Original Authors: Terry Dawson (main author), VK2KTJ; Alessandro Rubini (maintainer)
Former Maintainer: Joshua Drake (Poet)
The Linux Operating System boasts kernel based networking support written almost entirely from scratch. The performance of the tcp/ip implementation in recent kernels makes it a worthy alternative to even the best of its peers. This document aims to describe how to install and configure the Linux networking software and associated tools.
AF_APPLETALK)
AF_AX25)
AF_IPX)
AF_NETROM)
AF_ROSE)
This is the first release since LinuxPorts has become the author of this document. First let me say that we hope that over the next few months you will find this document to be of use and that we are able to provide accurate and timely information in regards to networking issues with Linux.
This document like the other howto's that we manage is going to become very different, this document will shortly become the Networking-HOWTO not just the Net-3(4) Howto. We will cover such items as PPP, VPN, and others...
The original NET-FAQ was written by Matt Welsh and Terry Dawson to answer frequently asked questions about networking for Linux at a time before the Linux Documentation Project had formally started. It covered the very early development versions of the Linux Networking Kernel. The NET-2-HOWTO superceded the NET-FAQ and was one of the original LDP HOWTO documents, it covered what was called version 2 and later version 3 of the Linux kernel Networking software. This document in turn supercedes it and relates only to version 4 of the Linux Networking Kernel or more specifically kernel releases 2.x and 2.2.x.
Previous versions of this document became quite large because of the enormous amount of material that fell within its scope. To help reduce this problem a number of HOWTO's dealing with specific networking topics have been produced. This document will provide pointers to them where relevant and cover those areas not yet covered by other documents.
We are always interested in feedback. Please contact us at: feedback@en.tldp.org.
Again, if you find anything erroneous or anything you would like to see added, please contact us.
This document is organized top-down. The first sections include informative material and can be skipped if you are not interested; what follows is a generic discussion of networking issues, and you must ensure you understand this before proceeding to more specific parts. The rest, ``technology specific'' information is grouped in three main sections: Ethernet and IP-related information, technologies pertaining to widespread PC hardware and seldom-used technologies.
The suggested path through the document is thus the following:
These sections apply to every, or nearly every, technology described later and so are very important for you to understand. On the other hand, I expect many of the readers to be already confident with this material.
You should know how your network is, or will be, designed and exactly what hardware and technology types you will be implementing.
This section describes basic Ethernet configuration and the various features that Linux offers for IP networks, like firewalling, advanced routing and so on.
The section describes PLIP, PPP, SLIP and ISDN, the widespread technologies used on personal workstations.
If your needs differ from IP and/or common hardware, the final section covers details specific to non-IP protocols and peculiar communication hardware.
You should actually try to configure your network and take careful note of any problems you have.
If you experience problems that this document does not help you to resolve then read the section related to where to get help or where to report bugs.
Networking is fun, enjoy it.
No special convention is used here, but you must be warned about
the way commands are shown. Following the classic Unix documentation,
any command you should type to your shell is prefixed by a
prompt. This howto shows "user%" as the prompt for commands
that do not require superuser privileges, and "root#" as the
prompt for commands that need to run as root. I chose to use
"root#" instead of a plain "#" to prevent confusion
with snapshots from shell scripts, where the hash mark is used to
define comment lines.
When ``Kernel Compile Options'' are shown, they are represented in the format used by menuconfig. They should be understandable even if you (like me) are not used to menuconfig. If you are in doubt about the options' nesting, running the program once can't but help.
Note that any link to other HOWTO's is local to help you browsing
your local copy of the LDP documents, in case you are using the html
version of this document. If you don't have a complete set of
documents, every HOWTO can be retrieved from metalab.unc.edu
(directory /pub/Linux/HOWTO) and its countless mirrors.
Developing a brand new kernel implementation of the tcp/ip protocol stack that would perform as well as existing implementations was not an easy task. The decision not to port one of the existing implementations was made at a time when there was some uncertainty as to whether the existing implementations may become encumbered by restrictive copyrights because of the court case put by U.S.L. and when there was a lot of fresh enthusiasm for doing it differently and perhaps even better than had already been done.
The original volunteer to lead development of the kernel network code was
Ross Biro <biro@yggdrasil.com>. Ross produced a simple and
incomplete but mostly usable implementation set of routines that were
complemented by an ethernet driver for the WD-8003 network interface card.
This was enough to get many people testing and experimenting with the software
and some people even managed to connect machines in this configuration to live
internet connections. The pressure within the Linux community driving
development for networking support was building and eventually the cost of a
combination of some unfair pressure applied to Ross and his own personal
commitments outweighed the benefit he was deriving and he stepped down as
lead developer. Ross's efforts in getting the project started and accepting
the responsibility for actually producing something useful in such
controversial circumstances were what catalyzed all future work and were
therefore an essential component of the success of the current product.
Orest Zborowski <obz@Kodak.COM> produced the original BSD socket
programming interface for the Linux kernel. This was a big step forward
as it allowed many of the existing network applications to be ported to
linux without serious modification.
Somewhere about this time Laurence Culhane <loz@holmes.demon.co.uk>
developed the first drivers for Linux to support the SLIP protocol. These
enabled many people who did not have access to Ethernet networking to
experiment with the new networking software. Again, some people took this
driver and pressed it into service to connect them to the Internet. This
gave many more people a taste of the possibilities that could be realized
if Linux had full networking support and grew the number of users actively
using and experimenting with the networking software that existed.
One of the people that had also been actively working on the task of building
networking support was Fred van Kempen <waltje@uwalt.nl.mugnet.org>.
After a period of some uncertainty following Ross's resignation from the lead
developer position Fred offered his time and effort and accepted the role
essentially unopposed. Fred had some ambitious plans for the direction that
he wanted to take the Linux networking software and he set about progressing
in those directions. Fred produced a series of networking code called the
`NET-2' kernel code (the `NET' code being Ross's) which many people were
able to use pretty much usefully. Fred formally put a number of innovations
on the development agenda, such as the dynamic device interface, Amateur Radio
AX.25 protocol support and a more modularly designed networking implementation.
Fred's NET-2 code was used by a fairly large number of enthusiasts, the number
increasing all the time as word spread that the software was working.
The networking software at this time was still a large number of patches to
the standard release of kernel code and was not included in the normal release.
