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<H2><A NAME="s7">7. Using common PC hardware</A></H2>
<P>
<H2><A NAME="ss7.1">7.1 ISDN</A>
</H2>
<P>The Integrated Services Digital Network (ISDN) is a series of standards
that specify a general purpose switched digital data network. An ISDN `call'
creates a synchronous point to point data service to the destination. ISDN
is generally delivered on a high speed link that is broken down into a number
of discrete channels. There are two different types of channels, the
`B Channels' which will actually carry the user data and a single channel
called the `D channel' which is used to send control information to the ISDN
exchange to establish calls and other functions. In Australia for example,
ISDN may be delivered on a 2Mbps link that is broken into 30 discrete 64kbps
B channels with one 64kbps D channel. Any number of channels may be used at a
time and in any combination. You could for example establish 30 separate calls
to 30 different destinations at 64kbps each, or you could establish 15 calls
to 15 different destinations at 128kbps each (two channels used per call), or
just a small number of calls and leave the rest idle. A channel may be used
for either incoming or outgoing calls. The original intention of ISDN was to
allow Telecommunications companies to provide a single data service which could
deliver either telephone (via digitised voice) or data services to your home
or business without requiring you to make any special configuration changes.
<P>There are a few different ways to connect your computer to an ISDN service.
One way is to use a device called a `Terminal Adaptor' which plugs into the
Network Terminating Unit that you telecommunications carrier will have
installed when you got your ISDN service and presents a number of serial
interfaces. One of those interfaces is used to enter commands to establish
calls and configuration and the others are actually connected to the network
devices that will use the data circuits when they are established. Linux will
work in this sort of configuration without modification, you just treat
the port on the Terminal Adaptor like you would treat any other serial device.
Another way, which is the way the kernel ISDN support is designed for allows
you to install an ISDN card into your Linux machine and then has your Linux
software handle the protocols and make the calls itself.
<P><B>Kernel Compile Options</B>:
<BLOCKQUOTE><CODE>
<PRE>
ISDN subsystem --->
&lt;*> ISDN support
[ ] Support synchronous PPP
[ ] Support audio via ISDN
&lt; > ICN 2B and 4B support
&lt; > PCBIT-D support
&lt; > Teles/NICCY1016PC/Creatix support
</PRE>
</CODE></BLOCKQUOTE>
<P>The Linux implementation of ISDN supports a number of different types of
internal ISDN cards. These are those listed in the kernel configuration
options:
<UL>
<LI>ICN 2B and 4B</LI>
<LI>Octal PCBIT-D</LI>
<LI>Teles ISDN-cards and compatibles</LI>
</UL>
<P>Some of these cards require software to be downloaded to them to make them
operational. There is a separate utility to do this with.
<P>Full details on how to configure the Linux ISDN support is available from
the <CODE>/usr/src/linux/Documentation/isdn/</CODE> directory and an FAQ dedicated
to <EM>isdn4linux</EM> is available at
<A HREF="http://www.lrz-muenchen.de/~ui161ab/www/isdn/">www.lrz-muenchen.de</A>.
(You can click on the english flag to get an english version).
<P><B>A note about PPP</B>. The PPP suite of protocols will operate over
either asynchronous or synchronous serial lines. The commonly distributed
PPP daemon for Linux `<EM>pppd</EM>' supports only asynchronous mode. If you wish
to run the PPP protocols over your ISDN service you need a specially modified
version. Details of where to find it are available in the documentation
referred to above.
<H2><A NAME="ss7.2">7.2 PLIP for Linux-2.0</A>
</H2>
<P>PLIP device names are `<CODE>plip0</CODE>', `<CODE>plip1</CODE> and <CODE>plip2</CODE>.
<P><B>Kernel Compile Options</B>:
<BLOCKQUOTE><CODE>
<PRE>
Network device support --->
&lt;*> PLIP (parallel port) support
</PRE>
</CODE></BLOCKQUOTE>
<P><EM>plip</EM> (Parallel Line IP), is like SLIP, in that it is used for
providing a <EM>point to point</EM> network connection between two machines,
except that it is designed to use the parallel printer ports on your machine
instead of the serial ports (a cabling diagram in included in the cabling
diagram section later in this document). Because it is possible to transfer
more than one bit at a time with a parallel port, it is possible to attain
higher speeds with the <EM>plip</EM> interface than with a standard serial device.
In addition, even the simplest of parallel ports, printer ports, can be used
in lieu of you having to purchase comparatively expensive 16550AFN UART's for
your serial ports. PLIP uses a lot of CPU compared to a serial link and is
most certainly not a good option if you can obtain some cheap ethernet cards,
but it will work when nothing else is available and will work quite well.
You should expect a data transfer rate of about 20 kilobytes per second when
a link is running well.
<P>The PLIP device drivers competes with the parallel device driver for the
parallel port hardware. If you wish to use both drivers then you should
compile them both as modules to ensure that you are able to select which
port you want to use for PLIP and which ports you want for the printer driver.
Refer to the ``Mudules mini-HOWTO'' for more
information on kernel module configuration.
<P>Please note that some laptops use chipsets that will not work with PLIP
because they do not allow some combinations of signals that PLIP relies on,
that printers don't use.
<P>The Linux <EM>plip</EM> interface is compatible with the <EM>Crynwyr Packet
Driver PLIP</EM> and this will mean that you can connect your Linux machine
to a DOS machine running any other sort of tcp/ip software via <EM>plip</EM>.
