minicom -o console |
|Welcome to minicom |
|Press CTRL-A Z for help on special keys |
| |
|AT &F &Y0 &W &W1 Enter |
|OK |
|AT Z Enter |
|OK |
| |
|Alt-A Q |
|Leave without reset? Yes |
+---------------------------------------------------------------------------+
If this fails then you will need to clear the modem's configuration memory.
The procedure for this varies by manufacturer, and probably requires the
disassembly of the modem.
-----------------------------------------------------------------------------
12.4. Internal modems
An internal modem is basically an external modem and serial port mounted upon
a PC bus card. These are cheaper than external modems as they do not require
a power supply or a chassis.
Internal modems work fine for remote serial console applications. They are
especially attractive for computers at co-location sites, as those sites
charge according to space and power consumption.
Check that your internal modem preserves its setting across a power cycle.
Ensure that the interrupt line and port address space used by the internal
modem's serial port do not conflict with that used by any other pre-existing
serial ports. Alternatively, ensure that the internal serial port can be
disabled, freeing its interrupt line and port address space for use by the
internal modem.
Be careful not to confuse an internal modem with a WinModem. An internal
modem does not need a special device driver, but appears to Linux as a
stardard serial port.
-----------------------------------------------------------------------------
12.5. WinModems
If you look at a modem, with it's small central processing unit and
special-purpose digital signal processor, and then look at a modern PC, with
it's large CPU and general-purpose DSP on the sound card, you may wonder if
the hardware duplication of an external modem is necessary.
A "WinModem" incorporates the CPU and DSP of the modem into the
slightly-enhanced fabric of a PC. They are called "WinModems" because they
originally only shipped with Microsoft Windows device drivers. These device
drivers presented the illusion of a serial port attached to a Hayes AT-style
modem. For a long time only Windows versions of these drivers where
available. Some manufacturers now provide Linux versions of their device
drivers as well, these modems are jokingly called "LinModems".
It is probably possible to use a LinModem as a Linux console. At the most
this would require altering the source code to dumb-down the AT command
emulation of the modem and recompiling the kernel.
Boot loaders, however, work in a very confined software environment and
struggle to support a simple serial chip. Considering that some boot loaders
do not even handle interrupts, handling the complex DSP of a LinModem is well
beyond what is practical.
-----------------------------------------------------------------------------
Appendix A. Bugs and annoyances
A.1. Flow control in Linux kernel
The Linux kernel can be asked to do CTS/RTS flow control using the r option
on the console= parameter. For example, a serial link at 9600bps with 8 data
bits, no parity and CTS/RTS flow control is configured as shown in Figure A-1
.
Figure A-1. A kernel console parameter with CTS/RTS flow control
console=9600n8r
Because the Linux kernel only ever sends data, CTS/RTS flow control is
implemented by checking that Clear to Send is not asserted. The code which
does is found in /usr/src/linux/drivers/char/serial.c, the relevant portion
can be seen in Figure A-2.
Figure A-2. Kernel source code for console CTS/RTS flow control
static inline void wait_for_xmitr(struct async_struct *info)
{
??
/* Wait for flow control if necessary */
if (info->flags & ASYNC_CONS_FLOW) {
tmout = 1000000;
while (--tmout &&
((serial_in(info, UART_MSR) & UART_MSR_CTS) == 0));
}
}
The loop driven by the tmout value of 1000000 results in a wait of about one
second for the CTS line to become asserted.
This code ignores the status of the RS-232 Data Set Ready and Data Carrier
Detect status lines. This has a number of consequences.
<EFBFBD><EFBFBD>*<2A>If the RS-232 cable is unplugged or the terminal server port is idle then
the code waits for CTS to be asserted for about one second for every
character written to the console. So the huge number of characters
written to the console when booting a machine can result in a very long
wait for a reboot.
<EFBFBD><EFBFBD>*<2A>Clear to Send is only validly asserted if Data Carrier Detect and Data
Set Ready are asserted. The code should allow for an unpowered device
which allows CTS to float.
<EFBFBD><EFBFBD>*<2A>After looping one million times, if Clear to Send is not assrted then the
character is sent in any case. Thus the kernel cannot be used on
multidrop RS-232 lines. The character should be dropped instead.
<EFBFBD><EFBFBD>*<2A>The character is sent even if Data Carrier Detect is not asserted. Thus
the attached modem may be in command mode. This results in a security
flaw if an attacker can get arbitrary text placed in a console messages.
As many console messages contain error text derived from user events, it
would not be too difficult to place AT&F in a console message and
unprogram the modem's auto-answer configuration.
As a result of these bugs this HOWTO no longer recommends the use of
kernel-level flow control. The author has a kernel patch which fixes all
current-reported bugs and is attempting to get that patch integrated into the
mainline kernel. Once the kernel bugs are corrected this HOWTO will once
again recommend kernel-level flow control.
-----------------------------------------------------------------------------
A.2. Red Hat Linux 7.1 and SysVinit
The System V init system shipped with Red Hat Linux 7.1 does not support
serial console correctly in single user mode. See Red Hat advisory RHBA-2001:
085-02 New SysVinit package to fix hangs on serial console. The advisory
announces an update to the package SysVinit-2.78-15.i386.rpm that is shipped
on the Red Hat Linux 7.1 CD.
-----------------------------------------------------------------------------
A.3. BIOSs, keyboards and video cards
Some BIOSs will not boot if the keyboard is not installed.
+---------------------------------------------------------------------------+
|Keyboard faulty, press F1 |
+---------------------------------------------------------------------------+
Most BIOSs have settings that will allow them to boot without a keybaord.
Some odd BIOSs will not boot if no video card is installed.
-----------------------------------------------------------------------------
A.4. Modem hangs up upon reboot
During reboot the serial controller is reset. This drops the modem control
line Data Terminal Ready. This in turn instructs the modem to hang up.
Avoid the temptation to configure the modem to ignore DTR. This leads to a
worse bug, where the telephone line does not clear down correctly, the modem
is engaged, and there is no way to clear it. Ignoring DTR also gives no way
to clear hostile callers from the line.
You may wish to record the amount of time that the computer takes from
Restarting system to the boot loader prompt.
The modem may also hang up during the boot process (as the serial chip is
reset) or when the init run level is changed (as getty is restarted).
-----------------------------------------------------------------------------
A.5. init and syslog output does not display on secondary consoles
The kernel can be configured to output messages to the serial port and to the
attached monitor. However messages from init and syslog only appear on the
last-listed console device, in our case the serial port.
This can confuse someone looking at the attached monitor, as the messages on
the monitor suggest that the machine has hung just before starting init.
Eventually the machine will finish booting and getty will display a login:
request. A Post-it Note on the monitor may reassure the impatient.
-----------------------------------------------------------------------------
A.6. The console is unresponsive after connecting
The terminal's screen may be blank after connecting to the machine. Pressing
Enter will usually bring up a login: request.
If no characters appear upon the screen after pressing Enter do not panic.
The machine must have power and the operating system must have booted: for
our call to be answered by the modem Data Terminal Ready must be active.
The most likely thing is that the machine booted and is running a fsck
filesystem check. These checks can take some considerable time, all with no
or very little output.
It will help your peace of mind considerably to record in the system log book
the time fsck takes to check each filesystem.
If you see garbled text after pressing Enter then there are mismatched bit
rates or parity parameters. Correct your terminal emulator's configuration.
-----------------------------------------------------------------------------
A.7. Modem hangs up during initialization
Using setserial will reset the serial port. This will hang up the modem.
setserial is sometimes used during the boot process, resulting in the output
seen in Figure A-3. Look into the file /etc/rc.serial and remove any
references to the port which is being used as the serial console.
Figure A-3. setserial causes a modem to hang up as the machine initializes
+---------------------------------------------------------------------------+
|?? |
|Mounting local filesystems: [ OK ] |
|Turning on user and group quotas for local filesystems: [ OK ] |
|Enabling swap space: [ OK ] |
|/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A |
| |
|NO CARRIER |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
A.8. Boot loader has no flow control
Most boot loaders do not support CTS/RTS flow control. This can cause some
data loss where large speed mis-matches exist, as is often the case with a
modern modem connected into a 9600bps fixed-speed port.
SYSLINUX 1.66 supports flow control.
-----------------------------------------------------------------------------
A.9. Boot loaders are vulnerable to line noise
Most boot loaders will sit at their prompt forever after receiving a single
character of line noise.
Some modems will let the RS-232 signals "float", sending noise when there is
no caller. Because the modem is not asserting Data Carrier Detect it expects
the receiver to discard the noise characters.
