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The Linux Networking Overview HOWTO
Daniel Lopez Ridruejo, ridruejo@rawbyte.com
v0.32, 8 July 2000
The purpose of this document is to give an overview of the networking
capabilities of the Linux Operating System and to provide pointers for
further information and implementation details.
______________________________________________________________________
Table of Contents
1. Introduction
2. Linux.
2.1 What is Linux?
2.2 What makes Linux different?
3. Networking protocols
3.1 TCP/IP
3.2 TCP/IP version 6
3.3 IPX/SPX
3.4 AppleTalk Protocol Suite
3.5 WAN Networking: X.25, Frame-relay, etc...
3.6 ISDN
3.7 PPP, SLIP, PLIP
3.8 Amateur Radio
3.9 ATM
4. Networking hardware supported
5. File Sharing and Printing
5.1 Apple environment
5.2 Windows Environment
5.3 Novell Environment
5.4 Unix Environment
6. Internet/Intranet
6.1 Mail
6.1.1 Mail servers
6.1.2 Remote access to mail
6.1.3 Mail User Agents
6.1.4 Mailing list software
6.1.5 Fetchmail
6.2 Web Servers
6.3 Web Browsers
6.4 FTP Servers and clients
6.5 News service
6.6 Domain Name System
6.7 DHCP, bootp
6.8 NIS
6.9 Authentication
7. Remote execution of applications
7.1 Telnet
7.2 Remote commands
7.3 The X Window System
7.4 VNC
8. Network Interconnection
8.1 Router
8.2 Bridge
8.3 IP Masquerade
8.4 IP Accounting
8.5 IP aliasing
8.6 Traffic Shaping
8.7 Firewall
8.8 Port forwarding
8.9 Load Balancing
8.10 EQL
8.11 Proxy Server
8.12 Diald on demand
8.13 Tunnelling, mobile IP and virtual private networks
9. Network Management
9.1 Network management applications
9.2 SNMP
10. Enterprise Linux Networking
10.1 High Availability
10.2 RAID
10.3 Redundant networking
11. Sources of Information
12. Document history
13. Acknowledgements and disclaimer
______________________________________________________________________
1. Introduction
The purpose of this document is to give an overview of the networking
capabilities of the Linux operating system. Although one of the
strengths of Linux is that plenty of information exists for nearly
every component of it, most of this information is focused on
implementation. New Linux users, particularly those coming from a
Windows environment, are often unaware of the networking possibilities
of Linux. This document aims to show a general picture of such
possibilities with a brief description of each one and pointers for
further information. The information has been gathered from many
sources: HOWTOs, faqs, projects' web pages and my own hands-on
experience. Full credit is given to the authors of these other
sources. Without them and their programs this document would have not
been possible or necessary.
2. Linux.
2.1. What is Linux?
The primary author of Linux is Linus Torvalds. Since his original
versions, it has been improved by countless numbers of people. It is a
clone, written entirely from scratch, of the Unix operating system.
One of the more interesting facts about Linux is that its development
occurs simultaneously around the world.
Linux has been copyrighted under the terms of the GNU General Public
License (GPL). This is a license written by the Free Software
Foundation (FSF) that is designed to prevent people from restricting
the distribution of software. In brief, it says that although money
can be charged for a copy, the person who received the copy can not be
prevented from giving it away for free. It also means that the source
code must be available. This is useful for programmers. Anybody can
modify Linux and even distribute his/her modifications, provided that
they keep the code under the same copyright.
2.2. What makes Linux different?
Why work on Linux? Linux is generally cheaper (or at least no more
expensive) than other operating systems and is frequently less
problematic than many commercial systems. But what makes Linux
different is not its price (after all, why would anyone want an OS -
even a free one - if it is not good enough?) but its outstanding
capabilities:
<20> Linux is a true 32-bit multitasking operating system, robust and
capable enough to be used in organizations ranging from
universities to large corporations.
<20> It runs on hardware ranging from low-end 386 boxes to massive
ultra-parallel machines in research centres.
<20> Out-of-the-box versions are available for Intel, Sparc, and Alpha
architectures, and experimental support exists for Power PC and
embedded systems, among others such as SGI, Ultra Sparc, AP1000+,
Strong ARM, and MIPS R3000/R4000.
<20> Finally, when it comes to networking, Linux is choice. Not only
because networking is tightly integrated with the OS itself and a
plethora of applications is freely available, but for the
robustness under heavy loads that can only be achieved after years
of debugging and testing in an Open Source project.
3. Networking protocols
Linux supports many different networking protocols:
3.1. TCP/IP
The Internet Protocol was originally developed two decades ago for the
United States Department of Defense (DoD), mainly for the purpose of
interconnecting different-brand computers. The TCP/IP suite of
protocols allowed, through its layered structure, to insulate
applications from networking hardware.
Although it is based on a layered model, it is focused more on
delivering interconnectivity than on rigidly adhering to functional
layers. This is one of the reasons why TCP/IP has become the de facto
standard internetworking protocol as opposed to OSI.
TCP/IP networking has been present in Linux since its beginnings. It
has been implemented from scratch. It is one of the most robust, fast
and reliable implementations and is one of the key factors of the
success of Linux.