<P>In the 2.0.* series kernel the plip devices are mapped to i/o port and IRQ as
follows:
<BLOCKQUOTE><CODE>
<PRE>
device i/o IRQ
------ ----- ---
plip0 0x3bc 5
plip1 0x378 7
plip2 0x278 2
</PRE>
</CODE></BLOCKQUOTE>
<P>If your parallel ports don't match any of the above combinations then you
can change the IRQ of a port using the <EM>ifconfig</EM> command using the
`<CODE>irq</CODE>' parameter (be sure to enable IRQ's on your printer ports in your
ROM BIOS if it supports this option). As an alternative, you
can specify ``<CODE>io=</CODE>'' annd ``<CODE>irq=</CODE>'' options on the <EM>insmod</EM>
command line, if you use modules. For example:
<BLOCKQUOTE><CODE>
<PRE>
root# insmod plip.o io=0x288 irq=5
</PRE>
</CODE></BLOCKQUOTE>
<P>PLIP operation is controlled by two timeouts, whose default values
are probably ok in most cases. You will probably need to increase them
if you have an especially slow computer, in which case the timers to
increase are actually on the <B>other</B> computer. A program called
<EM>plipconfig</EM> exists that allows you to change these timer settings
without recompiling your kernel. It is supplied with many Linux
distributions.
<P>To configure a <EM>plip</EM> interface, you will need to invoke the
following commands (or <B>add</B> them to your initialization scripts):
<BLOCKQUOTE><CODE>
<PRE>
root# /sbin/ifconfig plip1 localplip pointopoint remoteplip
root# /sbin/route add remoteplip plip1
</PRE>
</CODE></BLOCKQUOTE>
<P>Here, the port being used is the one at I/O address 0x378;
<EM>localplip</EM> amd <EM>remoteplip</EM> are the names or IP addresses used
over the PLIP cable. I personally keep them in my <CODE>/etc/hosts</CODE>
database:
<BLOCKQUOTE><CODE>
<PRE>
# plip entries
192.168.3.1 localplip
192.168.3.2 remoteplip
</PRE>
</CODE></BLOCKQUOTE>
<P>The <EM>pointopoint</EM> parameter has the same meaning as for SLIP,
in that it specifies the address of the machine at the other end of the
link.
<P>In almost all respects you can treat a <EM>plip</EM> interface as though it
were a <EM>SLIP</EM> interface, except that neither <EM>dip</EM> nor
<EM>slattach</EM> need be, nor can be, used.
<P>Further information on PLIP may be obtained from the
``PLIP mini-HOWTO''.
<H2><A NAME="ss7.3">7.3 PLIP for Linux-2.2</A>
</H2>
<P>During development of the 2.1 kernel versions, support for the
parallel port was changed to a better setup.
<P><B>Kernel Compile Options</B>:
<BLOCKQUOTE><CODE>
<PRE>
General setup --->
[*] Parallel port support
Network device support --->
&lt;*> PLIP (parallel port) support
</PRE>
</CODE></BLOCKQUOTE>
<P>The new code for PLIP behaves like the old one (use the same <EM>ifconfig</EM>
and <EM>route</EM> commands as in the previous section, but initialization
of the device is different due to the advanced parallel port support.
<P>The ``first'' PLIP device is always called ``plip0'', where first
is the first device detected by the system, similarly to what happens
for Ethernet devices. The actual parallel port being used is one of
the available ports, as shown in <CODE>/proc/parport</CODE>. For example,
if you have only one parallel port, you'll only have a directory
called <CODE>/proc/parport/0</CODE>.
<P>If your kernel didn't detect the IRQ number used by your port,
``<CODE>insmod plip</CODE>'' will fail; in this case just write the right
number to <CODE>/proc/parport/0/irq</CODE> and reinvoke <EM>insmod</EM>.
<P>Complete information about parallel port management is available in
the file <CODE>Documentation/parport.txt</CODE>, part of your kernel sources.
<H2><A NAME="ss7.4">7.4 PPP</A>
</H2>
<P>PPP devices names are `<CODE>ppp0</CODE>', `<CODE>ppp1</CODE>, etc. Devices are numbered
sequentially with the first device configured receiving `<CODE>0</CODE>'.
<P><B>Kernel Compile Options</B>:
<BLOCKQUOTE><CODE>
<PRE>
Networking options --->
&lt;*> PPP (point-to-point) support
</PRE>
</CODE></BLOCKQUOTE>
<P>PPP configuration is covered in detail in the
<A HREF="PPP-HOWTO.html">PPP-HOWTO</A>.
<H3>Maintaining a permanent connection to the net with <EM>pppd</EM>.</H3>
<P>If you are fortunate enough to have a semi permanent connection to the net and
would like to have your machine automatically redial your PPP connection if it
is lost then here is a simple trick to do so.
<P>Configure PPP such that it can be started by the <CODE>root</CODE> user by issuing the
command:
<BLOCKQUOTE><CODE>
<PRE>
# pppd
</PRE>
</CODE></BLOCKQUOTE>
<B>Be sure</B> that you have the `<CODE>-detach</CODE>' option configured in your
<CODE>/etc/ppp/options</CODE> file. Then, insert the following line into your
<CODE>/etc/inittab</CODE> file, down with the <EM>getty</EM> definitions:
<BLOCKQUOTE><CODE>
<PRE>
pd:23:respawn:/usr/sbin/pppd
</PRE>
</CODE></BLOCKQUOTE>
This will cause the <EM>init</EM> program to spawn and monitor the <EM>pppd</EM>
program and automatically restart it if it dies.
<H2><A NAME="ss7.5">7.5 SLIP client</A>
</H2>
<P>SLIP devices are named `<CODE>sl0</CODE>', `<CODE>sl1</CODE>' etc. with the first device
configured being assigned `<CODE>0</CODE>' and the rest incrementing sequentially
as they are configured.