The NET-FAQ and subsequent NET-2-HOWTO's described the then fairly complex
procedure to get it all working. Fred's focus was on developing innovations to
the standard network implementations and this was taking time. The community
of users was growing impatient for something that worked reliably and satisfied
the 80% of users and, as with Ross, the pressure on Fred as lead developer rose.
Alan Cox <iialan@www.uk.linux.org> proposed a solution to the
problem designed to resolve the situation. He proposed that he would take
Fred's NET-2 code and debug it, making it reliable and stable so that it
would satisfy the impatient user base while relieving that pressure from
Fred allowing him to continue his work. Alan set about doing this, with some
good success and his first version of Linux networking code was called
`Net-2D(ebugged)'. The code worked reliably in many typical configurations and
the user base was happy. Alan clearly had ideas and skills of his own to
contribute to the project and many discussions relating to the direction the
NET-2 code was heading ensued. There developed two distinct schools within the
Linux networking community, one that had the philosophy of `make it work
first, then make it better' and the other of `make it better first'. Linus
ultimately arbitrated and offered his support to Alan's development efforts
and included Alan's code in the standard kernel source distribution.
This placed Fred in a difficult position. Any continued development would
lack the large user base actively using and testing the code and this would
mean progress would be slow and difficult. Fred continued to work for a short
time and eventually stood down and Alan came to be the new leader of the Linux
networking kernel development effort.
Donald Becker <becker@cesdis.gsfc.nasa.gov> soon revealed his
talents in the low level aspects of networking and produced a huge range of
ethernet drivers, nearly all of those included in the current kernels were
developed by Donald. There have been other people that have made significant
contributions, but Donald's work is prolific and so warrants special mention.
Alan continued refining the NET-2-Debugged code for some time while working on
progressing some of the matters that remained unaddressed on the `TODO' list.
By the time the Linux 1.3.* kernel source had grown its teeth the kernel
networking code had migrated to the NET-3 release on which current versions
are based. Alan worked on many different aspects of the networking code and
with the assistance of a range of other talented people from the Linux
networking community grew the code in all sorts of directions. Alan produced
dynamic network devices and the first standard AX.25 and IPX implementations.
Alan has continued tinkering with the code, slowly restructuring and enhancing
it to the state it is in today.
PPP support was added by Michael Callahan <callahan@maths.ox.ac.uk>
and Al Longyear <longyear@netcom.com> this too was critical to
increasing the number of people actively using linux for networking.
Jonathon Naylor <jsn@cs.nott.ac.uk> has contributed by significantly
enhancing Alan's AX.25 code, adding NetRom and Rose protocol support.
The AX.25/NetRom/Rose support itself is quite significant, because no other
operating system can boast standard native support for these protocols beside
Linux.
There have of course been hundreds of other people who have made significant contribution to the development of the Linux networking software. Some of these you will encounter later in the technology specific sections, other people have contributed modules, drivers, bug-fixes, suggestions, test reports and moral support. In all cases each can claim to have played a part and offered what they could. The Linux kernel networking code is an excellent example of the results that can be obtained from the Linux style of anarchic development, if it hasn't yet surprised you, it is bound to soon enough, the development hasn't stopped.
There are a number of places where you can find good information about Linux networking.
There are a wealth of Consultants available. A listing can be found at LinuxPorts Consultants Database
Alan Cox, the current maintainer of the Linux kernel networking code maintains a world wide web page that contains highlights of current and new developments in linux Networking at: www.uk.linux.org.
Another good place is a book written by Olaf Kirch entitled the
Network Administrators Guide. It is a work of the
Linux Documentation Project
and you can read it interactively at
Network Administrators Guide HTML version
or you can obtain it in various formats by ftp from the
metalab.unc.edu LDP ftp archive. Olaf's book is quite
comprehensive and provides a good high level overview of network configuration
under linux.
There is a newsgroup in the Linux news hierarchy dedicated to networking and related matters, it is: comp.os.linux.networking
There is a mailing list to which you can subscribe where you may ask questions relating to Linux networking. To subscribe you should send a mail message:
To: majordomo@vger.rutgers.edu
Subject: anything at all
Message:
subscribe linux-net
On the various IRC networks there are often #linux channels on
which people will be able to answer questions on linux networking.
Please remember when reporting any problem to include as much relevant detail about the problem as you can. Specifically you should specify the versions of software that you are using, especially the kernel version, the version of tools such as pppd or dip and the exact nature of the problem you are experiencing. This means taking note of the exact syntax of any error messages you receive and of any commands that you are issuing.
If you are after some basic tutorial information on tcp/ip networking generally, then I recommend you take a look at the following documents:
this document comes as both a text version and a postscript version.
this document comes as both a text version and a postscript version.
If you are after some more detailed information on tcp/ip networking then I highly recommend:
Internetworking with TCP/IP, Volume 1: principles, protocols and architecture, by Douglas E. Comer, ISBN 0-13-227836-7, Prentice Hall publications, Third Edition, 1995.If you are wanting to learn about how to write network applications in a Unix compatible environment then I also highly recommend:
Unix Network Programming, by W. Richard Stevens, ISBN 0-13-949876-1, Prentice Hall publications, 1990.A second edition of this book is appearing on the bookshelves; the new book is made up of three volumes: check Prenice-Hall's web site to probe further.
You might also try the comp.protocols.tcp-ip newsgroup.
An important source of specific technical information relating to the Internet and the tcp/ip suite of protocols are RFC's. RFC is an acronym for `Request For Comment' and is the standard means of submitting and documenting Internet protocol standards. There are many RFC repositories. Many of these sites are ftp sites and other provide World Wide Web access with an associated search engine that allows you to search the RFC database for particular keywords.
One possible source for RFC's is at Nexor RFC database.
The following subsections you will pretty much need to know and understand before you actually try to configure your network. They are fundamental principles that apply regardless of the exact nature of the network you wish to deploy.
Before you start building or configuring your network you will need some things. The most important of these are:
Please note:
The majority of current distributions come with networking enabled, therefore it may not be required to recompile the kernel. If you are running well known hardware you should be just fine. For example: 3COM NIC, NE2000 NIC, or a Intel NIC. However if you find yourself in the position that you do need to update the kernel, the following information is provided.