Related HOWTO: http://metalab.unc.edu/mdw/HOWTO/NET3-4-HOWTO.html
3.2. TCP/IP version 6
IPv6, sometimes also referred to as IPng (IP Next Generation) is an
upgrade to the IPv4 protocol in order to address many issues. These
issues include: shortage of available IP addresses, lack of mechanisms
to handle time-sensitive traffic, lack of network layer security, etc.
The larger name space will be accompanied by an improved addressing
scheme, which will have a great impact on routing performance. A beta
implementation exists for Linux, and a production version is expected
for the 2.2.0 Linux kernel release.
<20> Linux IPv6 HOWTO:http://www.wcug.wwu.edu/ipv6/faq/
3.3. IPX/SPX
IPX/SPX (Internet Packet Exchange/Sequenced Packet Exchange) is a
proprietary protocol stack developed by Novell and based on the Xerox
Network Systems (XNS) protocol. IPX/SPX became prominent during the
early 1980s as an integral part of Novell, Inc.'s NetWare. NetWare
became the de facto standard network operating system (NOS) of first
generation LANs. Novell complemented its NOS with a business-oriented
application suite and client-side connection utilities.
Linux has a very clean IPX/SPX implementation, allowing it to be
configured as an:
<20> IPX router
<20> IPX bridge
<20> NCP client and/or NCP Server (for sharing files)
<20> Novell Print Client, Novell Print Server
And to:
<20> Enable PPP/IPX, allowing a Linux box to act as a PPP server/client
<20> Perform IPX tunnelling through IP, allowing the connection of two
IPX networks through an IP only link
Additionally, Caldera <http://www.caldera.com> offers commercial
support for Novell NetWare under Linux. Caldera provides a fully
featured Novell NetWare client built on technology licensed from
Novell Corporation. The client provides full client access to Novell
3.x and 4.x fileservers and includes features such as NetWare
Directory Service (NDS) and RSA encryption.
<20> IPX HOWTO: http://metalab.unc.edu/mdw/HOWTO/IPX-HOWTO.html
3.4. AppleTalk Protocol Suite
Appletalk is the name of Apple's internetworking stack. It allows a
peer-to-peer network model which provides basic functionality such as
file and printer sharing. Each machine can simultaneously act as a
client and a server, and the software and hardware necessary are
included with every Apple computer.
Linux provides full Appletalk networking. Netatalk is a kernel-level
implementation of the AppleTalk Protocol Suite, originally for BSD-
derived systems. It includes support for routing AppleTalk, serving
Unix and AFS filesystems over AFP (AppleShare), serving Unix printers
and accessing AppleTalk printers over PAP.
See section 5.1 for more information.
3.5. WAN Networking: X.25, Frame-relay, etc...
Several third parties provide T-1, T-3, X.25 and Frame Relay products
for Linux. Generally special hardware is required for these types of
connections. Vendors that provide the hardware also provide the
drivers with protocol support.
<20> WAN resources for Linux:
http://www.secretagent.com/networking/wan.html
3.6. ISDN
The Linux kernel has built-in ISDN capabilies. Isdn4linux controls
ISDN PC cards and can emulate a modem with the Hayes command set ("AT"
commands). The possibilities range from simply using a terminal
program to connections via HDLC (using included devices) to full
connection to the Internet with PPP to audio applications.
<20> FAQ for isdn4linux: http://ww.isdn4linux.de/faq/
3.7. PPP, SLIP, PLIP
The Linux kernel has built-in support for PPP (Point-to-Point-
Protocol), SLIP (Serial Line IP) and PLIP (Parallel Line IP). PPP is
the most popular way individual users access their ISPs (Internet
Service Providers). PLIP allows the cheap connection of two machines.
It uses a parallel port and a special cable, achieving speeds of
10kBps to 20kBps.
<20> Linux PPP HOWTO <http://metalab.unc.edu/mdw/HOWTO/PPP-HOWTO.html>
<20> PPP/SLIP emulator <http://metalab.unc.edu/mdw/HOWTO/mini/SLIP-PPP-
Emulator.html>
<20> PLIP information can be found in The Network Administrator Guide
<http://metalab.unc.edu/mdw/LDP/nag/nag.html>
3.8. Amateur Radio
The Linux kernel has built-in support for amateur radio protocols.
Especially interesting is the AX.25 support. The AX.25 protocol offers
both connected and connectionless modes of operation, and is used
either by itself for point-point links, or to carry other protocols
such as TCP/IP and NetRom.
It is similar to X.25 level 2 in structure, with some extensions to
make it more useful in the amateur radio environment.
<20> Amateur radio on Linux web site <http://radio.linux.org.au/>
3.9. ATM
ATM support for Linux is currently in pre-alpha stage. There is an
experimental release, which supports raw ATM connections (PVCs and
SVCs), IP over ATM, LAN emulation...
<20> Linux ATM-Linux home page <http://lrcwww.epfl.ch/linux-atm/>
4. Networking hardware supported
Linux supports a great variety of networking hardware, including some
obsolete equipment.