<P><B>Kernel Compile Options</B>:
<BLOCKQUOTE><CODE>
<PRE>
Network device support --->
[*] Network device support
&lt;*> SLIP (serial line) support
[ ] CSLIP compressed headers
[ ] Keepalive and linefill
[ ] Six bit SLIP encapsulation
</PRE>
</CODE></BLOCKQUOTE>
<P>SLIP (Serial Line Internet Protocol) allows you to use tcp/ip over a serial
line, be that a phone line with a dialup modem, or a leased line of some sort.
Of course to use SLIP you need access to a <EM>SLIP-server</EM> in your
area. Many universities and businesses provide SLIP access all over the
world.
<P>Slip uses the serial ports on your machine to carry IP datagrams. To do this
it must take control of the serial device. Slip device names are named
<EM>sl0</EM>, <EM>sl1</EM> etc. How do these correspond to your serial
devices ? The networking code uses what is called an <EM>ioctl</EM> (i/o
control) call to change the serial devices into SLIP devices. There are
two programs supplied that can do this, they are called <EM>dip</EM> and
<EM>slattach</EM>
<H3>dip</H3>
<P><EM>dip</EM> (Dialup IP) is a smart program that is able to set the speed of
the serial device, command your modem to dial the remote end of the link,
automatically log you into the remote server, search for messages sent to you
by the server and extract information for them such as your IP address and
perform the <EM>ioctl</EM> necessary to switch your serial port into
SLIP mode. <EM>dip</EM> has a powerful scripting ability and it is this
that you can exploit to automate your logon procedure.
<P>You can find it at:
<A HREF="ftp://metalab.unc.edu/pub/Linux/system/Network/serial/dip/dip337o-uri.tgz">metalab.unc.edu</A>.
<P>To install it, try the following:
<BLOCKQUOTE><CODE>
<PRE>
user% tar xvzf dip337o-uri.tgz
user% cd dip-3.3.7o
user% vi Makefile
root# make install
</PRE>
</CODE></BLOCKQUOTE>
<P>The <CODE>Makefile</CODE> assumes the existence of a group called <EM>uucp</EM>,
but you might like to change this to either <EM>dip</EM> or <EM>SLIP</EM>
depending on your configuration.
<H3>slattach</H3>
<P><EM>slattach</EM> as contrasted with <EM>dip</EM> is a very simple program,
that is very easy to use, but does not have the sophistication of <EM>dip</EM>.
It does not have the scripting ability, all it does is configure your
serial device as a SLIP device. It assumes you have all the information you
need and the serial line is established before you invoke it. <EM>slattach</EM>
is ideal to use where you have a permanent connection to your server, such as
a physical cable, or a leased line.
<H3>When do I use which ?</H3>
<P>You would use <EM>dip</EM> when your link to the machine that is your SLIP
server is a dialup modem, or some other temporary link. You would use
<EM>slattach</EM> when you have a leased line, perhaps a cable, between your
machine and the server and there is no special action needed to get the link
working. See section `Permanent Slip connection' for more information.
<P>Configuring SLIP is much like configuring an Ethernet interface
(read section `Configuring an ethernet device' above). However there
are a few key differences.
<P>First of all, SLIP links are unlike ethernet networks in that there
is only ever two hosts on the network, one at each end of the
link. Unlike an ethernet that is available for use as soon are you are
cabled, with SLIP, depending on the type of link you have, you may
have to initialize your network connection in some special way.
<P>If you are using <EM>dip</EM> then this would not normally be done
at boot time, but at some time later, when you were ready to use the
link. It is possible to automate this procedure. If you are using
<EM>slattach</EM> then you will probably want to add a section to your
<EM>rc.inet1</EM> file. This will be described soon.
<P>There are two major types of SLIP servers: Dynamic IP address
servers and static IP address servers. Almost every SLIP server will
prompt you to login using a username and password when dialing
in. <EM>dip</EM> can handle logging you in automatically.
<H3>Static SLIP server with a dialup line and DIP.</H3>
<P>A static SLIP server is one in which you have been supplied an IP address
that is exclusively yours. Each time you connect to the server, you will
configure your SLIP port with that address. The static SLIP server will
answer your modem call, possibly prompt you for a username and password,
and then route any datagrams destined for your address to you via that
connection. If you have a static server, then you may want to put entries
for your hostname and IP address (since you know what it will be) into your
<CODE>/etc/hosts</CODE>. You should also configure some other files such as:
<CODE>rc.inet2</CODE>, <CODE>host.conf</CODE>, <CODE>resolv.conf</CODE>,
<CODE>/etc/HOSTNAME</CODE> and <CODE>rc.local</CODE>. Remember that when configuring
<CODE>rc.inet1</CODE>, you don't need to add any special commands for your SLIP
connection since it is <EM>dip</EM> that does all of the hard work for you in
configuring your interface. You will need to give <EM>dip</EM> the appropriate
information and it will configure the interface for you after commanding the
modem to establish the call and logging you into your SLIP server.
<P>If this is how your SLIP server works then you can move to section
`Using Dip' to learn how to configure <EM>dip</EM> appropriately.
<H3>Dynamic SLIP server with a dialup line and DIP.</H3>
<P>A <EM>dynamic</EM> SLIP server is one which allocates you an IP
address randomly, from a pool of addresses, each time you logon. This
means that there is no guarantee that you will have any particular
address each time, and that address may well be used by someone else
after you have logged off. The network administrator who configured
the SLIP server will have assigned a pool of address for the SLIP
server to use, when the server receives a new incoming call, it finds
the first unused address, guides the caller through the login process
and then prints a welcome message that contains the IP address it has
allocated and will proceed to use that IP address for the duration of
that call.