Because the kernel you are running now might not yet have support for the network types or cards that you wish to use you will probably need the kernel source so that you can recompile the kernel with the appropriate options.
For users of the major distributions such as Redhat, Caldera, Debian, or Suse this no longer holds true. As long as you stay within the mainstream of hardware there should be no need to recompile your kernel unless there is a very specific feature that you need.
You can always obtain the latest kernel source from ftp.cdrom.com. This is not the official site but they have LOTS of bandwidth and ALOT of users allowed. The official site is kernel.org but please use the above if you can. Please remember that ftp.kernel.org is seriously overloaded. Use a mirror.
Normally the kernel source will be untarred into the
/usr/src/linux directory. For information on how to apply
patches and build the kernel you should read the
Kernel-HOWTO. For information on how
to configure kernel modules you should read the ``Modules
mini-HOWTO''. Also, the README file found in the kernel
sources and the Documentation directory are very informative
for the brave reader.
Unless specifically stated otherwise, I recommend you stick with the standard kernel release (the one with the even number as the second digit in the version number). Development release kernels (the ones with the odd second digit) may have structural or other changes that may cause problems working with the other software on your system. If you are uncertain that you could resolve those sorts of problems in addition to the potential for there being other software errors, then don't use them.
On the other hand, some of the features described here have been introduced during the development of 2.1 kernels, so you must take your choice: you can stick to 2.0 while wait for 2.2 and an updated distribution with every new tool, or you can get 2.1 and look around for the various support programs needed to exploit the new features. As I write this paragraph, in August 1998, 2.1.115 is current and 2.2 is expected to appear pretty soon.
The network tools are the programs that you use to configure linux network devices. These tools allow you to assign addresses to devices and configure routes for example.
Most modern linux distributions are supplied with the network tools, so if you have installed from a distribution and haven't yet installed the network tools then you should do so.
If you haven't installed from a distribution then you will need to source and compile the tools yourself. This isn't difficult.
The network tools are now maintained by Bernd Eckenfels and are available at: ftp.inka.de and are mirrored at: ftp.uk.linux.org.
You can also get the latest RedHat packages from net-tools-1.51-3.i386.rpm
Be sure to choose the version that is most appropriate for the kernel you wish to use and follow the instructions in the package to install.
To install and configure the version current at the time of the writing you need do the following:
user% tar xvfz net-tools-1.33.tar.gz
user% cd net-tools-1.33
user% make config
user% make
root# make install
Or to use the Redhat packahges:
root# rpm -U net-tools-1.51-3.i386.rpm
Additionally, if you intend configuring a firewall or using the IP masquerade feature you will require the ipfwadm command. The latest version of it may be obtained from: ftp.xos.nl. Again there are a number of versions available. Be sure to pick the version that most closely matches your kernel. Note that the firewalling features of Linux changed during 2.1 development and has been superceded by ipchains in v2.2 of the kernel. ipfwadm only applies to version 2.0 of the kernel. The following are known to be distributions with version 2.0 or below of the kernel.
Redhat 5.2 or below
Caldera pre version 2.2
Slackware pre version 4.x
Debian pre version 2.x
To install and configure the version current at the time of this writing you need to read the IPChains howto located at The Linux Documentation Project
Note that if you run version 2.2 (or late 2.1) of the kernel, ipfwadm is not the right tool to configure firewalling. This version of the NET-3-HOWTO currently doesn't deal with the new firewalling setup. If you need more detailed information on ipchains please refer to the above.
The network application programs are programs such as
telnet and ftp and their respective server
programs. David Holland has been managing a distribution of the most
common of these, which is now maintained by
netbug@ftp.uk.linux.org. You may obtain the distribution from:
ftp.uk.linux.org.
Internet Protocol Addresses are composed of four bytes. The convention is to write addresses in what is called `dotted decimal notation'. In this form each byte is converted to a decimal number (0-255) dropping any leading zero's unless the number is zero and written with each byte separated by a `.' character. By convention each interface of a host or router has an IP address. It is legal for the same IP address to be used on each interface of a single machine in some circumstances but usually each interface will have its own address.
Internet Protocol Networks are contiguous sequences of IP addresses. All addresses within a network have a number of digits within the address in common. The portion of the address that is common amongst all addresses within the network is called the `network portion' of the address. The remaining digits are called the `host portion'. The number of bits that are shared by all addresses within a network is called the netmask and it is role of the netmask to determine which addresses belong to the network it is applied to and which don't. For example, consider the following:
----------------- ---------------
Host Address 192.168.110.23
Network Mask 255.255.255.0
Network Portion 192.168.110.
Host portion .23
----------------- ---------------
Network Address 192.168.110.0
Broadcast Address 192.168.110.255
----------------- ---------------
Any address that is 'bitwise anded' with its netmask will reveal the address of the network it belongs to. The network address is therefore always the lowest numbered address within the range of addresses on the network and always has the host portion of the address coded all zeroes.
The broadcast address is a special address that every host on the network
listens to in addition to its own unique address. This address is the one
that datagrams are sent to if every host on the network is meant to receive
it. Certain types of data like routing information and warning messages
are transmitted to the broadcast address so that every host on the network
can receive it simultaneously. There are two commonly used standards for
what the broadcast address should be. The most widely accepted one is to
use the highest possible address on the network as the broadcast address.
In the example above this would be 192.168.110.255. For some reason
other sites have adopted the convention of using the network address as the
broadcast address. In practice it doesn't matter very much which you use
but you must make sure that every host on the network is configured with the
same broadcast address.