The combination of an unfortunate boot loader with an unfortunate modem can
result in a machine that will regularly hang during booting.
If you cannot configure your boot loader to obey DCD then be careful to test
any modem you intend to purchase to ensure that it does not generate
characters when their is no caller. At the present only SYSLINUX implements
full RS-232 status signals.
-----------------------------------------------------------------------------
A.10. Advanced Power Management
APM allows control of the power from software. This can be a blessing and a
curse.
The blessing is that the machine can be cleanly and totally shut down
remotely. You may want to do this if the remote site is maintaining their
power supply.
The curse is that once powered down the machine will not start up again until
the Power button is physically pressed. Some machines have a BIOS or
motherboard setting to defeat this unhelpful behaviour.
Caution Errors when typing shutdown are worse with APM
<EFBFBD> Be careful not to confuse shutdown -r now, which cleanly reboots the
machine, with shutdown -h now, which cleanly powers down the machine.
Someone will need to physically press the Power button if you choose
wrongly.
If you are serious about remote site computing then you should investigate
remote power switches from companies like [http://www.wti.com/] Western
Telematic, [http://www.servertech.com/] Server Technology and many others.
Some models include built-in terminal servers, built-in modems and RS-232
lines to simulate a UPS input power failure (and thus shut the Linux system
down cleanly before removing power).
-----------------------------------------------------------------------------
A.11. Modems and overseas telecommunications requirements
There is no world-wide approval processes to certify that a modem is suitable
for connection to the telephone network. This is despite the presence of a
common set of technical standards that modems must meet for use on the global
switched telephone network. There is little or no recognition of one nation's
approvals by other national regulators.
There are national technical requirements concerning the use of modems.
Common requirements are to set the modem and its software to answer after the
second ring and never to dial the same engaged or faulty number more than
five times in a row.
Warning Telecommunications device approvals
<EFBFBD> Using or importing unapproved telecommications equipment is a
criminal offense in most countries.
Additionally, the operator of some types of equipment may require
certification.
Privacy laws may control what can be done with calling line identification
records.
Do not assume that Touch Tone dialling is globally available. There is no
common standard for decadic dialling: some countries have the longest
sequence for zero, other countries have the shortest sequence for zero.
There is little coordination of national numbering plans. Be careful not to
call a national emergency services number when intending to dial the
international access code. Common emergency services numbers are: 112, 911,
000. International access codes vary by country.
Intelligent network features such as toll-free numbers are usually not
available to calls originating from abroad.
International calls may be routed through fiber optical submarine cable,
satelite or High Frequency radio. The possible bit rates vary considerably
between these options. Expect the maximum throughput with no errors from
fiber optical submarine cable. Expect 1200bps to 2400bps with some errors
from satelite. Expect 75bps to 300bps with many errors from HF radio.
There will be considerable latency depending upon the distance. If the
latency becomes greater than the modem's error correction window then you
will get better Zmodem file transfer performance if you disable the HDLC
-based error correction in the modems.
International calls may have their signal altered considerably.
Traditionally, international calls are placed through analogue conditioning
circuits to minimise echo. This conditioning limits the maximum bit rate a
modem can achieve, probably to less than 9600bps. You may be able to program
a guard tone to disable analogue conditioning, this will vary by carrier and
the commands to send the guard tone vary by modem.
On some modern international circuits, particularly those accessed by
international calling cards, digital voice compression is used. No reliable
modem connection can be established over these digitally-compressed circuits.
The best current tactic for identifying these digitally compressed circuits
is to listen to the background noise ?? when no-one is speaking the real
background noise will be replaced by a synthesized background noise (a
compression technique called silence suppression).
-----------------------------------------------------------------------------
Appendix B. Uploading files from a serial console
There are many scenarios where the machine is dead in the water and you need
to upload a file to correct that. In many of these scenarios the only way to
upload the file is via the serial port being used as the console.
Moving files about over serial links has a long history in microcomputing and
this section goes back in time to uncover the tools commonly used in the
pre-Internet age of the Bulletin Board System.
-----------------------------------------------------------------------------
B.1. Disable logging to console
Before attempting to upload or download files it is a good idea to prevent
messages from appearing on the console. These messages will corrupt files
moved using cat and will cause Xmodem and similar protocols to take much,
much longer.
Alter your system's configuration to give klogd the -c 1 parameter,
inhibiting the display of kernel messages directly to the console. Kernel
messages will still go to the system logger.
Figure B-1. Supressing kernel messages to the console in Red Hat Linux
+---------------------------------------------------------------------------+
|bash# vi /etc/sysconfig/syslog |
+---------------------------------------------------------------------------+
KLOGD_OPTIONS="-2 -c 1"
+---------------------------------------------------------------------------+
|bash# /etc/init.d/syslog restart |
+---------------------------------------------------------------------------+
Also modify the system logger's configuration not to send messages to the
console. Edit /etc/syslog.conf, altering lines sending output to /dev/
console. Send this output to a file instead.
-----------------------------------------------------------------------------
B.2. ASCII upload and cat
cat is available on every UNIX-like system. It copies the data received from
the keyboard to a file. Minicom and other terminal emulators have an "ASCII
upload" facility that will send a file up the serial link as though it had
been typed.
+---------------------------------------------------------------------------+
|remote bash$ cat > upload.txt |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|Alt-S Upload ascii |
|[ascii upload - Press CTRL-C to quit] |
+---------------------------------------------------------------------------+
Wait<EFBFBD>for<EFBFBD>upload<EFBFBD>to<EFBFBD>complete??
+---------------------------------------------------------------------------+
|ASCII upload of "upload.txt" |
|10.0 Kbytes transferred at 3900 CPS... Done. |
|READY: press any key to continue... |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|Ctrl-D |
|remote bash$ |
+---------------------------------------------------------------------------+
Without hardware flow control ASCII upload will drop the occassional
character.
To upload binary files encode them into ASCII, upload them, and then decode
them into binary again.
+---------------------------------------------------------------------------+
|localhost bash$ uuencode upload.bin < upload.bin > upload.txt |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|Alt-S Upload ascii |
|[ascii upload - Press CTRL-C to quit] |
+---------------------------------------------------------------------------+
Wait<EFBFBD>for<EFBFBD>upload<EFBFBD>to<EFBFBD>complete??
+---------------------------------------------------------------------------+
|ASCII upload of "upload.txt" |
|10.0 Kbytes transferred at 3900 CPS... Done. |
|READY: press any key to continue... |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|Ctrl-D |
|remote bash$ |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|remote bash$ uudecode < upload.txt |
+---------------------------------------------------------------------------+
You can detect transmission errors by using a checksum program such as sum,
cksum or md5sum. Print the ckecksum of the file before it is sent from the
local machine and after it is recieved upon the remote machine.
+---------------------------------------------------------------------------+
|localhost bash$ cksum upload.bin |
|1719761190 76 upload.bin |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|remote bash$ cksum upload.bin |
|1719761190 76 upload.bin |
+---------------------------------------------------------------------------+
There are a number of checksumming programs. The sum command should be used
with caution, as there are versions for BSD and System V UNIX which give
differing results. cksum is the attempt by the POSIX standards developers to
correct that mess: it gives the same result for the same file on all POSIX
machines.
If the checksums of the original and uploaded files do not match then the
file will have to be uploaded again. If the link is noisy and the file is big
then you may never get a successful upload. What is needed in this case is to
divide the file into many small parts, upload a part, check its checksum, and
if it is fine proceed to the next part.
This sounds like something that should be automated. Entering from stage left
is Xmodem.
-----------------------------------------------------------------------------
B.3. Xmodem, Ymodem and Zmodem
Xmodem sends 128 bytes and a checksum, waits for a Acknowledgment to say all
is well and sends the next block. If a negative acknowledgement is received
or if no ACK or NAK ever appears then the block is sent again.
Xmodem is a simple protocol, as you would expect of a program written for
8-bit computers running CP/M. It has lots of inefficiencies and minor
problems, such as rounding up the file size to the next 128 byte boundary.
These deficiencies lead to an evolution of the protocol with revisions of
Xmodem, then Ymodem and finishing with Zmodem. Zmodem is substantially faster
than Xmodem and has no niggling problems. The Zmodem protocol is
substantially more complex than the Xmodem protocol, but since we only seek
to at most compile the code, that complexity need not concern us.