Some interesting documents:
<20> Hardware HOWTO <http://metalab.unc.edu/mdw/HOWTO/Hardware-
HOWTO.html>
<20> Ethernet HOWTO <http://metalab.unc.edu/mdw/HOWTO/Ethernet-
HOWTO.html>
5. File Sharing and Printing
The primary purpose of many PC based Local Area Networks is to provide
file and printer sharing services to the users. Linux as a corporate
file and print server turns out to be a great solution.
5.1. Apple environment
As outlined in previous sections, Linux supports the Appletalk family
of protocols. Linux netatalk allows Macintosh clients to see Linux
Systems as another Macintosh on the network, share files and use
printers connected to Linux servers.
Netatalk faq and HOWTO:
<20> http://thehamptons.com/anders/netatalk/
<20> http://www.umich.edu/~rsug/netatalk/
<20> http://www.umich.edu/~rsug/netatalk/faq.html
5.2. Windows Environment
Samba is a suite of applications that allow most Unices (and in
particular Linux) to integrate into a Microsoft network both as a
client and a server. Acting as a server it allows Windows 95, Windows
for Workgroups, DOS and Windows NT clients to access Linux files and
printing services. It can completely replace Windows NT for file and
printing services, including the automatic downloading of printer
drivers to clients. Acting as a client allows the Linux workstation to
mount locally exported windows file shares.
According to the SAMBA Meta-FAQ:
"Many users report that compared to other SMB implementations Samba is more stable,
faster, and compatible with more clients. Administrators of some large installations say
that Samba is the only SMB server available which will scale to many tens of thousands
of users without crashing"
<20> Samba project home page <http://samba.anu.edu.au/samba/>
<20> SMB HOWTO <http://metalab.unc.edu/mdw/HOWTO/SMB-HOWTO.html>
<20> Printing HOWTO <http://metalab.unc.edu/mdw/HOWTO/Printing-
HOWTO.html>
5.3. Novell Environment
As stated in previous sections, Linux can be configured to act as an
NCP client or server, thus allowing file and printing services over a
Novell network for both Novell and Unix clients.
<20> IPX HOWTO <http://metalab.unc.edu/mdw/HOWTO/IPX-HOWTO.html>
5.4. Unix Environment
The preferred way to share files in a Unix networking environment is
through NFS. NFS stands for Network File Sharing and it is a protocol
originally developed by Sun Microsystems. It is a way to share files
between machines as if they were local. A client "mounts" a filesystem
"exported" by an NFS server. The mounted filesystem will appear to the
client machine as if it was part of the local filesystem.
It is possible to mount the root filesystem at startup time, thus
allowing diskless clients to boot up and access all files from a
server. In other words, it is possible to have a fully functional
computer without a hard disk.
Coda is a network filesystem (like NFS) that supports disconnected
operation, persistant caching, among other goodies. It's included in
2.2.x kernels. Really handy for slow or unreliable networks and
laptops.
NFS-related documents:
<20> http://metalab.unc.edu/mdw/HOWTO/mini/NFS-Root.html
<20> http://metalab.unc.edu/mdw/HOWTO/Diskless-HOWTO.html
<20> http://metalab.unc.edu/mdw/HOWTO/mini/NFS-Root-Client-mini-
HOWTO/index.html
<20> http://www.redhat.com/support/docs/rhl/NFS-Tips/NFS-Tips.html
<20> http://metalab.unc.edu/mdw/HOWTO/NFS-HOWTO.html
CODA can be found at: http://www.coda.cs.cmu.edu/
6. Internet/Intranet
Linux is a great platform to act as an Intranet / Internet server. The
term Intranet refers to the application of Internet technologies
inside an organisation mainly for the purpose of distributing and
making available information inside the company. Internet and Intranet
services offered by Linux include mail, news, WWW servers and many
more that will be outlined in the next sections.
6.1. Mail
6.1.1. Mail servers
Sendmail is the de facto standard mail server program (called an MTA,
or Mail Transport Agent) for Unix platforms. It is robust, scalable,
and properly configured and with the necessary hardware, can handle
loads of thousands of users without blinking. Alternative mail
servers, such as smail and qmail, are also available.
<20> Sendmail web site <http://www.sendmail.org/>
<20> Smail faq <http://www.sbay.org/smail-faq.html>
<20> Qmail web site <http://www.qmail.org>
Mail HOWTOs:
<20> http://metalab.unc.edu/mdw/HOWTO/Mail-User-HOWTO.html
<20> http://metalab.unc.edu/mdw/HOWTO/mini/Qmail+MH.html
<20> http://metalab.unc.edu/mdw/HOWTO/mini/Sendmail+UUCP.html
<20> http://metalab.unc.edu/mdw/HOWTO/mini/Mail-Queue.html
6.1.2. Remote access to mail
In an organisation or ISP, users will likely access their mail
remotely from their desktops. Several alternatives exist in Linux,
including POP (Post Office Protocol) and IMAP (Internet Message Access
Protocol) servers. The POP protocol is usually used to transfer
messages from the server to the client. IMAP permits also manipulation
of the messages in the server, remote creation and deletion of folders
in the server, concurrent access to shared mail folders, etc.