For administrative reasons some time early in the development of the IP protocol some arbitrary groups of addresses were formed into networks and these networks were grouped into what are called classes. These classes provide a number of standard size networks that could be allocated. The ranges allocated are:
----------------------------------------------------------
| Network | Netmask | Network Addresses |
| Class | | |
----------------------------------------------------------
| A | 255.0.0.0 | 0.0.0.0 - 127.255.255.255 |
| B | 255.255.0.0 | 128.0.0.0 - 191.255.255.255 |
| C | 255.255.255.0 | 192.0.0.0 - 223.255.255.255 |
|Multicast| 240.0.0.0 | 224.0.0.0 - 239.255.255.255 |
----------------------------------------------------------
What addresses you should use depends on exactly what it is that you are doing. You may have to use a combination of the following activities to get all the addresses you need:
If you wish to install a linux machine onto an existing IP network then you should contact whoever administers the network and ask them for the following information:
If you are building a private network and you never intend that network to be connected to the Internet then you can choose whatever addresses you like. However, for safety and consistency reasons there have been some IP network addresses that have been reserved specifically for this purpose. These are specified in RFC1597 and are as follows:
-----------------------------------------------------------
| RESERVED PRIVATE NETWORK ALLOCATIONS |
-----------------------------------------------------------
| Network | Netmask | Network Addresses |
| Class | | |
-----------------------------------------------------------
| A | 255.0.0.0 | 10.0.0.0 - 10.255.255.255 |
| B | 255.255.0.0 | 172.16.0.0 - 172.31.255.255 |
| C | 255.255.255.0 | 192.168.0.0 - 192.168.255.255 |
-----------------------------------------------------------
There are a few different approaches to Linux system boot
procedures. After the kernel boots, it always executes a program
called `init'. The init program then reads its configuration
file called /etc/inittab and commences the boot
process. There are a few different flavours of init around,
although everyone is now converging to the System V (Five) flavor,
developed by Miguel van Smoorenburg.
Despite the fact that the init program is always the same, the setup of system boot is organized in a different way by each distribution.
Usually the /etc/inittab file contains an entry looking something
like:
si::sysinit:/etc/init.d/boot
This line specifies the name of the shell script file that actually manages
the boot sequence. This file is somewhat equivalent to the AUTOEXEC.BAT
file in MS-DOS.
There are usually other scripts that are called by the boot script and often the network is configured within one of many of these.
The following table may be used as a guide for your system:
---------------------------------------------------------------------------
Distrib. | Interface Config/Routing | Server Initialization
---------------------------------------------------------------------------
Debian | /etc/init.d/network | /etc/rc2.d/*
---------------------------------------------------------------------------
Slackware| /etc/rc.d/rc.inet1 | /etc/rc.d/rc.inet2
---------------------------------------------------------------------------
RedHat | /etc/rc.d/init.d/network | /etc/rc.d/rc3.d/*
---------------------------------------------------------------------------
Note that Debian and Red Hat use a whole directory to host scripts
that fire up system services (and usually information does not lie
within these files, for example Red Hat systems store all of system
configuration in files under /etc/sysconfig, whence it is
retrieved by boot scripts). If you want to grasp the details of the
boot process, my suggestion is to check /etc/inittab and the
documentation that accompanies init. Linux Journal is also
going to publish an article about system initialization, and this
document will point to it as soon as it is available on the web.
Most modern distributions include a program that will allow you to configure many of the common sorts of network interfaces. If you have one of these then you should see if it will do what you want before attempting a manual configuration.
-----------------------------------------
Distrib | Network configuration program
-----------------------------------------
RedHat | /usr/bin/netcfg
Slackware | /sbin/netconfig
-----------------------------------------
In many Unix operating systems the network devices have appearances in the /dev directory. This is not so in Linux. In Linux the network devices are created dynamically in software and do not require device files to be present.
In the majority of cases the network device is automatically created by the
device driver while it is initializing and has located your hardware. For
example, the ethernet device driver creates eth[0..n] interfaces
sequentially as it locates your ethernet hardware. The first ethernet card
found becomes eth0, the second eth1 etc.
In some cases though, notably slip and ppp, the network devices are created through the action of some user program. The same sequential device numbering applies, but the devices are not created automatically at boot time. The reason for this is that unlike ethernet devices, the number of active slip or ppp devices may vary during the uptime of the machine. These cases will be covered in more detail in later sections.
When you have all of the programs you need and your address and network information you can configure your network interfaces. When we talk about configuring a network interface we are talking about the process of assigning appropriate addresses to a network device and to setting appropriate values for other configurable parameters of a network device. The program most commonly used to do this is the ifconfig (interface configure) command.
Typically you would use a command similar to the following:
root# ifconfig eth0 192.168.0.1 netmask 255.255.255.0 up
In this case I'm configuring an ethernet interface `eth0' with the
IP address `192.168.0.1' and a network mask of `255.255.255.0'.
The `up' that trails the command tells the interface that it should
become active, but can usually be omitted, as it is the default. To
shutdown an interface, you can just call ``ifconfig eth0 down''.
The kernel assumes certain defaults when configuring interfaces. For example,
you may specify the network address and broadcast address for an interface,
but if you don't, as in my example above, then the kernel will make reasonable
guesses as to what they should be based on the netmask you supply and if you
don't supply a netmask then on the network class of the IP address configured.
In my example the kernel would assume that it is a class-C network
being configured on the interface and configure a network address of
`192.168.0.0' and a broadcast address of `192.168.0.255' for the
interface.
There are many other options to the ifconfig command. The most important of these are:
this option activates an interface (and is the default).
this option deactivates an interface.
this option enables or disables use of the address resolution protocol on this interface
this option enables or disables the reception of all hardware multicast packets. Hardware multicast enables groups of hosts to receive packets addressed to special destinations. This may be of importance if you are using applications like desktop videoconferencing but is normally not used.
this parameter allows you to set the MTU of this device.
this parameter allows you to set the network mask of the network this device belongs to.
this parameter only works on certain types of hardware and allows you to set the IRQ of the hardware of this device.
this parameter allows you to enable and set the accepting of datagrams destined to the broadcast address, or to disable reception of these datagrams.
this parameter allows you to set the address of the machine at the remote end of a point to point link such as for slip or ppp.
this parameter allows you to set the hardware address of certain types of network devices. This is not often useful for ethernet, but is useful for other network types such as AX.25.
You may use the ifconfig command on any network interface. Some user programs such as pppd and dip automatically configure the network devices as they create them, so manual use of ifconfig is unnecessary.
The `Name Resolver' is a part of the linux standard library. Its prime
function is to provide a service to convert human-friendly hostnames like
`ftp.funet.fi' into machine friendly IP addresses such as
128.214.248.6.
You will probably be familiar with the appearance of Internet host names, but may not understand how they are constructed, or deconstructed. Internet domain names are hierarchical in nature, that is, they have a tree-like structure. A `domain' is a family, or group of names. A `domain' may be broken down into `subdomain'. A `toplevel domain' is a domain that is not a subdomain. The Top Level Domains are specified in RFC-920. Some examples of the most common top level domains are:
Commercial Organizations
Educational Organizations
Government Organizations
Military Organizations
Other organizations
Internet-Related Organizations
these are two letters codes that represent a particular country.