+---------------------------------------------------------------------------+
|remote bash$ rz |
|... waiting to receive.**B0100000023be50 |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|Alt-S Upload zmodem |
|[zmodem upload - Press CTRL-C to quit] |
|Sending: upload.bin |
|Bytes Sent: 3072/ 10000 BPS:2185 ETA 00:09 |
+---------------------------------------------------------------------------+
If an upload fails and you are left with rz waiting to recieve a file then
typing Ctrl-X a number of times will return you to the command prompt. This
also works for Xmodem's rx and Ymodem's ry.
Useful Zmodem abilities are resuming failed uploads and sending multiple
files in a single upload session.
An implementation of Xmodem, Ymodem and Zmodem for POSIX computers is
available from [http://www.ohse.de/uwe/software/lrzsz.html] http://
www.ohse.de/uwe/software/lrzsz.html. Red Hat Linux distribute this in the
lrzsz RPM package. lrzsz is a enhanced free software branch of the public
domain version of rzsz from Omen Technology.
-----------------------------------------------------------------------------
B.4. Kermit
Kermit is a terminal emulator and file transfer program delevoped by [http://
www.columbia.edu/] Columbia University. It's popularity springs from the
large range of computers that Kermit could be used to access, from IBM
mainframes to MS-DOS PCs.
A Kermit variant named G-Kermit was released under the GNU Public License.
This is available in most Linux distributions.
The recent Kermit and Zmodem protocols are built upon the same technologies.
Zmodem has better performance in calls with high error rates. Kermit has been
ported to more host platforms.
-----------------------------------------------------------------------------
Appendix C. Upgrading Red Hat Linux from a serial console
Upgrades to Linux distributions are frequently released. A machine is not
remotely manageable unless these upgrades can be installed without needing to
physically touch the machine.
This section examines the remote installation and remote upgrade of Red Hat
Linux.
Red Hat Linux can be installed over the network from a HTTP server using an
install diskette. We modify this diskette to use the serial console. If we
can control whether to boot from this diskette or from the hard disk then we
can remotely upgrade the Red Hat Linux distribution from the serial port. If
a blank diskette is placed in the drive when the machine is deployed then no
on-site intervention is needed to upgrade the operating system.
If you have upgrade procedures for other Linux distributions please
contribute them to the HOWTO maintainer.
-----------------------------------------------------------------------------
C.1. Select boot disk
The key to a remote upgrade is to be able to boot from floppy disk to perform
the upgrade, and then to reboot from the hard disk. The possibilities are:
1. Most BIOSs allow the boot disk order to be controlled through the BIOS'
configuration. If the BIOS supports a serial console then the machine can
be upgraded whilst leaving the floppy disk in the drive. No one need
attend the site to upgrade the operating system
2. Someone can insert a floppy disk before the upgrade and remove it
afterwards. Most co-location sites will provide this level of
"board-swap" technical support.
3. Two records of the CMOS memory which stores the BIOS configuration can be
made: one for booting from floppy and another for booting from hard disk.
Unfortunately the nvram device driver does not yet work on a wide enough
variety of machines for this HOWTO to pursue this option further.
-----------------------------------------------------------------------------
C.2. Configure the BIOS to use the serial port
Many servers allow the BIOS to be configured from the serial port, especially
on systems designed for rack mounting. At the moment few machines designed to
be used as desktop systems allow the BIOS to be accessed from the serial
port.
Refer to your vendor's documentation to set the BIOS to use the serial port.
Some vendors call this feature "console redirection". Unfortunately, the
meaning of this term varies by vendor. Some vendors use it to mean the
redirection of the VGA output and keyboard to a remote PC using a proprietary
serial protocol. This feature can only be used in conjunction with the Linux
serial console if the BIOS can be instructed to disable the serial
redirection after booting.
As an example of the confusion, Dell uses "console redirection" when
describing the Dell 2400 and the Dell 2450. The Dell 2450 BIOS can be
configured from the serial port. The Dell 2400's "console redirection" is
additional hardware that remotely replicates the computer's VGA monitor and
keyboard.
An example of a BIOS configuration is given in Figure C-1.
Figure C-1. Configuring BIOS to use serial link
+---------------------------------------------------------------------------+
|BIOS setup console redirection |
| |
|Enter BIOS setup during boot when |
| Keyboard: [Ctrl+Alt+Esc pressed] |
| Serial port: ["HAL" is typed] |
| |
|Serial port |
| Port: [COM1] |
| Speed [9600] bps |
| Data: [8] bits |
| Parity: [None] |
| Stop: [1] bits |
| Handshaking: [Full CTS/RTS handshaking] |
| Terminal: [Dumb] |
+---------------------------------------------------------------------------+
Many BIOSs will enter their configuration dialogs if a particular terminal
key is pressed during the BIOS boot. This can be a problem if the modem link
is noisy.
For normal operation, set the boot order to attempt to boot from the hard
disk first.
Figure C-2. Configuring BIOS to boot from hard disk
+---------------------------------------------------------------------------+
|BIOS setup boot order |
| |
|First: [Hard disk] |
|Second: [CD-ROM] |
|Third: [Floppy disk] |
+---------------------------------------------------------------------------+
-----------------------------------------------------------------------------
C.3. Configure modem to ignore DTR and assert DCD
The computer reboots a few times during the upgrade. These reboots hang up
the modem. Having to dial in a number of times during the upgrade can become
annoying. Altering the modem's configuration to ignore Data Terminal Ready
will cause the modem not to hang up when the computer is rebooted. To ignore
DTR send the command AT&D0 to the modem.
We may also wish to disconnect during the install to reduce transmission
charges. Configuring the modem to hold Data Carrier Detect on will prevent
any disconnection and reconnection from being apparent to the installer. Use
the command AT&C0 to always hold DCD high.
Apply these changes using the procedure in Section 12.3, retaining all of the
other AT commands.
-----------------------------------------------------------------------------
C.4. Prepare a network install floppy diskette
The Red Hat Linux web site has a floppy diskette image for a network
installation. For Red Hat Linux 7.1 the image is ftp://ftp.redhat.com/pub/
redhat/linux/7.1/en/os/i386/images/bootnet.img.
Install this image on a floppy disk.
+---------------------------------------------------------------------------+
|bash# mkfs -t msdos -c /dev/fd0 |
|mkfs.msdos 2.2 (06 Jul 1999) |
|bash# dd if=bootnet.img of=/dev/fd0 bs=1440k |
|1+0 records in |
|1+0 records out |
|bash# sync |
+---------------------------------------------------------------------------+
Now mount the diskette and check that the installer files are present.
+---------------------------------------------------------------------------+
|bash# mount -t vfat /dev/fd0 /mnt/floppy |
|bash# ls /mnt/floppy |
|boot.msg general.msg ldlinux.sys rescue.msg vmlinuz |
|expert.msg initrd.img param.msg syslinux.cfg |
+---------------------------------------------------------------------------+
This floppy disk uses the SYSLINUX boot loader which was discussed in Section
4.3 and in Section 5.3. Firstly, we alter the boot loader configuration file
/mnt/floppy/syslinux.cfg to use the serial port. If you are going to use the
vi editor to alter this file, use the -n option to avoid writing a swap file
to the floppy disk.
+---------------------------------------------------------------------------+
|bash# vi -n /mnt/floppy/syslinux.cfg |
+---------------------------------------------------------------------------+
serial 0 9600
Secondly we add a new boot option. This is modeled upon the other boot
options in the file. Our variant passes the serial console parameters to the
kernel, the same parameters that we pass during normal operation when using
serial console. "serial" seems an appropriate name for the boot option.
label serial
kernel vmlinuz
append initrd=initrd.img lang= text serial expert devfs=nomount console=ttyS0,9600n8
text, serial and expert are parameters to the Red Hat anaconda installer.
Specifying text ensures that the graphical installer does not start.
Specifying serial prevents scans for possibly non-existent video hardware.
You will need to run Xconfigurator manually if you do have a video card.
Specifying expert allows all the configuration options to be seen, giving one
floppy image that can be used for all purposes.
Thirdly, we make this new configuration start automatically. As there is
no-one at the site, there's no need to issue a boot: prompt.
default serial
prompt 0
Fourthy, we write the new configuration to diskette.
+---------------------------------------------------------------------------+
|bash# umount /mnt/floppy |
+---------------------------------------------------------------------------+
Check that the diskette boots. If it does not then write a new boot sector by
downloading and running the most recent SYSLINUX.
+---------------------------------------------------------------------------+
|bash# syslinux /dev/fd0 |
+---------------------------------------------------------------------------+
Finally, create a new boot image for copying to the computers to be upgraded.