<20> Brief comparison IMAP and POP
<http://www.imap.org/imap.vs.pop.brief.html>
Mail related HOWTOs:
<20> http://metalab.unc.edu/mdw/HOWTO/Mail-User-HOWTO.html
<20> http://metalab.unc.edu/mdw/HOWTO/Cyrus-IMAP.html
6.1.3. Mail User Agents
There are a number of MUA (Mail User Agents) in Linux, both graphical
and text mode. The most widely used ones include: pine, elm, mutt and
Netscape.
<20> List of mail related software
<http://www.linuxlinks.com/Software/Internet/Mail/>
<20> http://metalab.unc.edu/mdw/HOWTO/mini/TkRat.html
6.1.4. Mailing list software
There are many MLM (Mail List Management) programs available for Unix
in general and for Linux in particular.
<20> A good comparison of existing MLMs may be found
at:ftp://ftp.uu.net/usenet/news.answers/mail/list-admin/
<20> Listserv <http://www.lsoft.com/>
<20> Majordomo home page <http://www.greatcircle.com/majordomo/>
6.1.5. Fetchmail
One userful mail-related utility is fetchmail. Fetchmail is a free,
full-featured, robust, well-documented remote-mail retrieval and
forwarding utility intended to be used over on-demand TCP/IP links
(such as SLIP or PPP connections). It supports every remote-mail
protocol now in use on the Internet. It can even support IPv6 and
IPSEC.
Fetchmail retrieves mail from remote mail servers and forwards it via
SMTP, so it can then be be read by normal mail user agents such as
mutt, elm or BSD Mail. It allows all the system MTA's filtering,
forwarding, and aliasing facilities to work just as they would on
normal mail.
Fetchmail can be used as a POP/IMAP-to-SMTP gateway for an entire DNS
domain, collecting mail from a single drop box on an ISP and SMTP-
forwarding it based on header addresses.
A small company may centralise its mail in a single mailbox, configure
fetchmail to collect all outgoing mail, send it via a single mailbox
at their ISP and retrieve all incoming mail from the same mailbox.
<20> Fetchmail home page <http://www.tuxedo.org/~esr/fetchmail/>
6.2. Web Servers
Most Linux distributions include Apache <http://www.apache.org>.
Apache is the number one server on the internet according to
http://www.netcraft.co.uk/survey/ . More than a half of all internet
sites are running Apache or one of it derivatives. Apache's advantages
include its modular design, stability and speed. Given the appropriate
hardware and configuration it can support the highest loads: Yahoo,
Altavista, GeoCities, and Hotmail are based on customized versions of
this server.
Optional support for SSL (which enables secure transactions) is also
available at:
<20> http://www.apache-ssl.org/
<20> http://raven.covalent.net/
<20> http://www.c2.net/
Related HOWTOs:
<20> http://metalab.unc.edu/mdw/HOWTO/WWW-HOWTO.html
<20> http://metalab.unc.edu/mdw/HOWTO/Virtual-Services-HOWTO.html
<20> http://metalab.unc.edu/mdw/HOWTO/Intranet-Server-HOWTO.html
<20> Web servers for Linux
<http://www.linuxlinks.com/Software/Internet/WebServers/>
6.3. Web Browsers
A number of web browsers exist for the Linux platform. Netscape
Navigator has been one of the choices from the very beginning and the
upcoming Mozilla (http://www.mozilla.org) will have a Linux version.
Another popular text based web browser is lynx. It is fast and handy
when no graphical environment is available.
<20> Browser software for Linux
<http://www.linuxlinks.com/Software/Internet/WebBrowsers/>
<20> http://metalab.unc.edu/mdw/HOWTO/mini/Public-Web-Browser.html
6.4. FTP Servers and clients
FTP stands for File Transfer Protocol. An FTP server allows clients to
connect to it and retrieve (download) files. Many ftp servers and
clients exist for Linux and are included with most distributions.
There are text-based clients as well as GUI based ones. FTP related
software (servers and clients) for Linux may be found at:
http://metalab.unc.edu/pub/Linux/system/network/file-transfer/
6.5. News service
Usenet (also known as news) is a big bulletin board system that covers
all kinds of topics and it is organised hierarchically. A network of
computers across the internet (Usenet) exchange articles through the
NNTP protocol. Several implementations exist for Linux, either for
heavily loaded sites or for small sites receiving only a few
newsgroups.
<20> INN home page <http://www.isc.org/>
<20> Linux news related software
<http://www.linuxlinks.com/Software/Internet/News/>
6.6. Domain Name System
A DNS server has the job of translating names (readable by humans) to
IP addresses. A DNS server does not know all the IP addresses in the
world; rather, it is able to request other servers for the unknown
addresses. The DNS server will either return the wanted IP address to
the user or report that the name cannot be found in the tables.
Name serving on Unix (and on the vast majority of the Internet) is
done by a program called named. This is a part of the bind package of
The Internet Software Consortium.