For historical reasons most domains belonging to one of the
non-country based top level domains were used by organizations within
the United States, although the United States also has its own country
code `.us'. This is not true any more for .com and .org
domains, which are commonly used by non-us companies.
Each of these top level domains has subdomains. The top level
domains based on country name are often next broken down into
subdomains based on the com, edu, gov, mil and
org domains. So for example you end up with: com.au and
gov.au for commercial and government organizations in Australia;
note that this is not a general rule, as actual policies depend on the
naming authority for each domain.
The next level of division usually represents the name of the organization. Further subdomains vary in nature, often the next level of subdomain is based on the departmental structure of the organization but it may be based on any criterion considered reasonable and meaningful by the network administrators for the organization.
The very left-most portion of the name is always the unique name assigned to the host machine and is called the `hostname', the portion of the name to the right of the hostname is called the `domainname' and the complete name is called the `Fully Qualified Domain Name'.
To use Terry's host as an example, the fully qualified domain name
is `perf.no.itg.telstra.com.au'. This means that the host name is
`perf' and the domain name is `no.itg.telstra.com.au'. The
domain name is based on a top level domain based on his country,
Australia and as his email address belongs to a commercial
organization, `.com' is there as the next level domain. The name
of the company is (was) `telstra' and their internal naming
structure is based on organizational structure, in this case the
machine belongs to the Information Technology Group, Network
Operations section.
Usually, the names are fairly shorter; for example, my ISP is
called ``systemy.it'' and my non-profit organization is called
``linux.it'', without any com and org subdomain, so
that my own host is just called ``morgana.systemy.it'' and
rubini@linux.it is a valid email address. Note that the owner
of a domain has the rights to register hostnames as well as subdomains;
for example, the LUG I belong to uses the domain pluto.linux.it,
because the owners of linux.it agreed to open a subdomain for the LUG.
You will need to know what domain your hosts name will belong to. The name resolver software provides this name translation service by making requests to a `Domain Name Server', so you will need to know the IP address of a local nameserver that you can use.
There are three files you need to edit, I'll cover each of these in turn.
The /etc/resolv.conf is the main configuration file for
the name resolver code. Its format is quite simple. It is a text file
with one keyword per line. There are three keywords typically used,
they are:
this keyword specifies the local domain name.
this keyword specifies a list of alternate domain names to search for a hostname
this keyword, which may be used many times, specifies an IP address of a domain name server to query when resolving names
An example /etc/resolv.conf might look something like:
domain maths.wu.edu.au
search maths.wu.edu.au wu.edu.au
nameserver 192.168.10.1
nameserver 192.168.12.1
This example specifies that the default domain name to append to unqualified
names (ie hostnames supplied without a domain) is maths.wu.edu.au and
that if the host is not found in that domain to also try the wu.edu.au
domain directly. Two nameservers entry are supplied, each of which may be
called upon by the name resolver code to resolve the name.
The /etc/host.conf file is where you configure some items that
govern the behaviour of the name resolver code. The format of this file
is described in detail in the `resolv+' man page. In nearly all
circumstances the following example will work for you:
order hosts,bind
multi on
This configuration tells the name resolver to check the /etc/hosts
file before attempting to query a nameserver and to return all valid addresses
for a host found in the /etc/hosts file instead of just the first.
The /etc/hosts file is where you put the name and IP
address of local hosts. If you place a host in this file then you do
not need to query the domain name server to get its IP Address. The
disadvantage of doing this is that you must keep this file up to date
yourself if the IP address for that host changes. In a well managed
system the only hostnames that usually appear in this file are an
entry for the loopback interface and the local hosts name.
# /etc/hosts
127.0.0.1 localhost loopback
192.168.0.1 this.host.name
You may specify more than one host name per line as demonstrated by the first entry, which is a standard entry for the loopback interface.
If you want to run a local nameserver, you can do it easily. Please refer to the DNS-HOWTO and to any documents included in your version of BIND (Berkeley Internet Name Domain).
The `loopback' interface is a special type of interface that allows you
to make connections to yourself. There are various reasons why you might want
to do this, for example, you may wish to test some network software without
interfering with anybody else on your network. By convention the IP address
`127.0.0.1' has been assigned specifically for loopback. So no matter
what machine you go to, if you open a telnet connection to 127.0.0.1
you will always reach the local host.
Configuring the loopback interface is simple and you should ensure you do (but note that this task is usually performed by the standard initialization scripts).
root# ifconfig lo 127.0.0.1
root# route add -host 127.0.0.1 lo
We'll talk more about the route command in the next section.
Routing is a big topic. It is easily possible to write large volumes of text about it. Most of you will have fairly simple routing requirements, some of you will not. I will cover some basic fundamentals of routing only. If you are interested in more detailed information then I suggest you refer to the references provided at the start of the document.
Let's start with a definition. What is IP routing ? Here is one that I'm using:
IP Routing is the process by which a host with multiple network connections decides where to deliver IP datagrams it has received.
It might be useful to illustrate this with an example. Imagine a typical office router, it might have a PPP link off the Internet, a number of ethernet segments feeding the workstations and another PPP link off to another office. When the router receives a datagram on any of its network connections, routing is the mechanism that it uses to determine which interface it should send the datagram to next. Simple hosts also need to route, all Internet hosts have two network devices, one is the loopback interface described above and the other is the one it uses to talk to the rest of the network, perhaps an ethernet, perhaps a PPP or SLIP serial interface.
Ok, so how does routing work ? Each host keeps a special list of routing rules, called a routing table. This table contains rows which typically contain at least three fields, the first is a destination address, the second is the name of the interface to which the datagram is to be routed and the third is optionally the IP address of another machine which will carry the datagram on its next step through the network. In linux you can see this table by using the following command:
user% cat /proc/net/route
or by using either of the following commands:
user% /sbin/route -n
user% netstat -r
The routing process is fairly simple: an incoming datagram is received, the destination address (who it is for) is examined and compared with each entry in the table. The entry that best matches that address is selected and the datagram is forwarded to the specified interface. If the gateway field is filled then the datagram is forwarded to that host via the specified interface, otherwise the destination address is assumed to be on the network supported by the interface.