+---------------------------------------------------------------------------+
|bash# dd if=/dev/fd0 of=bootserialnet.img bs=1440k |
|1+0 records in |
|1+0 records out |
+---------------------------------------------------------------------------+
If you test the new boot floppy on a machine with a serial console you should
briefly see SYSLINUX booting
+---------------------------------------------------------------------------+
|SYSLINUX 1.52 2001-02-07 Copyright (C) 1994-2001 H. Peter Anvin |
+---------------------------------------------------------------------------+
and then presenting the boot.msg file and then the Linux kernel should be
loaded
+---------------------------------------------------------------------------+
|Loading initrd.img.............. |
|Loading vmlinuz............. ready. |
+---------------------------------------------------------------------------+
and run.
+--------------------------------------------------------------------------------+
|Linux version 2.4.2-2BOOT (root@porky.devel.redhat.com) (gcc version 2.96 200001|
+--------------------------------------------------------------------------------+
Next the init system flashes by
+-----------------------------------------------------------------------------+
|Greetings. |
|Red Hat install init version 7.0 starting |
|mounting /proc filesystem... done |
|mounting /dev/pts (unix98 pty) filesystem... done |
|Red Hat install init version 7.0 using a serial console |
|remember, cereal is an important part of a nutritionally balanced breakfast. |
|checking for NFS root filesystem...no |
|trying to remount root filesystem read write... done |
|checking for writeable /tmp... yes |
|running install... |
|running /sbin/loader |
+-----------------------------------------------------------------------------+
before the installation application, called anaconda, is started
+---------------------------------------------------------------------------+
|Welcome to Red Hat Linux |
|+----------+ Devices +-----------+ |
|| | |
|| Do you have a driver disk? | |
|| | |
|| +-----+ +----+ | |
|| | Yes | | No | | |
|| +-----+ +----+ | |
|| | |
|| | |
|+--------------------------------+ |
|<Tab>/<Alt-Tab> between elements | <Space> selects | <F12> next screen |
+---------------------------------------------------------------------------+
There does not seem to be a way to access the function keys, fortunately the
user interface does not require their use.
Now that the floppy has been tested, eject the disk and reboot the machine
into normal operation.
-----------------------------------------------------------------------------
C.5. Prepare HTTP server
It is best if the web server runs the version of Red Hat Linux as is being
upgraded to. If it runs an earlier version, then do not rebuild the operating
system on this machine and install anaconda-runtime from the later operating
system.
Copy the Linux distribution to a local web server using a mirroring utility
like wget. Alternatively the files can be copied from the distribution CDs to
the web server.
+---------------------------------------------------------------------------------------------------------------------------------------------------+
|bash$ mkdir --mode=664 --parents /var/www/html/redhat/linux/7.1/en/os/i386 |
|bash$ umask 002 |
|bash$ wget -nh -nH -r -N -nr -l0 -k -np -X SRPMS ftp://ftp.redhat.com/pub/redhat/linux/7.1/en/os/i386/ -P /var/www/html/redhat/linux/7.1/en/os/i386|
+---------------------------------------------------------------------------------------------------------------------------------------------------+
It's best to use a mirror site in place of Red Hat's FTP site used in the
example above.
It is very important not to gain files along the way. Delete any files
generated by FTP servers, web servers and CD-ROMs.
+---------------------------------------------------------------------------+
|bash$ cd /var/www/html/redhat |
|bash$ # Files added by FTP server |
|bash$ find . -name '.listing' -print -exec rm {} \; |
|bash$ find . -name 'ls-*' -print -exec rm {} \; |
|bash$ # Files added by a wget from a HTTP server |
|bash$ find . -name '\?*' -print -exec rm {} \; |
|bash$ # Files added by a CD-ROM |
|bash$ find . -name 'TRANS.TBL' -print -exec rm {} \; |
+---------------------------------------------------------------------------+
We now need to add the latest updates to the distributed software. This is
done to avoid the machine being compromised immediately following the
upgrade.
Adding the updates is essential for Red Hat Linux 7.1, see Section A.2.
Collect together the updates RPMs from ftp://ftp.redhat.com/pub/updates/7.1/
en/os/ in the subdirectories i386, i486, i586 i686, images and noarch.
Merge these updates into the copy of the distribution. For each updated RPM
file, remove the original RPM file then replace it with the updated RPM file.
For example:
+---------------------------------------------------------------------------------+
|bash$ cd /var/www/html/redhat/linux/7.1/en/os/i386/RedHat/RPMS |
|bash$ ls /var/www/html/redhat/updates/7.1/en/os/i386 |
|SysVinit-2.78-17.i386.rpm |
|bash$ ls SysVinit-*.rpm |
|SysVinit-2.78-15.i386.rpm |
|bash$ rm SysVinit-2.78-15.i386.rpm |
|bash$ cp /var/www/html/redhat/updates/7.1/en/os/i386/SysVinit-2.78-17.i386.rpm . |
|bash$ chmod u=rw,g=r,o=r SysVinit-2.78-17.i386.rpm |
+---------------------------------------------------------------------------------+
Merge the RPMs from the updates subdirectories i386, i686 and noarch into /
var/www/html/redhat/linux/7.1/en/os/i386/RedHat/RPMS. Merge the files from
the directory /var/www/html/redhat/updates/7.1/en/os/images into the
directory /var/www/html/redhat/linux/7.1/en/os/i386/images.
The file /var/www/html/redhat/linux/7.1/en/os/i386/RedHat/base/hdlist and
hdlist2 contain the list of the RPMs to install. This needs to be modified to
contain the names of the updated RPMs.
Install the anaconda-runtime RPM on the HTTP server. This RPM should be the
same version as the Red Hat Linux being upgraded to.
Now create a new hdlist with the commands:
+-----------------------------------------------------------------------------------------------------------------------------------------------+
|bash$ cd /usr/lib/anaconda-runtime |
|bash$ rm /var/www/html/redhat/linux/7.1/en/os/i386/RedHat/base/hdlist* |
|bash$ umask 002 |
|bash$ ./genhdlist --withnumbers --hdlist /var/www/html/redhat/linux/7.1/en/os/i386/RedHat/base/hdlist /var/www/html/redhat/linux/7.1/en/os/i386|
+-----------------------------------------------------------------------------------------------------------------------------------------------+
The distribution plus the updates can now be used for a network install. They
cannot be used for a CD install, but that doesn't concern us.
As the distribution plus the updates is different from the original
distribution, we should not use the version number of the original
distribution. Appending the date to which the updates have been applied seems
best.
bash$ cd /var/www/html/redhat/linux/
bash$ mv 7.1 7.1-20020202
-----------------------------------------------------------------------------
C.6. Record network configuration
If the machine does not use the Dynamic Host Configuration Protocol then
record the current network configuration. This is used to complete the
installer's Configure TCP/IP screen.
Example C-1. Displaying the Internet Protocol configuration
bash$ ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:11:22:33:44:55
inet addr:10.1.2.3 Bcast:10.1.2.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:344233 errors:0 dropped:0 overruns:0 frame:0
TX packets:285750 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
Interrupt:10 Base address:0x9000
bash$ netstat -r -n (1)
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
10.1.2.0 0.0.0.0 255.255.255.0 U 40 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo
0.0.0.0 10.1.2.254 0.0.0.0 UG 40 0 0 eth0
bash$ cat /etc/resolv.conf
domain example.edu.au
nameserver 10.255.1.1
nameserver 10.255.2.1
nameserver 172.16.1.1
(1) The value of inet addr is the "IP address". Our example shows 10.1.2.3.
The value of Mask is the "Netmask". Our example shows 255.255.255.0.
(2) The value in the Gateway column for Destination 0.0.0.0 is the "Default
gateway". Our example shows 10.1.2.254.
(3) The value of the first listed nameserver is the "Primary nameserver". Our
example shows 10.255.1.1.
-----------------------------------------------------------------------------
C.7. Record LILO configuration
Record the current value of append=, boot= and linear in /etc/lilo.conf.
Example C-2. Displaying the LILO configuration
+---------------------------------------------------------------------------+
|bash# fgrep append= /etc/lilo.conf |
|append="console=tty0 console=ttyS0,9600n8" |
|bash# fgrep boot= /etc/lilo.conf |
|boot=/dev/hda |
|bash# fgrep linear /etc/lilo.conf |
|bash# |
+---------------------------------------------------------------------------+
If the boot= parameter points to a hard disk then LILO is installed in the
master boot record, or MBR. It can also point to a partition.
If the linear parameter is present then the hard disk that is booted from
uses linear block addressing, or LBA.
-----------------------------------------------------------------------------
C.8. Upgrade Red Hat distribution
In this section it all comes together. We will walk through an entire serial
console upgrade, not that it differs much from a standard text mode upgrade.