<P>Configuring for this type of server is similar to configuring for a
static server, except that you must add a step where you obtain the IP
address that the server has allocated for you and configure your SLIP
device with that.
<P>Again, <EM>dip</EM> does the hard work and new versions are smart
enough to not only log you in, but to also be able to automatically
read the IP address printed in the welcome message and store it so
that you can have it configure your SLIP device with it.
<P>If this is how your SLIP server works then you can move to section
`Using Dip' to learn how to configure <EM>dip</EM> appropriately.
<H3>Using DIP.</H3>
<P>As explained earlier, <EM>dip</EM> is a powerful program that can simplify
and automate the process of dialing into the SLIP server, logging you in,
starting the connection and configuring your SLIP devices with the
appropriate <EM>ifconfig</EM> and <EM>route</EM> commands.
<P>Essentially to use <EM>dip</EM> you'll write a `dip script', which
is basically a list of commands that <EM>dip</EM> understands that
tell <EM>dip</EM> how to perform each of the actions you want it to
perform. See <CODE>sample.dip</CODE> that comes supplied with <EM>dip</EM>
to get an idea of how it works. <EM>dip</EM> is quite a powerful
program, with many options. Instead of going into all of them here
you should look at the <EM>man</EM> page, README and sample files that
will have come with your version of <EM>dip</EM>.
<P>You may notice that the <CODE>sample.dip</CODE> script assumes that
you're using a static SLIP server, so you know what your IP address is
beforehand. For dynamic SLIP servers, the newer versions of
<EM>dip</EM> include a command you can use to automatically read and
configure your SLIP device with the IP address that the dynamic server
allocates for you. The following sample is a modified version of the
<CODE>sample.dip</CODE> that came supplied with <EM>dip337j-uri.tgz</EM>
and is probably a good starting point for you. You might like to save
it as <CODE>/etc/dipscript</CODE> and edit it to suit your configuration:
<BLOCKQUOTE><CODE>
<PRE>
#
# sample.dip Dialup IP connection support program.
#
# This file (should show) shows how to use the DIP
# This file should work for Annex type dynamic servers, if you
# use a static address server then use the sample.dip file that
# comes as part of the dip337-uri.tgz package.
#
#
# Version: @(#)sample.dip 1.40 07/20/93
#
# Author: Fred N. van Kempen, &lt;waltje@uWalt.NL.Mugnet.ORG>
#
main:
# Next, set up the other side's name and address.
# My dialin machine is called 'xs4all.hacktic.nl' (== 193.78.33.42)
get $remote xs4all.hacktic.nl
# Set netmask on sl0 to 255.255.255.0
netmask 255.255.255.0
# Set the desired serial port and speed.
port cua02
speed 38400
# Reset the modem and terminal line.
# This seems to cause trouble for some people!
reset
# Note! "Standard" pre-defined "errlevel" values:
# 0 - OK
# 1 - CONNECT
# 2 - ERROR
#
# You can change those grep'ping for "addchat()" in *.c...
# Prepare for dialing.
send ATQ0V1E1X4\r
wait OK 2
if $errlvl != 0 goto modem_trouble
dial 555-1234567
if $errlvl != 1 goto modem_trouble
# We are connected. Login to the system.
login:
sleep 2
wait ogin: 20
if $errlvl != 0 goto login_trouble
send MYLOGIN\n
wait ord: 20
if $errlvl != 0 goto password_error
send MYPASSWD\n
loggedin:
# We are now logged in.
wait SOMEPROMPT 30
if $errlvl != 0 goto prompt_error
# Command the server into SLIP mode
send SLIP\n
wait SLIP 30
if $errlvl != 0 goto prompt_error
# Get and Set your IP address from the server.
# Here we assume that after commanding the SLIP server into SLIP
# mode that it prints your IP address
get $locip remote 30
if $errlvl != 0 goto prompt_error
# Set up the SLIP operating parameters.
get $mtu 296
# Ensure "route add -net default xs4all.hacktic.nl" will be done
default
# Say hello and fire up!
done:
print CONNECTED $locip ---> $rmtip
mode CSLIP
goto exit
prompt_error:
print TIME-OUT waiting for sliplogin to fire up...
goto error
login_trouble:
print Trouble waiting for the Login: prompt...
goto error
password:error:
print Trouble waiting for the Password: prompt...
goto error
modem_trouble:
print Trouble occurred with the modem...
error:
print CONNECT FAILED to $remote
quit
exit:
exit
</PRE>
</CODE></BLOCKQUOTE>
<P>The above example assumes you are calling a <EM>dynamic</EM> SLIP server, if
you are calling a <EM>static</EM> SLIP server, then the <CODE>sample.dip</CODE>
file that comes with <EM>dip337j-uri.tgz</EM> should work for you.
<P>When <EM>dip</EM> is given the <EM>get $local</EM> command it
searches the incoming text from the remote end for a string that looks like an
IP address, ie strings numbers separated by `.' characters. This modification
was put in place specifically for <EM>dynamic</EM> SLIP servers, so that the
process of reading the IP address granted by the server could be automated.
<P>The example above will automatically create a default route via your SLIP link,
if this is not what you want, you might have an ethernet connection that should
be your default route, then remove the <EM>default</EM> command from the script.
After this script has finished running, if you do an <EM>ifconfig</EM> command,
you will see that you have a device <EM>sl0</EM>. This is your SLIP device.