To manipulate this table a special command is used. This command takes command line arguments and converts them into kernel system calls that request the kernel to add, delete or modify entries in the routing table. The command is called `route'.
A simple example. Imagine you have an ethernet network. You've been told
it is a class-C network with an address of 192.168.1.0. You've been
supplied with an IP address of 192.168.1.10 for your use and have
been told that 192.168.1.1 is a router connected to the Internet.
The first step is to configure the interface as described earlier. You would use a command like:
root# ifconfig eth0 192.168.1.10 netmask 255.255.255.0 up
You now need to add an entry into the routing table to tell the kernel that
datagrams for all hosts with addresses that match 192.168.1.* should
be sent to the ethernet device. You would use a command similar to:
root# route add -net 192.168.1.0 netmask 255.255.255.0 eth0
Note the use of the `-net' argument to tell the route program that this
entry is a network route. Your other choice here is a `-host' route which
is a route that is specific to one IP address.
This route will enable you to establish IP connections with all of the hosts on your ethernet segment. But what about all of the IP hosts that aren't on your ethernet segment ?
It would be a very difficult job to have to add routes to every possible
destination network, so there is a special trick that is used to simplify this
task. The trick is called the `default' route. The default route
matches every possible destination, but poorly, so that if any other entry
exists that matches the required address it will be used instead of the
default route. The idea of the default route is simply to enable
you to say "and everything else should go here". In the example I've contrived
you would use an entry like:
root# route add default gw 192.168.1.1 eth0
The `gw' argument tells the route command that the next argument is
the IP address, or name, of a gateway or router machine which all datagrams
matching this entry should be directed to for further routing.
So, your complete configuration would look like:
root# ifconfig eth0 192.168.1.10 netmask 255.255.255.0 up
root# route add -net 192.168.1.0 netmask 255.255.255.0 eth0
root# route add default gw 192.168.1.1 eth0
If you take a close look at your network `rc' files you will find
that at least one of them looks very similar to this. This is a very common
configuration.
Let's now look at a slightly more complicated routing configuration. Let's imagine we are configuring the router we looked at earlier, the one supporting the PPP link to the Internet and the lan segments feeding the workstations in the office. Lets imagine the router has three ethernet segments and one PPP link. Our routing configuration would look something like:
root# route add -net 192.168.1.0 netmask 255.255.255.0 eth0
root# route add -net 192.168.2.0 netmask 255.255.255.0 eth1
root# route add -net 192.168.3.0 netmask 255.255.255.0 eth2
root# route add default ppp0
Each of the workstations would use the simpler form presented
above, only the router needs to specify each of the network routes
separately because for the workstations the default route
mechanism will capture all of them letting the router worry about
splitting them up appropriately. You may be wondering why the default
route presented doesn't specify a `gw'. The reason for this is
simple, serial link protocols such as PPP and slip only ever have two
hosts on their network, one at each end. To specify the host at the
other end of the link as the gateway is pointless and redundant as
there is no other choice, so you do not need to specify a gateway for
these types of network connections. Other network types such as
ethernet, arcnet or token ring do require the gateway to be specified
as these networks support large numbers of hosts on them.
The routing configuration described above is best suited to simple network arrangements where there are only ever single possible paths to destinations. When you have a more complex network arrangement things get a little more complicated. Fortunately for most of you this won't be an issue.
The big problem with `manual routing' or `static routing' as described, is that if a machine or link fails in your network then the only way you can direct your datagrams another way, if another way exists, is by manually intervening and executing the appropriate commands. Naturally this is clumsy, slow, impractical and hazard prone. Various techniques have been developed to automatically adjust routing tables in the event of network failures where there are alternate routes, all of these techniques are loosely grouped by the term `dynamic routing protocols'.
You may have heard of some of the more common dynamic routing protocols. The most common are probably RIP (Routing Information Protocol) and OSPF (Open Shortest Path First Protocol). The Routing Information Protocol is very common on small networks such as small-medium sized corporate networks or building networks. OSPF is more modern and more capable at handling large network configurations and better suited to environments where there is a large number of possible paths through the network. Common implementations of these protocols are: `routed' - RIP and `gated' - RIP, OSPF and others. The `routed' program is normally supplied with your Linux distribution or is included in the `NetKit' package detailed above.
An example of where and how you might use a dynamic routing protocol might look something like the following:
192.168.1.0 / 192.168.2.0 /
255.255.255.0 255.255.255.0
- -
| |
| /-----\ /-----\ |
| | |ppp0 // ppp0| | |
eth0 |---| A |------//---------| B |---| eth0
| | | // | | |
| \-----/ \-----/ |
| \ ppp1 ppp1 / |
- \ / -
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
ppp0\ /ppp1
/-----\
| |
| C |
| |
\-----/
|eth0
|
|---------|
192.168.3.0 /
255.255.255.0
We have three routers A, B and C. Each supports one ethernet segment with a Class C IP network (netmask 255.255.255.0). Each router also has a PPP link to each of the other routers. The network forms a triangle.
It should be clear that the routing table at router A could look like:
root# route add -net 192.168.1.0 netmask 255.255.255.0 eth0
root# route add -net 192.168.2.0 netmask 255.255.255.0 ppp0
root# route add -net 192.168.3.0 netmask 255.255.255.0 ppp1
This would work just fine until the link between router A and B should fail. If that link failed then with the routing entry shown above hosts on the ethernet segment of A could not reach hosts on the ethernet segment on B because their datagram would be directed to router A's ppp0 link which is broken. They could still continue to talk to hosts on the ethernet segment of C and hosts on the C's ethernet segment could still talk to hosts on B's ethernet segment because the link between B and C is still intact.
But wait, if A can talk to C and C can still talk to B, why shouldn't A route its datagrams for B via C and let C send them to B ? This is exactly the sort of problem that dynamic routing protocols like RIP were designed to solve. If each of the routers A, B and C were running a routing daemon then their routing tables would be automatically adjusted to reflect the new state of the network should any one of the links in the network fail. To configure such a network is simple, at each router you need only do two things. In this case for Router A:
root# route add -net 192.168.1.0 netmask 255.255.255.0 eth0
root# /usr/sbin/routed
The `routed' routing daemon automatically finds all active network ports when it starts and sends and listens for messages on each of the network devices to allow it to determine and update the routing table on the host.