Configure BIOS to boot from floppy or insert the floppy disk. Now reboot the
machine.
+----------------------------------------------------------------------------------------------------------------------------------------------+
|bash# shutdown -r now |
|SYSLINUX 1.64 1.64-pre2 Copyright (C) 1994-2001 H. Peter Anvin |
| Welcome to Red Hat Linux 7.1! |
| - To install or upgrade Red Hat Linux in graphical mode, |
| press the <ENTER> key. |
| - To install or upgrade Red Hat Linux in text mode, type: text <ENTER>. |
| - To enable low resolution mode, type: lowres <ENTER>. |
| Press <F2> for more information about low resolution mode. |
| - To disable framebuffer mode, type: nofb <ENTER>. |
| Press <F2> for more information about disabling framebuffer mode. |
| - To enable expert mode, type: expert <ENTER>. |
| Press <F3> for more information about expert mode. |
| - To enable rescue mode, type: linux rescue <ENTER>. |
| Press <F5> for more information about rescue mode. |
| - If you have a driver disk, type: linux dd <ENTER>. |
| - Use the function keys listed below for more information. |
|[F1-Main] [F2-General] [F3-Expert] [F4-Kernel] [F5-Rescue] |
|boot: |
|Loading initrd.img.............. |
|Loading vmlinuz............. ready. |
|Linux version 2.4.2-2BOOT (root@porky.devel.redhat.com) (gcc version 2.96 20000731 (Red Hat Linux 7.1 2.96-79)) #1 Sun Apr 8 18:24:33 EDT 2001|
+----------------------------------------------------------------------------------------------------------------------------------------------+
Because we have booted into expert mode, the menus differ slightly from the
standard upgrade. For example, you probably don't have a driver disk.
+---------------------------------------------------------------------------+
| Welcome to Red Hat Linux |
|+----------+ Devices +-----------+ |
|| | |
|| Do you have a driver disk? | |
|| | |
|| +-----+ +----+ | |
|| | Yes | |[No]| | |
|| +-----+ +----+ | |
|| | |
|+--------------------------------+ |
+---------------------------------------------------------------------------+
The upgrade then continues in the usual fashion.
+---------------------------------------------------------------------------+
|+--------+ Choose a Language +---------+ |
|| | |
|| What language should be used during | |
|| the installation process? | |
|| | |
|| Czech : | |
|| [ English : ] | |
|| Danish : | |
|| French : | |
|| German : | |
|| Hungarian : | |
|| Icelandic : | |
|| Italian : | |
|| | |
|| +----+ | |
|| |[OK]| | |
|| +----+ | |
|| | |
|+--------------------------------------+ |
+---------------------------------------------------------------------------+
Select HTTP to upgrade from the web server we prepared previously.
+---------------------------------------------------------------------------+
|+-----+ Installation Method +------+ |
|| | |
|| What type of media contains the | |
|| packages to be installed? | |
|| | |
|| NFS image | |
|| FTP | |
|| [ HTTP ] | |
|| | |
|| +----+ +------+ | |
|| |[OK]| | Back | | |
|| +----+ +------+ | |
|| | |
|+----------------------------------+ |
+---------------------------------------------------------------------------+
Here we supply the network details recorded in Example C-1. If your network
supports Dynamic Host Configuration Protocol or the Bootstrap Protocol then
these work fine too.
+---------------------------------------------------------------------------+
|+--------------------+ Configure TCP/IP +--------------------+ |
|| | |
|| Please enter the IP configuration for this machine. Each | |
|| item should be entered as an IP address in dotted-decimal | |
|| notation (for example, 1.2.3.4). | |
|| | |
|| [ ] Use dynamic IP configuration (BOOTP/DHCP) | |
|| | |
|| IP address: 10.1.2.3________ | |
|| Netmask: 255.255.255.0___ | |
|| Default gateway (IP): 10.1.2.254______ | |
|| Primary nameserver: 10.255.1.1______ | |
|| | |
|| +----+ +------+ | |
|| |[OK]| | Back | | |
|| +----+ +------+ | |
|| | |
|+------------------------------------------------------------+ |
+---------------------------------------------------------------------------+
Provide the name of the pre-prepared web server. Note that the response to
Red Hat directory must start with a /.
+---------------------------------------------------------------------------+
|+-----------------+ HTTP Setup +-----------------------------------+ |
|| | |
|| Please enter the following information: | |
|| | |
|| o the name or IP number of your web server | |
|| o the directory on that server containing | |
|| Red Hat Linux for your architecure | |
|| | |
|| Web site name: www.example.edu.au_______________________ | |
|| Red Hat directory: /redhat/linux/7.1-20020202/en/os/i386____ | |
|| | |
|| +----+ +------+ | |
|| |[OK]| | Back | | |
|| +----+ +------+ | |
|| | |
|+------------------------------------------------------------------+ |
+---------------------------------------------------------------------------+
The following status messages then fly by before the welcome screen appears.
+----------------------------------------------------------------------------+
|Retrieving base/netstg1.img... |
|Loading /mnt/runtime ramdisk... |
|Retrieving base/netstg2.img... |
|Loading /mnt/runtime/usr ramdisk... |
|Running anaconda - please wait... |
|Graphical installation not available for http installs. Starting text mode.|
+----------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|+----------------+ Red Hat Linux +-----------------+ |
|| | |
|| Welcome to Red Hat Linux! | |
|| | |
|| This installation process is outlined in detail | |
|| in the Official Red Hat Linux Installation | |
|| Guide available from Red Hat Software. If you | |
|| have access to this manual, you should read the | |
|| installation section before continuing. | |
|| | |
|| If you have purchased Official Red Hat Linux, | |
|| be sure to register your purchase through our | |
|| web site, http://www.redhat.com/. | |
|| | |
|| +----+ +------+ | |
|| |[OK]| | Back | | |
|| +----+ +------+ | |
|| | |
|+--------------------------------------------------+ |
+---------------------------------------------------------------------------+
Select Upgrade Existing Installation, although this procedure works fine for
installations as well.
+---------------------------------------------------------------------------+
|+--------------+ Installation Type +--------------+ |
|| | |
|| What type of system would you like to install? | |
|| | |
|| Workstation | |
|| Server System | |
|| Laptop | |
|| Custom System | |
|| [ Upgrade Existing Installation ] | |
|| | |
|| +----+ +------+ | |
|| | OK | | Back | | |
|| +----+ +------+ | |
|| | |
|+-------------------------------------------------+ |
+---------------------------------------------------------------------------+
The upgrade continues. When the LILO Configuration screen appears insert the
kernel parameters recorded from Example C-2. These parameters should include
console=ttyS??.
+---------------------------------------------------------------------------+
|+---------------------+ LILO Configuration +---------------------+ |
|| | |
|| A few systems will need to pass special options to the kernel | |
|| at boot time for the system to function properly. If you need | |
|| to pass boot options to the kernel, enter them now. If you | |
|| don't need any or aren't sure, leave this blank. | |
|| | |
|| [ ] Use linear mode (needed for some SCSI drives) | |
|| | |
|| console=tty0 console=ttyS0,9600n8_______________ | |
|| | |
|| +----+ +------+ +------+ | |
|| | OK | | Skip | | Back | | |
|| +----+ +------+ +------+ | |
|| | |
|+----------------------------------------------------------------+ |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|+-------------+ LILO Configuration +--------------+ |
|| | |
|| Where do you want to install the bootloader? | |
|| | |
||[/dev/hda Master Boot Record (MBR) ]| |
|| /dev/hda1 First sector of boot partition | |
|| | |
|| +----+ +------+ | |
|| | OK | | Back | | |
|| +----+ +------+ | |
|| | |
|+-------------------------------------------------+ |
+---------------------------------------------------------------------------+
+---------------------------------------------------------------------------+
|+----------------------+ LILO Configuration +-----------------------+ |
|| | |
|| The boot manager Red Hat uses can boot other operating systems | |
|| as well. You need to tell me what partitions you would like to | |
|| be able to boot and what label you want to use for each of them. | |
|| | |
|| Device Partition type Default Boot label | |
||[/dev/hda6 Linux Native * linux ] : | |
|| : | |
|| : | |
|| : | |
|| : | |
|| | |
|| +----+ +------+ +------+ | |
|| | Ok | | Edit | | Back | | |
|| +----+ +------+ +------+ | |
|| | |
|| | |
|+-------------------------------------------------------------------+ |
+---------------------------------------------------------------------------+
The upgrade continues. As installing the packages may take a few hours, you
can disconnect.