<20> BIND <http://www.isc.org/>
<20> DNS HOWTO <http://metalab.unc.edu/mdw/HOWTO/DNS-HOWTO.html>
6.7. DHCP, bootp
DHCP and bootp are protocols that allow a client machine to obtain
network information (such as their IP number) from a server. Many
organisations are starting to use it because it eases network
administration, especially in large networks or networks which have
lots of mobile users.
Related documents:
<20> DHCP mini-HOWTO
<http://metalab.unc.edu/mdw/HOWTO/mini/DHCP/index.html>
6.8. NIS
The Network Information Service (NIS) provides a simple network lookup
service consisting of databases and processes. Its purpose is to
provide information that has to be known throughout the network to all
machines on the network. For example, it enables an administrator to
allow users access to any machine in a network running NIS without a
password entry existing on each machine; only the main database needs
to be maintained.
Related HOWTO:
<20> NIS HOWTO <http://metalab.unc.edu/mdw/HOWTO/NIS-HOWTO.html>
6.9. Authentication
There are also various ways of authenticating users in mixed networks.
<20> For Linux/Windows NT:http://www.mindware.com.au/ftp/smb-NT-
verify.1.1.tar.gz
<20> The PAM (pluggable authentication module) which is a flexible
method of Unix authentication: PAM library
<http://www.kernel.org/pub/linux/libs/pam/index.html>.
<20> Finally, LDAP in Linux
<http://www.umich.edu/~dirsvcs/ldap/index.html>
7. Remote execution of applications
One of the most amazing features of Unix (yet one of the most unknown
to new users) is its great support for remote and distributed
execution of applications.
7.1. Telnet
Telnet is a program that allows a person to use a remote computer as
if that person were actually at the remote site. Telnet is one of the
most powerful tools for Unix, allowing for true remote administration.
It is also an interesting program from the point of view of users,
because it allows remote access to all their files and programs from
anywhere in the Internet. Combined with an X server, there is no
difference (apart from the delay) between being at the console or on
the other side of the planet. Telnet daemons and clients are available
with most Linux distributions.
Encrypted remote shell sessions are available through SSH
(http://www.ssh.fi/sshprotocols2/index.html
<http://www.ssh.fi/sshprotocols2/index.html>) thus effectively
allowing secure remote administration.
<20> Telnet related software
<http://metalab.unc.edu/pub/Linux/system/network/telnet/>
7.2. Remote commands
In Unix, and in particular in Linux, remote commands exist that allow
for interaction with other computers from the shell prompt. Examples
are: rlogin, which allows for login in a remote machine in a similar
way to telnet, rcp, which allows for the remote transfer of files
among machines, etc. Finally, the remote shell command rsh allows the
execution of a command on a remote machine without actually logging
onto that machine.
7.3. The X Window System
The X Window System was developed at MIT in the late 1980s, rapidly
becoming the industry standard windowing system for Unix graphics
workstations. The software is freely available, very versatile, and is
suitable for a wide range of hardware platforms. Any X environment
consists of two distinct parts, the X server and one or more X
clients. It is important to realise the distinction between the server
and the client. The server controls the display directly and is
responsible for all input/output via the keyboard, mouse or display.
The clients, on the other hand, do not access the screen directly -
they communicate with the server, which handles all input and output.
It is the clients which do the "real" computing work - running
applications or whatever. The clients communicate with the server,
causing the server to open one or more windows to handle input and
output for that client.
In short, the X Window System allows a user to log in into a remote
machine, execute a process (for example, open a web browser) and have
the output displayed on his own machine. Because the process is
actually being executed on the remote system, very little CPU power is
needed in the local one. Indeed, computers exist whose primary purpose
is to act as pure X servers. Such systems are called X terminals.
A free port of the X Window System exists for Linux and can be found
at: Xfree <http://www.xfree86.org/>. It is included in most Linux
distributions.
Related HOWTO:
<20> Remote X Apps HOWTO <http://metalab.unc.edu/mdw/HOWTO/mini/Remote-
X-Apps.html>
7.4. VNC
VNC stands for Virtual Network Computing. It is, in essence, a remote
display system which allows one to view a computing 'desktop'
environment not only on the machine where it is running, but from
anywhere on the Internet and from a wide variety of machine
architectures. Both clients and servers exist for Linux as well as for
many other platforms. It is possible to execute MS-Word in a Windows
NT or 95 machine and have the output displayed in a Linux machine. The
opposite is also true; it is possible to execute an application in a
Linux machine and have the output displayed in any other Linux or
Windows machine. One of the available clients is a Java applet,
allowing the remote display to be run inside a web browser. Another
client is a port for Linux using the SVGAlib graphics library,
allowing 386s with as little as 4 MB of RAM to become fully functional
X-Terminals.
<20> VNC web site <http://www.realvnc.com/>
8. Network Interconnection
Linux networking is rich in features. A Linux box can be configured so
it can act as a router, bridge, etc... Some of the available options
are described below.
8.1. Router
The Linux kernel has built-in support for routing functions. A Linux
box can act either as an IP or IPX router for a fraction of the cost
of a commercial router. Recent kernels include special options for
machines acting primarily as routers:
<20> Multicasting: Allows the Linux machine to act as a router for IP
packets that have several destination addresses. It is needed on
the MBONE, a high bandwidth network on top of the Internet which
carries audio and video broadcasts.