This has been a very brief explanation of dynamic routing and where you would use it. If you want more information then you should refer to the suggested references listed at the top of the document.
The important points relating to dynamic routing are:
Network servers and services are those programs that allow a remote user to make user of your Linux machine. Server programs listen on network ports. Network ports are a means of addressing a particular service on any particular host and are how a server knows the difference between an incoming telnet connection and an incoming ftp connection. The remote user establishes a network connection to your machine and the server program, the network daemon program, listening on that port accepts the connection and executes. There are two ways that network daemons may operate. Both are commonly employed in practice. The two ways are:
the network daemon program listens on the designated network port and when an incoming connection is made it manages the network connection itself to provide the service.
the inetd server is a special network daemon program that specializes in managing incoming network connections. It has a configuration file which tells it what program needs to be run when an incoming connection is received. Any service port may be configured for either of the tcp or udp protcols. The ports are described in another file that we will talk about soon.
There are two important files that we need to configure. They are the
/etc/services file which assigns names to port numbers and the
/etc/inetd.conf file which is the configuration file for the
inetd network daemon.
/etc/servicesThe /etc/services file is a simple database that associates a
human friendly name to a machine friendly service port. Its format is
quite simple. The file is a text file with each line representing and
entry in the database. Each entry is comprised of three fields separated by
any number of whitespace (tab or space) characters. The fields
are:
name port/protocol aliases # comment
a single word name that represents the service being described.
this field is split into two subfields.
a number that specifies the port number
the named service will be available on. Most
of the common services have assigned service
numbers. These are described in
RFC-1340.
this subfield may be set to either
tcp or udp.
It is important to note that an entry of 18/tcp is
very different from an entry of 18/udp and that there
is no technical reason why the same service needs to exist on
both. Normally common sense prevails and it is only if a
particular service is available via both tcp and
udp that you will see an entry for both.
other names that may be used to refer to this service entry.
Any text appearing in a line after a `#' character is ignored and treated
as a comment.
/etc/services file.All modern linux distributions provide a good /etc/services file.
Just in case you happen to be building a machine from the ground up, here is
a copy of the /etc/services file supplied with an old
Debian distribution:
# /etc/services:
# $Id: NET3-4-HOWTO.sgml,v 1.2 2000/07/19 15:33:03 gferg dead $
#
# Network services, Internet style
#
# Note that it is presently the policy of IANA to assign a single well-known
# port number for both TCP and UDP; hence, most entries here have two entries
# even if the protocol doesn't support UDP operations.
# Updated from RFC 1340, ``Assigned Numbers'' (July 1992). Not all ports
# are included, only the more common ones.
tcpmux 1/tcp # TCP port service multiplexer
echo 7/tcp
echo 7/udp
discard 9/tcp sink null
discard 9/udp sink null
systat 11/tcp users
daytime 13/tcp
daytime 13/udp
netstat 15/tcp
qotd 17/tcp quote
msp 18/tcp # message send protocol
msp 18/udp # message send protocol
chargen 19/tcp ttytst source
chargen 19/udp ttytst source
ftp-data 20/tcp
ftp 21/tcp
ssh 22/tcp # SSH Remote Login Protocol
ssh 22/udp # SSH Remote Login Protocol
telnet 23/tcp
# 24 - private
smtp 25/tcp mail
# 26 - unassigned
time 37/tcp timserver
time 37/udp timserver
rlp 39/udp resource # resource location
nameserver 42/tcp name # IEN 116
whois 43/tcp nicname
re-mail-ck 50/tcp # Remote Mail Checking Protocol
re-mail-ck 50/udp # Remote Mail Checking Protocol
domain 53/tcp nameserver # name-domain server
domain 53/udp nameserver
mtp 57/tcp # deprecated
bootps 67/tcp # BOOTP server
bootps 67/udp
bootpc 68/tcp # BOOTP client
bootpc 68/udp
tftp 69/udp
gopher 70/tcp # Internet Gopher
gopher 70/udp
rje 77/tcp netrjs
finger 79/tcp
www 80/tcp http # WorldWideWeb HTTP
www 80/udp # HyperText Transfer Protocol
link 87/tcp ttylink
kerberos 88/tcp kerberos5 krb5 # Kerberos v5
kerberos 88/udp kerberos5 krb5 # Kerberos v5
supdup 95/tcp
# 100 - reserved
hostnames 101/tcp hostname # usually from sri-nic
iso-tsap 102/tcp tsap # part of ISODE.
csnet-ns 105/tcp cso-ns # also used by CSO name server
csnet-ns 105/udp cso-ns
rtelnet 107/tcp # Remote Telnet
rtelnet 107/udp
pop-2 109/tcp postoffice # POP version 2
pop-2 109/udp
pop-3 110/tcp # POP version 3
pop-3 110/udp
sunrpc 111/tcp portmapper # RPC 4.0 portmapper TCP
sunrpc 111/udp portmapper # RPC 4.0 portmapper UDP
auth 113/tcp authentication tap ident
sftp 115/tcp
uucp-path 117/tcp
nntp 119/tcp readnews untp # USENET News Transfer Protocol
ntp 123/tcp
ntp 123/udp # Network Time Protocol
netbios-ns 137/tcp # NETBIOS Name Service
netbios-ns 137/udp
netbios-dgm 138/tcp # NETBIOS Datagram Service
netbios-dgm 138/udp
netbios-ssn 139/tcp # NETBIOS session service
netbios-ssn 139/udp
imap2 143/tcp # Interim Mail Access Proto v2
imap2 143/udp
snmp 161/udp # Simple Net Mgmt Proto
snmp-trap 162/udp snmptrap # Traps for SNMP
cmip-man 163/tcp # ISO mgmt over IP (CMOT)
cmip-man 163/udp
cmip-agent 164/tcp
cmip-agent 164/udp
xdmcp 177/tcp # X Display Mgr. Control Proto
xdmcp 177/udp
nextstep 178/tcp NeXTStep NextStep # NeXTStep window
nextstep 178/udp NeXTStep NextStep # server
bgp 179/tcp # Border Gateway Proto.