+---------------------------------------------------------------------------+
|+-------------+ Package Installation +--------------+ |
|| | |
|| Name : | |
|| Size : | |
|| Summary: | |
|| | |
|| Packages Bytes Time | |
|| Total : 0 0M | |
|| Completed: 0 0M | |
|| Remaining: 0 0M | |
|| | |
|| | |
|+---------------------------------------------------+ |
+---------------------------------------------------------------------------+
If you disconnected, then when reconnecting it is best to press Tab rather
than pressing Return.
Pressing Return on the Bootdisk screen writes a boot disk. This will
overwrite the upgrade disk.
You may wish to deliberately create a boot disk if you cannot alter the BIOS
parameters to boot from the hard disk, or if you cannot wait for someone to
eject the floppy disk before rebooting.
+---------------------------------------------------------------------------+
|+------------------+ Bootdisk +-------------------+ |
|| | |
|| A custom boot disk provides a way of booting | |
|| into your Linux system without depending on | |
|| the normal bootloader. This is useful if you | |
|| don't want to install lilo on your system, | |
|| another operating system removes lilo, or lilo | |
|| doesn't work with your hardware configuration. | |
|| A custom boot disk can also be used with the | |
|| Red Hat rescue image, making it much easier to | |
|| recover from severe system failures. | |
|| | |
|| Would you like to create a boot disk for your | |
|| system? | |
|| | |
|| +-----+ +----+ | |
|| |[Yes]| | No | | |
|| +-----+ +----+ | |
|| | |
|+-------------------------------------------------+ |
+---------------------------------------------------------------------------+
When the Complete screen appears prepare to reboot into Linux. If you have a
serial BIOS be prepared to alter the BIOS parameters to boot from the hard
disk first. If you do not have a serial BIOS ask someone to eject the floppy
disk.
+---------------------------------------------------------------------------+
|+-----------------+ Complete +------------------+ |
|| | |
|| Congratulations, installation is complete. # | |
|| : | |
|| Press return to reboot, and be sure to : | |
|| remove your boot medium after the system : | |
|| reboots, or your system will rerun the : | |
|| install. For information on fixes which : | |
|| are available for this release of Red Hat : | |
|| Linux, consult the Errata available from : | |
|| http://www.redhat.com/errata. : | |
|| : | |
|| Information on configuring and using your : | |
|| Red Hat Linux system is contained in the : | |
|| | |
|| +----+ | |
|| |[OK]| | |
|| +----+ | |
|| | |
|+-----------------------------------------------+ |
+---------------------------------------------------------------------------+
+-------------------------------------------------------------------------------------------------------------------------------------------+
|sending termination signals...done |
|sending kill signals...done |
|disabling swap... |
| /tmp/swap/hda5 |
|unmounting filesystems... |
| /mnt/sysimage/var/www/html |
| /mnt/sysimage/boot |
| /mnt/sysimage/proc |
| /mnt/runtime/usr |
| /mnt/sysimage |
| /proc/bus/usb |
| /mnt/runtime |
| /dev/pts |
| /proc |
|rebooting system |
|Restarting system. |
| |
|LILO |
|Loading linux...................... |
|Linux version 2.4.3-12 (root@porky.devel.redhat.com) (gcc version 2.96 20000731 (Red Hat Linux 7.1 2.96-85)) #1 Fri Jun 8 15:05:56 EDT 2001|
+-------------------------------------------------------------------------------------------------------------------------------------------+
-----------------------------------------------------------------------------
C.9. Create boot disk for serial console
Once the upgrade has been sucessfully done create a boot floppy which has
serial console support. This is most simply done by creating a boot disk, as
done by the anaconda installer or as described in Section 2.1; modifying the
configuration file \SYSLINUX.CFG to configure the boot loader to use the
serial console, as described in Section 4.3; and finally configuring the
kernel to use the serial console, as described in Section 5.3.
An alternative is to create your own mkbootdisk RPM package containing a
modified copy of the shell script /sbin/mkbootdisk.
The \SYSLINUX.CFG file on the boot floppy is written by mkbootdisk using the
code in Figure C-3. We alter this code to use the serial console; the result
is shown in Figure C-4.
Figure C-3. Extract from Red Hat Linux 7.2 mkbootdisk which creates
SYSLINUX.CFG
cat > $MOUNTDIR/syslinux.cfg <<EOF
default linux
prompt 1
display boot.msg
timeout 100
label linux
kernel vmlinuz
append $INITRDARG root=$rootdev
EOF
Figure C-4. Altered extract from mkbootdisk, which creates a SYSLINUX.CFG
that uses a serial console
cat > $MOUNTDIR/syslinux.cfg <<EOF
serial 0 9600
default linux
prompt 1
display boot.msg
timeout 100
label linux
kernel vmlinuz
append $INITRDARG root=$rootdev console=tty0 console=ttyS0,9600n8
EOF
Created boot floppies will now use the serial console.
By far the best alternative would be the addition of parameters to mkbootdisk
to allow the kernel parameters and serial port, speed and flow control to be
given when the boot floppy is created. For this enhancement request see Red
Hat Bugzilla entry [https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=
59351] 59351.
-----------------------------------------------------------------------------
C.10. Further references
Sometimes the kernel on the installation CD won't boot on the machine to be
upgraded, or the filesystem requires modules that are not present. In this
case you will need to build a new kernel and rebuild the installation disk to
use the new kernel. This is documented in the RedHat7 CDs mini-HowTo. This is
an informal HOWTO not available through the Linux Documentation Project.
An older document that more fully describes an older Red Hat distribution
build process is Burning a RedHat CD HOWTO.
-----------------------------------------------------------------------------
Appendix D. Upgrading Debian GNU/Linux from a serial console
Make a boot disk and a root disk.
Boot the boot disk with the parameter console=ttyS0,9600.
Start the install program.
-----------------------------------------------------------------------------
Appendix E. Terminal server configuration
Terminal servers were originally designed for connecting terminals to
minicomputers. Each terminal would have an RS-232 port. The connection to the
minicomputer usually used an ethernet port. Connecting terminals would be
connected to a command line interface where they could select from a list of
predefined machines. A Telnet session would then be started to that machine.
Over time terminal servers gained more features. For example, modems could be
connected. These initially allowed people to dial in to the minicomputer but
grew in features until most terminal servers became routers with a great
number of serial ports.
As well as allowing the connection of many console to a single terminal, the
terminal server can be configured with user accounts and passwords,
preventing unauthenticated access to the console whilst still allowing the
console to be reached from any modem.
This remainder of this section lists the considerations when purchasing
terminal servers and the cabling pinouts and basic software configuration
needed for differing types of terminal servers.
Further contributions are welcome and should be e-mailed to the maintainer of
this HOWTO.
-----------------------------------------------------------------------------
E.1. Considerations when buying second-hand terminal servers
Internet Service Providers have been large users of terminal servers in the
past. Each modem would be connected to a terminal server port and incoming
users would be permitted to send IP packets anywhere, not just to some
predefined minicomputer. Manufacturers renamed the equipment to "access
servers" or "modem servers" to reflect this new use.
These access servers have been superseded by a new generation which allows
telephone trunks to be plugged directly into the ISP's router. There are no
discrete modems; the modem tones are decoded by digital signal processing
chips within the router. As a result terminal servers are currently readily
available on the second-hand market.
When purchasing a second-hand terminal server ensure that you are also buying
the rights to the software. Some companies license their software and have
contract terms which state that the license cannot be resold, but has to be
repurchased from the company if the terminal server changes hands.
Many vendors require a current maintenance contract to obtain software
updates. These maintenance agreements can be expensive, a common figure is
15% per annum of the manufacturer's retail price. You may be able to source a
cheaper software updates from a third-party maintenance supplier.
Many older terminal servers are no longer sold or supported by their vendors.
Search the vendor's web site for "end of life".
Vendor support can be a particular issue when the most-recently available
software does not fit within the RAM or flash memory contraints of the
terminal server you have purchased. You should check this before purchasing a
seond-hand terminal server. Upgrading flash memory can be particularly
difficult, as the ROM on the motherboard may also need to be replaced with
one aware of the new flash memory's characteristics.[9]
Third-party parts suppliers such as [http://www.kingston.com/] Kingston or
[http://www.memoryx.net/] MemoryX can usually provide dynamic RAM and flash
memory. They cannot usually supply ROMs or static RAM.
Most old terminal servers will not support Secure Shell. In this is the case
accessing the terminal server by its ethernet port is a poor idea: when you
login to the console you password will travel across the Internet in clear
text. Either dial in to the terminal server or use a one-time password system
such as the RADIUS protocol with S/KEY authentication.