<20> IP policy routing: Normally a router decides what to do with a
received packet based solely on the packet's final destination
address, but routing can also take into account the originating
address and the network device from which the packet reached it.
There are some related projects which include one aiming at building a
complete, running Linux router on a floppy disk: Linux router project
<http://www.linuxrouter.org>
8.2. Bridge
The Linux kernel has built-in support for acting as an Ethernet
bridge, which means that the different Ethernet segments it is
connected to will appear as one Ethernet to the participants. Several
bridges can work together to create even larger networks of Ethernets
using the IEEE802.1 spanning tree algorithm. As this is a standard,
Linux bridges will interoperate properly with other third party bridge
products. Additional packages allow filtering based on IP, IPX or MAC
addresses.
Related HOWTOs:
<20> Bridge+Firewall
<http://metalab.unc.edu/mdw/HOWTO/mini/Bridge+Firewall.html>
<20> Bridge <http://metalab.unc.edu/mdw/HOWTO/mini/Bridge.html>
8.3. IP Masquerade
IP Masquerade is a developing networking function in Linux. If a Linux
host is connected to the Internet with IP Masquerade enabled, then
computers connecting to it (either on the same LAN or connected with
modems) can reach the Internet as well, even though they have no
officially assigned IP addresses. This allows for reduction of costs,
since many people may be able to access the Internet using a single
modem connection as well as contributes to increased security (in some
way the machine is acting as a firewall, since unofficially assigned
addresses cannot be accessed outside of that network).
IP masquerade related pages and documents:
<20> http://ipmasq.home.ml.org/
<20> http://www.indyramp.com/masq/links.pfhtml
<20> http://metalab.unc.edu/mdw/HOWTO/IP-Masquerade-HOWTO.html
8.4. IP Accounting
This option of the Linux kernel keeps track of IP network traffic,
performs packet logging and produces some statistics. A series of
rules may be defined so when a packet matches a given pattern, some
action is performed: a counter is increased, it is accepted/rejected,
etc.
8.5. IP aliasing
This feature of the Linux kernel provides the possibility of setting
multiple network addresses on the same low-level network device driver
(e.g two IP addresses in one Ethernet card). It is typically used for
services that act differently based on the address they listen on
(e.g. "multihosting" or "virtual domains" or "virtual hosting
services".
Related HOWTO:
<20> IP Aliasing HOWTO <http://metalab.unc.edu/mdw/HOWTO/mini/IP-
Alias.html>
8.6. Traffic Shaping
The traffic shaper is a virtual network device that makes it possible
to limit the rate of outgoing data flow over another network device.
This is especially useful in scenarios such as ISPs, where it is
desirable to control and enforce policies regarding how much bandwidth
is used by each client. Another alternative (for web services only)
may be certain Apache modules which restrict the number of IP
connections by client or the bandwidth used.
<20> http://metalab.unc.edu/mdw/HOWTO/NET3-4-HOWTO-6.html#ss6.15
8.7. Firewall
A firewall is a device that protects a private network from the public
part (the internet as a whole). It is designed to control the flow of
packets based on the source, destination, port and packet type
information contained in each packet.
Different firewall toolkits exist for Linux as well as built-in
support in the kernel. Other firewalls are TIS and SOCKS. These
firewall toolkits are very complete and combined with other tools
allow blocking/redirection of all kinds of traffic and protocols.
Different policies can be implemented via configuration files or GUI
programs.
<20> TIS home page <http://www.tis.com>
<20> SOCKS <http://www.socks.nec.com/socksfaq.html>
<20> Firewall HOWTO <http://metalab.unc.edu/mdw/HOWTO/Firewall-
HOWTO.html>
8.8. Port forwarding
An increasing number of web sites are becoming interactive by having
cgi-bins or Java applets that access some database or other service.
Since this access may pose a security problem, the machine containing
the database should not be directly connected to the Internet.
Port Forwarding can provide an almost ideal solution to this access
problem. On the firewall, IP packets that come in to a specific port
number can be re-written and forwarded to the internal server
providing the actual service. The reply packets from the internal
server are re-written to make it appear that they came from the
firewall.
Port forwarding information may be found here
<http://www.ox.compsoc.net/~steve/portforwarding.html>
8.9. Load Balancing
Demand for load balancing usually arises in database/web access when
many clients make simultaneous requests to a server. It would be
desirable to have multiple identical servers and redirect requests to
the less loaded server. This can be achieved through Network Address
Translation techniques (NAT) of which IP masquerading is a subset.
Network administrators can replace a single server providing Web
services - or any other application - with a logical pool of servers
sharing a common IP address. Incoming connections are directed to a
particular server using one load-balancing algorithm. The virtual
server rewrites incoming and outgoing packets to give clients the
appearance that only one server exists.
Linux IP-NAT information may be found here <http://www.csn.tu-
chemnitz.de/HyperNews/get/linux-ip-nat.html>
8.10. EQL
EQL is integrated into the Linux kernel. If two serial connections
exist to some other computer (this usually requires two modems and two
telephone lines) and SLIP or PPP (protocols for sending Internet
traffic over telephone lines) are used on them, it is possible to make
them behave like one double speed connection using this driver.