Should you need to, you can modify its configuration manually, after the
<EM>dip</EM> command has finished, using the <EM>ifconfig</EM> and
<EM>route</EM> commands.
Please note that <EM>dip</EM> allows you to select a number of different
protocols to use with the <CODE>mode</CODE> command, the most common example is
<EM>cSLIP</EM> for SLIP with compression. Please note that both ends of the
link must agree, so you should ensure that whatever you select agrees with
what your server is set to.
The above example is fairly robust and should cope with most errors. Please
refer to the <EM>dip</EM> man page for more information. Naturally you could,
for example, code the script to do such things as redial the server if it
doesn't get a connection within a prescribed period of time, or even try
a series of servers if you have access to more than one.
<H3>Permanent SLIP connection using a leased line and slattach.</H3>
<P>If you have a cable between two machines, or are fortunate enough to have a
leased line, or some other permanent serial connection between your machine
and another, then you don't need to go to all the trouble of using
<EM>dip</EM> to set up your serial link. <EM>slattach</EM> is a very simple
to use utility that will allow you just enough functionality to configure your
connection.
Since your connection will be a permanent one, you will want to add some
commands to your <CODE>rc.inet1</CODE> file. In essence all you need to do for
a permanent connection is ensure that you configure the serial device to
the correct speed and switch the serial device into SLIP mode. <EM>slattach</EM>
allows you to do this with one command. <B>Add</B> the following to your
<CODE>rc.inet1</CODE> file:
<BLOCKQUOTE><CODE>
<PRE>
#
# Attach a leased line static SLIP connection
#
# configure /dev/cua0 for 19.2kbps and cslip
/sbin/slattach -p cslip -s 19200 /dev/cua0 &amp;
/sbin/ifconfig sl0 IPA.IPA.IPA.IPA pointopoint IPR.IPR.IPR.IPR up
#
# End static SLIP.
</PRE>
</CODE></BLOCKQUOTE>
Where:
<DL>
<P>
<DT><B>IPA.IPA.IPA.IPA</B><DD><P>represents your IP address.
<P>
<DT><B>IPR.IPR.IPR.IPR</B><DD><P>represents the IP address of the remote end.
</DL>
<P><EM>slattach</EM> allocates the first unallocated SLIP device to the serial
device specified. <EM>slattach</EM> starts with <EM>sl0</EM>. Therefore
the first <EM>slattach</EM> command attaches SLIP device <EM>sl0</EM> to
the serial device specified and <EM>sl1</EM> the next time, etc.
<P><EM>slattach</EM> allows you to configure a number of different
protocols with the <CODE>-p</CODE> argument. In your case you will use
either <EM>SLIP</EM> or <EM>cSLIP</EM> depending on whether you want
to use compression or not. Note: both ends must agree on whether you
want compression or not.
<H2><A NAME="ss7.6">7.6 SLIP server.</A>
</H2>
<P>If you have a machine that is perhaps network connected, that you'd like
other people be able to dial into and provide network services, then you
will need to configure your machine as a server. If you want to use SLIP
as the serial line protocol, then currently you have three options as to how
to configure your Linux machine as a SLIP server. My preference would be to
use the first presented, <EM>sliplogin</EM>, as it seems the easiest to
configure and understand, but I will present a summary of each, so you can
make your own decision.
<H3>Slip Server using <EM>sliplogin</EM>.</H3>
<P><EM>sliplogin</EM> is a program that you can use in place of the normal login
shell for SLIP users that converts the terminal line into a SLIP line. It
allows you to configure your Linux machine as either a <EM>static address
server</EM>, users get the same address everytime they call in, or a
<EM>dynamic address server</EM>, where users get an address allocated for
them which will not necessarily be the same as the last time they called.
<P>The caller will login as per the standard login process, entering their username
and password, but instead of being presented with a shell after their login,
<EM>sliplogin</EM> is executed which searches its configuration file
(<CODE>/etc/slip.hosts</CODE>) for an entry with a login name that matches that of
the caller. If it locates one, it configures the line as an 8bit clean line,
and uses an <EM>ioctl</EM> call to convert the line discipline to SLIP. When
this process is complete, the last stage of configuration takes place, where
<EM>sliplogin</EM> invokes a shell script which configures the SLIP interface
with the relevant ip address, netmask and sets appropriate routing in place.
This script is usually called <CODE>/etc/slip.login</CODE>, but in a similar manner
to <EM>getty</EM>, if you have certain callers that require special
initialization, then you can create configuration scripts called
<CODE>/etc/slip.login.loginname</CODE> that will be run instead of the default
specifically for them.
<P>There are either three or four files that you need to configure to get
<EM>sliplogin</EM> working for you. I will detail how and where to
get the software and how each is configured in detail. The files are:
<UL>
<LI><CODE>/etc/passwd</CODE>, for the dialin user accounts.</LI>
<LI><CODE>/etc/slip.hosts</CODE>, to contain the information unique to each
dial-in user.</LI>
<LI><CODE>/etc/slip.login</CODE>, which manages the configuration of the routing
that needs to be performed for the user.</LI>
<LI><CODE>/etc/slip.tty</CODE>, which is required only if you are configuring
your server for <EM>dynamic address allocation</EM> and contains a table
of addresses to allocate</LI>
<LI><CODE>/etc/slip.logout</CODE>, which contains commands to clean up after the
user has hung up or logged out.</LI>
</UL>
<H3>Where to get <EM>sliplogin</EM></H3>
<P>You may already have the <EM>sliplogin</EM> package installed as part of your
distribution, if not then <EM>sliplogin</EM> can be obtained from:
<A HREF="ftp://metalab.unc.edu/pub/linux/system/Network/serial/sliplogin-2.1.1.tar.gz">metalab.unc.edu</A>.