An alternative to using a terminal server is to use a multiport serial card
in another Linux system.
-----------------------------------------------------------------------------
E.2. Cisco 2511
The basic configuration for a Cisco 2511 access server is shown in Figure E-1
. A similar configuration will work for other Cisco access servers. Cisco has
excellent documentation at its [http://www.cisco.com/] web site; start by
finding the correct Configuration guide.
A current maintenance contract with Cisco or a reseller is required to
download software updates. This contract also includes the provision of ROMs
required for flash memory upgrades. In most jurisdictions Cisco software
licenses are not transferrable, so if you purcashed the access server on the
second-hand market you will need to purchase a software license from Cisco or
a reseller.
Figure E-1. Basic configuration for Cisco 2511 terminal server to Linux PC
interface Async1
description To Linux computer
ip unnumbered Loopback0
async mode interactive
no peer default ip address
line 1
location To Linux PC
session-timeout 30
no exec
login
modem InOut
terminal-type vt100
special-character-bits 8
transport preferred none
transport input telnet
telnet break-on-ip
telnet ip-on-break
stopbits 1
flowcontrol hardware
line vty 0 4
location Network
password PASSWORD
login local
terminal-type vt100
transport preferred none
transport output telnet
There is a [http://www.mcvax.org/~koen/uClinux-cisco2500/] port of Linux to
the Cisco 2500 series of routers. At the time of writing it did did not
support the asycnhronous ports on the Cisco 2511. The attractiveness of
running Linux instead of running Cisco's IOS is that Linux can support SSH.
At the time of writing Cisco were yet to release SSH on the Cisco 2500 series
of routers, although a unofficial beta version has been seen.
-----------------------------------------------------------------------------
E.3. Xyplex/iTouch MAXserver 1600
A good site for information on Xyplex terminal servers is [http://www.gno.org
/~gdr/xyplex/] http://www.gno.org/~gdr/xyplex/. Cabling is discussed at
[http://www.conserver.com/consoles/xyplexcons.html] http://www.conserver.com/
consoles/xyplexcons.html.
The Xyplex terminal servers are now manufacturered by [http://
www.itouchcom.com/] iTouch Communications. A current maintenance contract
with iTouch is required to download software updates.
-----------------------------------------------------------------------------
E.4. Xylogics/Bay/Nortel Annex
A good site for information on Annex terminal servers is [http://www.ofb.net/
~jheiss/annex/] http://www.ofb.net/~jheiss/annex/.
-----------------------------------------------------------------------------
E.5. Livingston/Lucent Portmaster
Firstly configure the terminal server, as shown in Figure E-2. This figure
uses the system name example, with IP address 10.1.2.3, address mask
255.255.255.0, gateway address 10.1.2.254, and DNS server address 10.1.1.1.
Replace these addresses with the addresses used in your network.
Figure E-2. Portmaster unit configuration
+---------------------------------------------------------------------------+
|set sysname example |
|set password PASSWORD |
|set ether0 address 10.1.2.3 |
|set ether0 netmask 255.255.255.0 |
|set ether0 broadcast high |
|set gateway 10.1.2.254 |
|set namesvc dns |
|set nameserver 10.1.1.1 |
|save all |
+---------------------------------------------------------------------------+
Now configure each serial port of the terminal server, as shown in Figure E-3
.
Figure E-3. Portmaster port configuration
+---------------------------------------------------------------------------+
|set s0 service_device telnet 2000 |
|set s0 device |
|reset s0 |
|set s1 service_device telnet 2001 |
|set s1 device |
|reset s1 |
|?? |
|set s29 service_device telnet 2029 |
|set s29 device |
|reset s29 |
|save all |
+---------------------------------------------------------------------------+
To connect to serial port 0 enter the command telnet example 2000. Use the
associated TCP port number to connect to telnet to the other serial devices.
-----------------------------------------------------------------------------
Appendix F. Gratuitous advice for developers
F.1. Advice for boot loader authors
Serial console support in a boot loader is very useful. Thank you for
supporting it.
The boot loader should support the 8250A UART and its programming-compatible
82510, 16450, 16550 and 16750 descendants. The serial chip used in the IBM PC
/XT, the 8250 (no A), and its 8250B descendant need not be supported. The
8250A data sheet is 82C50A CMOS Asynchronous Communications Element and is
updated by Intel's errata 82510 PC Software Compatibility. The 16550 data
sheet is PC16550D Universal Asynchronous Receiver/Transmitter with FIFOs.
To set the serial port and serial parameters, most Linux boot loaders use a
syntax modeled upon the kernel's console parameter. It would be nice to
retain this consistency, since the user needs to learn the kernel syntax in
any case.
The default value should be 9600bps, 8 data bits, no parity, 1 stop bit and
CTS/RTS flow control. This gives the maximum interoperability with the other
programs that use the serial console.
Please do not ignore the lower speeds, as remote serial console is at its
most valuable when the computer is located three days walk up a mountain in
the New Guinea highlands. It is difficult to get more than 75bps from HF
radio under adverse sky conditions.
Be conservative in your use of the modem status lines. Even if you are
ignoring incoming status (DSR, DCD) and handshaking lines (RTS) at least
assert the outgoing status (DTR) and handshaking (CTS) lines. Correctly
configured modems will not receive calls with DTR low, and dropping DTR will
cause the modem to hang up.
Consider that the BIOS may have already initialised the UART and provide a
configuration option to allow the boot loader to be informed of that. When
the boot loader initialises the UART, DTR will fall and the line will hang
up. In some scenarios each hang up requires the satelite circuit to be
re-booked before another call can be placed.
Cater for line noise. Imagine the boot loader starting and then being sent
nonsensical characters every few seconds. Although this is certainly wrong, a
fault in a modem is difficult to remotely diagnose and correct if the machine
is left stranded at the boot loader prompt. A solution is to boot the default
image upon the expiry of a timer; the boot occurring even if the user (or
line noise) has started to type. For example the boot loader configuration
could say:
# Start the machine regardless after 30 minutes
# 30 * 60 seconds per minute * units of tenths of seconds
lifetime 18000
The default should be no life timer. The timer is also useful in high
availability applications: when a machine is used in environments with an
planned availability of 99.999% the lifetime value should be configured to
three minutes or less.
Check information read from the BIOS for reasonablness. For example, if the
BIOS's Extended Data Area suggests 0x000 as the address for the serial port's
registers then don't try to initialise the registers.
-----------------------------------------------------------------------------
F.2. Advice for BIOS authors
Thank you for adding support for remote operations to your BIOS. A few points
will maximize the benefits of that support, most of them are listed in
Section F.1.
<EFBFBD><EFBFBD>*<2A>Keep the user interface simple. There is no need for fancy
cursor-addressed terminal support. Fancy features simply limit the number
of client terminal emulators that can be used. A surprising number of
these have very buggy DEC VT100 implementations.
In addition to supporting lower speeds, also test your user interface at
low data rates.
<EFBFBD><EFBFBD>*<2A>Don't do too much. In Linux the boot loader and operating system both
have explicit support for a serial console. So all the BIOS need do is to
support the a serial interface for itself. Linux has no need for a
generic serial redirection facility. If you do provide such a facility
for other operating systems, please allow it to be disabled after system
boot.
<EFBFBD><EFBFBD>*<2A>Don't allow line noise to prevent the computer from booting. Don't
require just one key to enter the BIOS configuration, make your users and
your marketing people happy by using a phrase like dell, hp or ibm. Copy
the lifetime idea from Section F.1.
<EFBFBD><EFBFBD>*<2A>Present a consistent prompt. Imagine a user with a supercomputer array of
five hundred PCs. You want to change a BIOS parameter. Make it easy for
[http://expect.nist.gov/] Expect to set those parameters.
<EFBFBD><EFBFBD>*<2A>Make sure the Linux utilities work. Check that the Linux nvram device
driver returns the full contents of CMOS. This makes it simple to set the
same CMOS settings on a large number of machines. The commands in Figure
F-2 and Figure F-3 should work to copy the BIOS settings from one machine
to another, where the make, model and BIOS versions of the machines are
the same.
Figure F-1. Configuring /dev/nvram to access the CMOS configuration
+---------------------------------------------------------------+
|bash# /dev/MAKEDEV nvram |
|bash# vi /etc/modules.conf |
+---------------------------------------------------------------+
alias char-major-10-144 nvram
+---------------------------------------------------------------+
|bash# depmod -a |
+---------------------------------------------------------------+
Figure F-2. Getting the CMOS configuration
+---------------------------------------------------------------+
|bash# cat /dev/nvram > /etc/nvram.bin |
+---------------------------------------------------------------+
Figure F-3. Setting the CMOS configuration
+---------------------------------------------------------------+
|bash# cat /etc/nvram.bin > /dev/nvram |
+---------------------------------------------------------------+
<EFBFBD><EFBFBD>*<2A>Have a flash BIOS upgrade program that works from Linux. Make the source
code to this available. Or publish the specifications so that one can be
written.