Naturally, this has to be supported at the other end as well.
<20> http://metalab.unc.edu/mdw/HOWTO/NET3-4-HOWTO-6.html#ss6.2
8.11. Proxy Server
The term proxy means "to do something on behalf of someone else." In
networking terms, a proxy server computer can act on the behalf of
several clients. An HTTP proxy is a machine that receives requests for
web pages from another machine (Machine A). The proxy gets the page
requested and returns the result to Machine A. The proxy may have a
cache with the requested pages, so if another machine asks for the
same page the copy in the cache will be returned instead. This allows
efficient use of bandwidth resources and less response time. As a side
effect, as client machines are not directly connected to the outside
world this is a way of securing the internal network. A well-
configured proxy can be as effective as a good firewall.
Several proxy servers exist for Linux. One popular solution is the
Apache proxy module. A more complete and robust implementation of an
HTTP proxy is SQUID.
<20> Apache <http://www.apache.org>
<20> Squid <http://squid.nlanr.net/>
8.12. Diald on demand
The purpose of dial on demand is to make it transparently appear that
the users have a permanent connection to a remote site. Usually,
there is a daemon who monitors the traffic of packets and where an
interesting packet (interesting is defined usually by a set of
rules/priorities/permissions) arrives it establishes a connection with
the remote end. When the channel is idle for a certain period of time,
it drops the connection.
<20> Diald HOWTO <http://metalab.unc.edu/mdw/HOWTO/Diald-HOWTO.html>
8.13. Tunnelling, mobile IP and virtual private networks
The Linux kernel allows the tunnelling (encapsulation) of protocols.
It can do IPX tunnelling through IP, allowing the connection of two
IPX networks through an IP only link. It can also do IP-IP tunnelling,
which it is essential for mobile IP support, multicast support and
amateur radio. (see
http://metalab.unc.edu/mdw/HOWTO/NET3-4-HOWTO-6.html#ss6.8)
Mobile IP specifies enhancements that allow transparent routing of IP
datagrams to mobile nodes in the Internet. Each mobile node is always
identified by its home address, regardless of its current point of
attachment to the Internet. While situated away from its home, a
mobile node is also associated with a care-of address, which provides
information about its current point of attachment to the Internet.
The protocol provides for registering the care-of address with a home
agent. The home agent sends datagrams destined for the mobile node
through a tunnel to the care-of address. After arriving at the end of
the tunnel, each datagram is then delivered to the mobile node.
Point-to-Point Tunneling Protocol (PPTP) is a networking technology
that allows the use of the Internet as a secure virtual private
network (VPN). PPTP is integrated with the Remote Access Services
(RAS) server which is built into Windows NT Server. With PPTP, users
can dial into a local ISP, or connect directly to the Internet, and
access their network as if they were at their desks. PPTP is a closed
protocol and its security has recently being compromised. It is highly
recomendable to use other Linux based alternatives, since they rely on
open standards which have been carefully examined and tested.
<20> A client implementation of the PPTP for Linux is available here
<http://www.pdos.lcs.mit.edu/~cananian/Projects/PPTP/>
<20> More on Linux PPTP can be found here
<http://bmrc.berkeley.edu/people/chaffee/linux_pptp.html>
Mobile IP:
<20> http://www.hpl.hp.com/personal/Jean_Tourrilhes/MobileIP/mip.html
<20> http://metalab.unc.edu/mdw/HOWTO/NET3-4-HOWTO-6.html#ss6.12
Virtual Private Networks related documents:
<20> http://metalab.unc.edu/mdw/HOWTO/mini/VPN.html
<20> http://sites.inka.de/sites/bigred/devel/cipe.html
9. Network Management
9.1. Network management applications
There is an impressive number of tools focused on network management
and remote administration. Some interesting remote administration
projects are linuxconf and webmin:
<20> Webmin <http://www.webmin.com/webmin/>
<20> Linuxconf <http://www.solucorp.qc.ca/linuxconf/>
Other tools include network traffic analysis tools, network security
tools, monitoring tools, configuration tools, etc. An archive of many
of these tools may be found at Metalab
<http://www.metalab.unc.edu/pub/Linux/system/network/>
9.2. SNMP
The Simple Network Management Protocol is a protocol for Internet
network management services. It allows for remote monitoring and
configuration of routers, bridges, network cards, switches, etc...
There is a large amount of libraries, clients, daemons and SNMP based
monitoring programs available for Linux. A good page dealing with SNMP
and Linux software may be found at : http://linas.org/linux/NMS.html
10. Enterprise Linux Networking
In certain situations it is necessary for the networking
infrastructure to have proper mechanisms to guarantee network
availability nearly 100% of the time. Some related techniques are
described in the following sections. Most of the following material
can be found at the excellent Linas website:
http://linas.org/linux/index.html and in the Linux High-Availability
HOWTO <http://metalab.unc.edu/pub/Linux/ALPHA/linux-ha/High-
Availability-HOWTO.html>
10.1. High Availability
Redundancy is used to prevent the overall IT system from having single
points of failure. A server with only one network card or a single
SCSI disk has two single points of failure. The objective is to mask
unplanned outages from users in a manner that lets users continue to
work quickly. High availability software is a set of scripts and tools
that automatically monitor and detect failures, taking the appropriate
steps to restore normal operation and to notifying system
administrators.