The tar file contains both source, precompiled binaries and a <EM>man</EM> page.
<P>To ensure that only authorized users will be able to run <EM>sliplogin</EM>
program, you should add an entry to your <CODE>/etc/group</CODE> file similar to
the following:
<BLOCKQUOTE><CODE>
<PRE>
..
slip::13:radio,fred
..
</PRE>
</CODE></BLOCKQUOTE>
<P>When you install the <EM>sliplogin</EM> package, the <CODE>Makefile</CODE> will
change the group ownership of the <EM>sliplogin</EM> program to <CODE>slip</CODE>,
and this will mean that only users who belong to that group will be able
to execute it. The example above will allow only users <CODE>radio</CODE> and <CODE>fred</CODE>
to execute <EM>sliplogin</EM>.
<P>To install the binaries into your <CODE>/sbin</CODE> directory and the <EM>man</EM>
page into section 8, do the following:
<BLOCKQUOTE><CODE>
<PRE>
# cd /usr/src
# gzip -dc .../sliplogin-2.1.1.tar.gz | tar xvf -
# cd sliplogin-2.1.1
# &lt;..edit the Makefile if you don't use shadow passwords..>
# make install
</PRE>
</CODE></BLOCKQUOTE>
<P>If you want to recompile the binaries before installation, add a
<CODE>make clean</CODE> before the <CODE>make install</CODE>. If you want to install
the binaries somewhere else, you will need to edit the <CODE>Makefile</CODE>
<EM>install</EM> rule.
<P>Please read the <CODE>README</CODE> files that come with the package for more
information.
<H3>Configuring <CODE>/etc/passwd</CODE> for Slip hosts.</H3>
<P>Normally you would create some special logins for Slip callers in your
<CODE>/etc/passwd</CODE> file. A convention commonly followed is to use the
<EM>hostname</EM> of the calling host with a capital `S' prefixing it. So,
for example, if the calling host is called <CODE>radio</CODE> then you could
create a <CODE>/etc/passwd</CODE> entry that looked like:
<BLOCKQUOTE><CODE>
<PRE>
Sradio:FvKurok73:1427:1:radio SLIP login:/tmp:/sbin/sliplogin
</PRE>
</CODE></BLOCKQUOTE>
It doesn't really matter what the account is called, so long as it is
meaningful to you.
<P>Note: the caller doesn't need any special home directory, as they will not
be presented with a shell from this machine, so <CODE>/tmp</CODE> is a good choice.
Also note that <EM>sliplogin</EM> is used in place of the normal login shell.
<H3>Configuring <CODE>/etc/slip.hosts</CODE></H3>
<P>The <CODE>/etc/slip.hosts</CODE> file is the file that <EM>sliplogin</EM>
searches for entries matching the login name to obtain configuration details
for this caller. It is this file where you specify the ip address and netmask
that will be assigned to the caller and configured for their use. Sample
entries for two hosts, one a static configuration for host <CODE>radio</CODE> and
another, a dynamic configuration for user host <CODE>albert</CODE> might look like:
<BLOCKQUOTE><CODE>
<PRE>
#
Sradio 44.136.8.99 44.136.8.100 255.255.255.0 normal -1
Salbert 44.136.8.99 DYNAMIC 255.255.255.0 compressed 60
#
</PRE>
</CODE></BLOCKQUOTE>
The <CODE>/etc/slip.hosts</CODE> file entries are:
<OL>
<LI>the login name of the caller.</LI>
<LI>ip address of the server machine, ie this machine.</LI>
<LI>ip address that the caller will be assigned. If this field is coded
<CODE>DYNAMIC</CODE> then an ip address will be allocated based on the information
contained in your <CODE>/etc/slip.tty</CODE> file discussed later. <B>Note:</B>
you must be using at least version 1.3 of sliplogin for this to work.</LI>
<LI>the netmask assigned to the calling machine in dotted decimal notation
eg 255.255.255.0 for a Class C network mask.</LI>
<LI>the slip mode setting which allows you to enable/disable compression and
slip other features. Allowable values here are "<CODE>normal</CODE>" or
"<CODE>compressed</CODE>".</LI>
<LI>a timeout parameter which specifies how long the line can remain idle (no
datagrams received) before the line is automatically disconnected. A negative
value disables this feature.</LI>
<LI>optional arguments.</LI>
</OL>
<P>Note: You can use either hostnames or IP addresses in dotted decimal notation
for fields 2 and 3. If you use hostnames then those hosts must be resolvable,
that is, your machine must be able to locate an ip address for those hostnames,
otherwise the script will fail when it is called. You can test this by
trying trying to telnet to the hostname, if you get the
`<EM>Trying nnn.nnn.nnn...</EM>' message then your machine has been able to find
an ip address for that name. If you get the message `<EM>Unknown host</EM>', then
it has not. If not, either use ip addresses in dotted decimal notation, or fix
up your name resolver configuration (See section <CODE>Name Resolution</CODE>).
<P>The most common slip modes are:
<DL>
<P>
<DT><B>normal</B><DD><P>to enable normal uncompressed SLIP.
<P>
<DT><B>compressed</B><DD><P>to enable van Jacobsen header compression (cSLIP)
</DL>
Naturally these are mutually exclusive, you can use one or the other. For more
information on the other options available, refer to the <EM>man</EM> pages.