Many flash BIOS update programs run from a Microsoft MS-DOS boot
diskette. Please check that the program also works with the similar Free
DOS operating system. Many Linux computers do not have licenses for
Microsoft operating system software, so legally creating a MS-DOS boot
diskette may not be possible.
<EFBFBD><EFBFBD>*<2A>Be clear in the documentation about what serial servies the BIOS
provides. Some BIOSs with a "serial redirection" feature don't allow the
BIOS to be redirected to a plain text terminal, but instead use a
proprietary protocol. This isn't of much use to Linux serial console
users.
-----------------------------------------------------------------------------
Appendix G. About this HOWTO
G.1. Copyright
The first edition of this document is copyright <20> 2001 Mark F. Komarinski and
is distributed under the terms of the Linux Documentation Project (LDP)
License, see Section G.1.1.
The revisions to this document for the second edition are copyright <20> AARNet
Pty Ltd (Australian Company Number 084 540 518), 2001-2003. These parts were
written by Glen Turner. He asserts his moral rights to be identified as one
of the authors of this work under the Copyright Act 1968 (Commonwealth of
Australia). The Australian Academic and Research Network and Glen Turner
distribute these parts under the terms of the Linux Documentation Project (
LDP) License, see Section G.1.1.
This license meets the Debian Free Software Guidelines, so you should find
this HOWTO in the Debian package doc-linux-html.
-----------------------------------------------------------------------------
G.1.1. Linux Documentation Project License
Unless otherwise stated, Linux HOWTO documents are copyrighted by their
respective authors. Linux HOWTO documents may be reproduced and distributed
in whole or in part, in any medium physical or electronic, as long as this
copyright notice is retained on all copies. Commercial redistribution is
allowed and encouraged; however, the author would like to be notified of any
such distributions.
All translations, derivative works, or aggregate works incorporating any
Linux HOWTO documents must be covered under this copyright notice. That is,
you may not produce a derivative work from a HOWTO and impose additional
restrictions on its distribution. Exceptions to these rules may be granted
under certain conditions; please contact the Linux HOWTO coordinator at the
address given below.
In short, we wish to promote dissemination of this information through as
many channels as possible. However, we do wish to retain copyright on the
HOWTO documents, and would like to be notified of any plans to redistribute
the HOWTOs.
If you have any questions, please contact <linux-howto@metalab.unc.edu>.
-----------------------------------------------------------------------------
G.2. Disclaimer
No liability for the contents of this documents can be accepted. Use the
concepts, examples and other content at your own risk. As this is a new
edition of this document, there may be errors and inaccuracies, that may of
course be damaging to your system. Proceed with caution, and although this is
highly unlikely, the author(s) do not take any responsibility for that.
All copyrights are held by their by their respective owners, unless
specifically noted otherwise. Use of a term in this document should not be
regarded as affecting the validity of any trademark or service mark.
Naming of particular products or brands should not be seen as endorsements.
You are strongly recommended to take a backup of your system before major
installation and backups at regular intervals.
-----------------------------------------------------------------------------
G.3. Acknowledgments
The first edition of this HOWTO was written by Mark Komarinski. It was based
upon /usr/src/linux/Documentation/serial-console.txt, which was written by
Miquel van Smoorenburg.
The second edition of this HOWTO was written by the staff of the Australian
Academic and Research Network, mainly Glen Turner and David Vu.
The following people have contributed to this HOWTO. They are listed in no
particular order.
LinuxSA mailing list
Proof reading of the Second Edition. [http://www.linuxsa.org.au/] LinuxSA
is a Linux user group based in South Australia.
David Lawyer
Technical review of the Second Edition and recommending the updated HOWTO
to the Linux Documentation Project. David is author of the
Text-Terminal-HOWTO.
Devin Reade
Xyplex terminal server information. Devin maintains information about
Xyplex terminal servers at [http://www.gno.org/~gdr/xyplex/] http://
www.gno.org/~gdr/xyplex/.
Michael Brown, Marc Mondragon and other members of the Linux on Dell
PowerEdge mailing list
Technically described how the BIOS redirects characters to the serial
port. The Linux on Dell PowerEdge list can be subscribed to by sending a
message containing subscribe linux-poweredge to <
linux-poweredge-request@dell.com>.
Thomas Lunde, Gabor Kiss and Carlo Belon
Noticed errors of grammar and typography.
Darren Young
Updates to /etc/security/console.perms for Red Hat Linux 7.2.
Yasufumi Haga
Spotted many errors whilst translating this HOWTO into Japanese for the
JF Linux documentation endeavour.
Thomas Horsley
Pointed out that the X Window System may still need to be running even if
a serial console is used. Supplied the gdm configuration used in Figure
7-3.
Greg Matthews, Nathan Neulinger and Romildo Wildgrube
Encountered and reported that machines hang when booting if kernel
parameter console=ttyS??r is used. This is due to a kernel bug which
loops testing CTS without firstly checking that DSR and DCD are asserted.
Shaun Karl and Keisuke Nakao
Procedures for Debian GNU/Linux.
Igor Sviridov
Configuration of Livingstone Portmaster terminal server in Section E.5.
Sue Bauer-Lee
Suggested using the off clause in /etc/inittab in Figure 6-9 rather than
commenting or deleting the excess mingetty invocations. This has the
advatage that no automated system administration tool will restore the
excess inittab entries.
Yasuhiro Suzuki
Noticed inconsistent descrtiptions of Clear to Send and Ready to Send.
-----------------------------------------------------------------------------
G.4. Comments and corrections
The current maintainer of this HOWTO is Glen Turner. Please send corrections,
additions, comments and criticisms to <glen.turner+howto@aarnet.edu.au>.
The maintainer would also appreciate e-mails from people that have
sucessfully used this HOWTO to configure serial consoles on their machines.
Please state the version of the HOWTO you used (see the cover page), your
Linux distribution and its version, and the number of machines involved. This
information allows the maintainer to show his employer sufficient public
benefit for his work on this HOWTO to continue and will not be used for any
other purpose.
Linux is continually improving, so please send those small alterations
required for the latest version of your Linux distribution.
The HOWTO's maintainer is not a professional writer. If you find some parts
of this HOWTO difficult to comprehend then let the maintainer know.
-----------------------------------------------------------------------------
Colophon
Written in DocBook 4.1 SGML. XEmacs and the PSGML package were used to create
the SGML source file. The HTML, PostScript and PDF output was generated from
the DocBook source by the Linux Documentation Project.
Notes
[1] The Linux 2.4 kernel also supports the output of console messages to
Centronics or IEEE 1284-2000 parallel printer interfaces.
[2] There is no good reason for this difference. Feel free to submit a patch
to the linux-kernel mailing list to correct this oddity.
[3] A bit-time is the time taken to transmit one bit. The distinction
between bit-times of signal and bits of data is apparent when you
consider that 1.5 bit-times of signal is possible but that 1.5 bits of
data is impossible.
[4] As usual with IBM PC/AT hardware "loads a boot loader from the MBR of
the first hard disk" is a simplification. BIOS settings permitting, the
MBR can be loaded from the first two detected hard disks of any
controller card containing a BIOS extension. Thus the MBR can be loaded
from one of the first two detected IDE disks and one of the first two
detected SCSI disks.
[5] Another simplification. A 512 byte MBR is too small to contain a program
big enough to load a complex operating system. Thus most boot loaders
have two stages, the first stage is located in the MBR and is only able
to load the second stage of the boot loader from somewhere on a disk
(such as the boot sector of the first partition). The second stage of
the boot loader presents the user interface and loads the operating
system.
[6] A serial console attached to a USB dongle is only available in Linux
kernel version 2.5.7 and later. The 2.5 series of kernels are
developer's kernels and are not ready for production use.
[7] This is not as inefficient as it may appear. The last 5% of a disk
formatted with a general purpose filesystem always performs poorly and
is best left empty.
[8] But don't submit your proposed password to a search engine! Sending
passwords in plain text across the Internet isn't good, nor the
possibility of having them appear in the logs of a search engine.
[9] This is a fault with the design of flash memory. It identifies itself
with a model designator rather than with the timings required to read
and write the memory. So to load software from flash memory the boot ROM
must have a table of flash memory models and timings.
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