10.2. RAID
RAID, short for Redundant Array of Inexpensive Disks, is a method
whereby information is spread across several disks, using techniques
such as disk striping (RAID Level 0) and disk mirroring (RAID level 1)
to achieve redundancy, lower latency and/or higher bandwidth for
reading and/or writing, and recoverability from hard-disk crashes.
Over six different types of RAID configurations have been defined.
There are three types of RAID solution options available to Linux
users: software RAID, outboard DASD boxes, and RAID disk controllers.
<20> Software RAID: Pure software RAID implements the various RAID
levels in the kernel disk (block device) code.
<20> Outboard DASD Solutions: DASD (Direct Access Storage Device) are
separate boxes that come with their own power supply, provide a
cabinet/chassis for holding the hard drives, and appear to Linux as
just another SCSI device. In many ways, these offer the most robust
RAID solution.
<20> RAID Disk Controllers: Disk Controllers are adapter cards that plug
into the ISA/EISA/PCI bus. Just like regular disk controller cards,
a cable attaches them to the disk drives. Unlike regular disk
controllers, the RAID controllers will implement RAID on the card
itself, performing all necessary operations to provide various RAID
levels.
Related HOWTOs:
<20> http://metalab.unc.edu/mdw/HOWTO/mini/DPT-Hardware-RAID.html
<20> http://metalab.unc.edu/mdw/HOWTO/Root-RAID-HOWTO.html
<20> http://metalab.unc.edu/mdw/HOWTO/Software-RAID-HOWTO.html
RAID at linas.org:
<20> http://linas.org/linux/raid.html
10.3. Redundant networking
IP Address Takeover (IPAT). When a network adapter card fails, its IP
address should be taken by a working network card in the same node or
in another node. MAC Address Takeover: when an IP takeover occurs, it
should be made sure that all the nodes in the network update their ARP
caches (the mapping between IP and MAC addresses).
See the High-Availability HOWTO for more details:
http://metalab.unc.edu/pub/Linux/ALPHA/linux-ha/High-Availability-
HOWTO.html
11. Sources of Information
If you have networking problems with Linux, please do not e-mail the
questions to me. I just simply do not have the time to answer them.
You have better chances to obtain help if you post a question in the
comp.os.linux.networking newsgroup (which you can access through
http://www.dejanews.com). Before posting there, make sure that you
have read the relevant documentation. Then search the news archive,
because chances are that somebody, sometime made the same question
(and somebody answered). When posting, remember to explain all the
steps you have followed and the error messages you got. Where to get
further information:
<20> Linux: http://www.linux.org
<20> Linux Documentation Project: http://metalab.unc.edu/mdw/linux.html
(check out the Linux Network Administrator Guide)
<20> Freshmeat: The latest releases of Linux Software.
http://www.freshmeat.net
<20> Linux links: http://www.linuxlinks.com/Networking/
12. Document history
<20> 0.32 Updated many links that have changed. Special thanks go here
to Kontiki <mailto:kontiki@lares.dtui.ne.jp> for his careful review
and detailed description of what needed to change. Many thanks also
to Anne <mailto:annabell@ixos.de> and Mathias
<hessler@isdn4linux.de> who pointed out other links that were no
longer valid.
<20> 0.31 (17 Sept 1999) Changed address for linux router project
(thanks to John Ellis) and added another PPTP link (thanks to
Benjamin Smith)
<20> 0.30 (6 April 1999) Included section on CODA (thanks to Brian
Ristuccia <mailto:brianr@osiris.978.org>
<20> 0.2-0.29 Bugfixes :-) (see acknowledgements, at the end of this
document)
<20> 0.1 (5 june 1998)
13. Acknowledgements and disclaimer
This document is based on the work of many other people who have made
it possible for Linux to be what it is now: one of the best network
operating systems. All credit is theirs. A lot of effort has been put
into this document to make it simple but accurate and complete but not
excessively long. Nevertheless, no liability will be assumed by the
author under any circumstance. Use the information contained here at
your own risk. Please feel free to e-mail me suggestions, corrections
or general comments about the document so I can improve it. Other
topics that will probably be included in futures revisions of this
document may include radius, web/ftp mirroring tools such as wget,
traffic analyzers, CORBA... and many others that may be suggested and
suitable. You can reach me at daniel@rawbyte.com.
Finally I would like to thank Finnbjorn av Teigum, Cesar Kant,
Mathieu Arnold and specially Hisakuni Nogami and Phil Garcia for their
careful reviews and comments on this HOWTO. Their help is greatly
appreciated.
You can find a version of this document at http://www.rawbyte.com/lno/
<http://www.rawbyte.com/lno>.
Daniel Lopez Ridruejo 8 July 2000
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