<H3>Configuring the <CODE>/etc/slip.login</CODE> file.</H3>
<P>After <EM>sliplogin</EM> has searched the <CODE>/etc/slip.hosts</CODE> and found
a matching entry, it will attempt to execute the <CODE>/etc/slip.login</CODE> file
to actually configure the SLIP interface with its ip address and netmask.
The sample <CODE>/etc/slip.login</CODE> file supplied with the <EM>sliplogin</EM>
package looks like this:
<BLOCKQUOTE><CODE>
<PRE>
#!/bin/sh -
#
# @(#)slip.login 5.1 (Berkeley) 7/1/90
#
# generic login file for a SLIP line. sliplogin invokes this with
# the parameters:
# $1 $2 $3 $4, $5, $6 ...
# SLIPunit ttyspeed pid the arguments from the slip.host entry
#
/sbin/ifconfig $1 $5 pointopoint $6 mtu 1500 -trailers up
/sbin/route add $6
arp -s $6 &lt;hw_addr> pub
exit 0
#
</PRE>
</CODE></BLOCKQUOTE>
You will note that this script simply uses the <EM>ifconfig</EM> and
<EM>route</EM> commands to configure the SLIP device with its ipaddress,
remote ip address and netmask and creates a route for the remote address via
the SLIP device. Just the same as you would if you were using the
<EM>slattach</EM> command.
<P>Note also the use of <EM>Proxy ARP</EM> to ensure that other hosts on the same
ethernet as the server machine will know how to reach the dial-in host.
The <CODE>&lt;hw_addr></CODE> field should be the hardware address of the ethernet
card in the machine. If your server machine isn't on an ethernet network then
you can leave this line out completely.
<H3>Configuring the <CODE>/etc/slip.logout</CODE> file.</H3>
<P>When the call drops out, you want to ensure that the serial device is restored
to its normal state so that future callers will be able to login correctly.
This is achieved with the use of the <CODE>/etc/slip.logout</CODE> file. It is
quite simple in format and is called with the same argument as the
<CODE>/etc/slip.login</CODE> file.
<BLOCKQUOTE><CODE>
<PRE>
#!/bin/sh -
#
# slip.logout
#
/sbin/ifconfig $1 down
arp -d $6
exit 0
#
</PRE>
</CODE></BLOCKQUOTE>
All it does is `down' the interface which will delete the manual route
previously created. It also uses the <EM>arp</EM> command to delete any proxy
arp put in place, again, you don't need the <EM>arp</EM> command in the script
if your server machine does not have an ethernet port.
<H3>Configuring the <CODE>/etc/slip.tty</CODE> file.</H3>
<P>If you are using dynamic ip address allocation (have any hosts configured
with the <CODE>DYNAMIC</CODE> keyword in the <CODE>/etc/slip.hosts</CODE> file, then
you must configure the <CODE>/etc/slip.tty</CODE> file to list what addresses
are assigned to what port. You only need this file if you wish your server
to dynamically allocate addresses to users.
<P>The file is a table that lists the <EM>tty</EM> devices that will support
dial-in SLIP connections and the ip address that should be assigned to users
who call in on that port.
Its format is as follows:
<BLOCKQUOTE><CODE>
<PRE>
# slip.tty tty -> IP address mappings for dynamic SLIP
# format: /dev/tty?? xxx.xxx.xxx.xxx
#
/dev/ttyS0 192.168.0.100
/dev/ttyS1 192.168.0.101
#
</PRE>
</CODE></BLOCKQUOTE>
<P>What this table says is that callers that dial in on port <CODE>/dev/ttyS0</CODE>
who have their remote address field in the <CODE>/etc/slip.hosts</CODE> file
set to <CODE>DYNAMIC</CODE> will be assigned an address of <CODE>192.168.0.100</CODE>.
<P>In this way you need only allocate one address per port for all users who do
not require an dedicated address for themselves. This helps you keep the number
of addresses you need down to a minimum to avoid wastage.
<H3>Slip Server using <EM>dip</EM>.</H3>
<P>Let me start by saying that some of the information below came from the
<EM>dip</EM> man pages, where how to run Linux as a SLIP server is briefly
documented. Please also beware that the following has been based on
the <EM>dip337o-uri.tgz</EM> package and probably will not apply to other
versions of <EM>dip</EM>.
<EM>dip</EM> has an input mode of operation, where it automatically locates
an entry for the user who invoked it and configures the serial line as
a SLIP link according to information it finds in the <CODE>/etc/diphosts</CODE>
file. This input mode of operation is activated by invoking <EM>dip</EM>
as <EM>diplogin</EM>. This therefore is how you use <EM>dip</EM> as a SLIP
server, by creating special accounts where <EM>diplogin</EM> is used as the
login shell.
The first thing you will need to do is to make a symbolic link as follows:
<BLOCKQUOTE><CODE>
<PRE>
# ln -sf /usr/sbin/dip /usr/sbin/diplogin
</PRE>
</CODE></BLOCKQUOTE>
You then need to add entries to both your <CODE>/etc/passwd</CODE> and your
<CODE>/etc/diphosts</CODE> files. The entries you need to make are formatted
as follows:
To configure Linux as a SLIP server with <EM>dip</EM>, you need to create some
special SLIP accounts for users, where <EM>dip</EM> (in input mode) is used as
the login shell. A suggested convention is that of having all SLIP accounts
begin with a capital `S', eg `Sfredm'.
A sample <CODE>/etc/passwd</CODE> entry for a SLIP user looks like:
<BLOCKQUOTE><CODE>
<PRE>
Sfredm:ij/SMxiTlGVCo:1004:10:Fred:/tmp:/usr/sbin/diplogin
^^ ^^ ^^ ^^ ^^ ^^ ^^
| | | | | | \__ diplogin as login shell
| | | | | \_______ Home directory
| | | | \____________ User Full Name
| | | \_________________ User Group ID
| | \_____________________ User ID
| \_______________________________ Encrypted User Password
\__________________________________________ Slip User Login Name
</PRE>
</CODE></BLOCKQUOTE>
<P>After the user logs in, the <EM>login</EM> program, if it finds and
verifies the user ok, will execute the <EM>diplogin</EM>
command. <EM>dip</EM>, when invoked as <EM>diplogin</EM> knows that it
should automatically assume that it is being used a login shell. When
it is started as <EM>diplogin</EM> the first thing it does is use the
<EM>getuid()</EM> function call to get the userid of whoever has
invoked it. It then searches the <CODE>/etc/diphosts</CODE> file for the
first entry that matches either the userid or the name of the
<EM>tty</EM> device that the call has come in on and configures itself
appropriately. By judicious decision as to whether to give a user an
entry in the <CODE>diphosts</CODE> file, or whether to let the user be
given the default configuration you can build your server in such a
way that you can have a mix of static and dynamically assigned address
users.
<EM>dip</EM> will automatically add a `Proxy-ARP' entry if invoked in input
mode, so you do not need to worry about manually adding such entries.
<H3>Configuring <CODE>/etc/diphosts</CODE></H3>
<P><CODE>/etc/diphosts</CODE> is used by <EM>dip</EM> to lookup preset
configurations for remote hosts. These remote hosts might be users
dialing into your linux machine, or they might be for machines that you dial
into with your linux machine.
The general format for <CODE>/etc/diphosts</CODE> is as follows:
<BLOCKQUOTE><CODE>
<PRE>
..
Suwalt::145.71.34.1:145.71.34.2:255.255.255.0:SLIP uwalt:CSLIP,1006
ttyS1::145.71.34.3:145.71.34.2:255.255.255.0:Dynamic ttyS1:CSLIP,296
..
</PRE>
</CODE></BLOCKQUOTE>
The fields are:
<OL>
<LI><CODE>login name</CODE>: as returned by getpwuid(getuid()) or tty name.</LI>
<LI><CODE>unused</CODE>: compat. with passwd</LI>
<LI><CODE>Remote Address</CODE>: IP address of the calling host, either numeric or by name</LI>
<LI><CODE>Local Address</CODE>: IP address of this machine, again numeric or by name</LI>
<LI><CODE>Netmask</CODE>: in dotted decimal notation</LI>
<LI><CODE>Comment field</CODE>: put whatever you want here.</LI>
<LI><CODE>protocol</CODE>: Slip, CSlip etc.</LI>
<LI><CODE>MTU</CODE>: decimal number</LI>
</OL>
An example <CODE>/etc/net/diphosts</CODE> entry for a remote SLIP user might be:
<BLOCKQUOTE><CODE>
<PRE>
Sfredm::145.71.34.1:145.71.34.2:255.255.255.0:SLIP uwalt:SLIP,296
</PRE>
</CODE></BLOCKQUOTE>
which specifies a SLIP link with remote address of 145.71.34.1 and MTU of 296,
or:
<BLOCKQUOTE><CODE>
<PRE>
Sfredm::145.71.34.1:145.71.34.2:255.255.255.0:SLIP uwalt:CSLIP,1006
</PRE>
</CODE></BLOCKQUOTE>
which specifies a cSLIP-capable link with remote address 145.71.34.1 and MTU
of 1006.
<P>Therefore, all users who you wish to be allowed a statically allocated dial-up
IP access should have an entry in the <CODE>/etc/diphosts</CODE>. If you want
users who call a particular port to have their details dynamically allocated
then you must have an entry for the <CODE>tty</CODE> device and do not configure a
user based entry. You should remember to configure at least one entry for
each <CODE>tty</CODE> device that your dialup users use to ensure that a suitable
configuration is available for them regardless of which modem they call in on.
<P>When a user logs in they will receive a normal login and password prompt at
which they should enter their SLIP-login userid and password. If these verify
ok then the user will see no special messages and they should just change into
SLIP mode at their end. The user should then be able to connect ok and be
configured with the relevant parameters from the <CODE>diphosts</CODE> file.
<H3>SLIP server using the <EM>dSLIP</EM> package.</H3>
<P>Matt Dillon <CODE>&lt;dillon@apollo.west.oic.com></CODE> has written a package
that does not only dial-in but also dial-out SLIP. Matt's package is
a combination of small programs and scripts that manage your connections
for you. You will need to have <EM>tcsh</EM> installed as at least one
of the scripts requires it. Matt supplies a binary copy of the <EM>expect</EM>
utility as it too is needed by one of the scripts. You will most likely need
some experience with <EM>expect</EM> to get this package working to your
liking, but don't let that put you off.
<P>Matt has written a good set of installation instructions in the
README file, so I won't bother repeating them.
<P>You can get the <EM>dSLIP</EM> package from its home site at:
<B>apollo.west.oic.com</B>
<BLOCKQUOTE><CODE>
<PRE>
/pub/linux/dillon_src/dSLIP203.tgz
</PRE>
</CODE></BLOCKQUOTE>
or from:
<B>metalab.unc.edu</B>
<BLOCKQUOTE><CODE>
<PRE>
/pub/Linux/system/Network/serial/dSLIP203.tgz
</PRE>
</CODE></BLOCKQUOTE>
Read the <CODE>README</CODE> file and create the <CODE>/etc/passwd</CODE> and
<CODE>/etc/group</CODE> entries <B>before</B> doing a <CODE>make install</CODE>.
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