bgp 179/udp
prospero 191/tcp # Cliff Neuman's Prospero
prospero 191/udp
irc 194/tcp # Internet Relay Chat
irc 194/udp
smux 199/tcp # SNMP Unix Multiplexer
smux 199/udp
at-rtmp 201/tcp # AppleTalk routing
at-rtmp 201/udp
at-nbp 202/tcp # AppleTalk name binding
at-nbp 202/udp
at-echo 204/tcp # AppleTalk echo
at-echo 204/udp
at-zis 206/tcp # AppleTalk zone information
at-zis 206/udp
z3950 210/tcp wais # NISO Z39.50 database
z3950 210/udp wais
ipx 213/tcp # IPX
ipx 213/udp
imap3 220/tcp # Interactive Mail Access
imap3 220/udp # Protocol v3
ulistserv 372/tcp # UNIX Listserv
ulistserv 372/udp
#
# UNIX specific services
#
exec 512/tcp
biff 512/udp comsat
login 513/tcp
who 513/udp whod
shell 514/tcp cmd # no passwords used
syslog 514/udp
printer 515/tcp spooler # line printer spooler
talk 517/udp
ntalk 518/udp
route 520/udp router routed # RIP
timed 525/udp timeserver
tempo 526/tcp newdate
courier 530/tcp rpc
conference 531/tcp chat
netnews 532/tcp readnews
netwall 533/udp # -for emergency broadcasts
uucp 540/tcp uucpd # uucp daemon
remotefs 556/tcp rfs_server rfs # Brunhoff remote filesystem
klogin 543/tcp # Kerberized `rlogin' (v5)
kshell 544/tcp krcmd # Kerberized `rsh' (v5)
kerberos-adm 749/tcp # Kerberos `kadmin' (v5)
#
webster 765/tcp # Network dictionary
webster 765/udp
#
# From ``Assigned Numbers'':
#
#> The Registered Ports are not controlled by the IANA and on most systems
#> can be used by ordinary user processes or programs executed by ordinary
#> users.
#
#> Ports are used in the TCP [45,106] to name the ends of logical
#> connections which carry long term conversations. For the purpose of
#> providing services to unknown callers, a service contact port is
#> defined. This list specifies the port used by the server process as its
#> contact port. While the IANA can not control uses of these ports it
#> does register or list uses of these ports as a convenience to the
#> community.
#
ingreslock 1524/tcp
ingreslock 1524/udp
prospero-np 1525/tcp # Prospero non-privileged
prospero-np 1525/udp
rfe 5002/tcp # Radio Free Ethernet
rfe 5002/udp # Actually uses UDP only
bbs 7000/tcp # BBS service
#
#
# Kerberos (Project Athena/MIT) services
# Note that these are for Kerberos v4 and are unofficial. Sites running
# v4 should uncomment these and comment out the v5 entries above.
#
kerberos4 750/udp kdc # Kerberos (server) udp
kerberos4 750/tcp kdc # Kerberos (server) tcp
kerberos_master 751/udp # Kerberos authentication
kerberos_master 751/tcp # Kerberos authentication
passwd_server 752/udp # Kerberos passwd server
krb_prop 754/tcp # Kerberos slave propagation
krbupdate 760/tcp kreg # Kerberos registration
kpasswd 761/tcp kpwd # Kerberos "passwd"
kpop 1109/tcp # Pop with Kerberos
knetd 2053/tcp # Kerberos de-multiplexor
zephyr-srv 2102/udp # Zephyr server
zephyr-clt 2103/udp # Zephyr serv-hm connection
zephyr-hm 2104/udp # Zephyr hostmanager
eklogin 2105/tcp # Kerberos encrypted rlogin
#
# Unofficial but necessary (for NetBSD) services
#
supfilesrv 871/tcp # SUP server
supfiledbg 1127/tcp # SUP debugging
#
# Datagram Delivery Protocol services
#
rtmp 1/ddp # Routing Table Maintenance Protocol
nbp 2/ddp # Name Binding Protocol
echo 4/ddp # AppleTalk Echo Protocol
zip 6/ddp # Zone Information Protocol
#
# Debian GNU/Linux services
rmtcfg 1236/tcp # Gracilis Packeten remote config server
xtel 1313/tcp # french minitel
cfinger 2003/tcp # GNU Finger
postgres 4321/tcp # POSTGRES
mandelspawn 9359/udp mandelbrot # network mandelbrot
# Local services
In the real world, the actual file is always growing as new
services are being created. If you fear your own copy is incomplete,
I'd suggest to copy a new /etc/services from a recent distribution.
/etc/inetd.confThe /etc/inetd.conf file is the configuration file for the
inetd server daemon. Its function is to tell inetd what to do
when it receives a connection request for a particular service. For each
service that you wish to accept connections for you must tell inetd
what network server daemon to run and how to run it.
Its format is also fairly simple. It is a text file with each line describing
a service that you wish to provide. Any text in a line following a `#'
is ignored and considered a comment. Each line contains seven fields separated
by any number of whitespace (tab or space) characters. The general format
is as follows:
service socket_type proto flags user server_path server_args
is the service relevant to this
configuration as taken from the /etc/services
file.
this field describes the type of socket
that this entry will consider relevant, allowable
values are: stream, dgram, raw,
rdm, or seqpacket. This is a little
technical in nature, but as a rule of thumb nearly all
tcp based services use stream and nearly all
udp based services use dgram. It is only
very special types of server daemons that would use
any of the other values.
the protocol to considered valid for this
entry. This should match the appropriate entry in the
/etc/services file and will typically be
either tcp or udp. Sun RPC (Remote Procedure
Call) based servers will use rpc/tcp or
rpc/udp.
there are really only two possible settings
for this field. This field setting tells inetd
whether the network server program frees the socket
after it has been started and therefore whether
inetd can start another one on the next
connection request, or whether inetd should wait
and assume that any server daemon already running will
handle the new connection request. Again this is a
little tricky to work out, but as a rule of thumb all
tcp servers should have this entry set to
nowait and most udp servers should have this
entry set to wait. Be warned there are some
notable exceptions to this, so let the example guide
you if you are not sure.
this field describes which user account from
/etc/passwd will be set as the owner of the
network daemon when it is started. This is often
useful if you want to safeguard against security
risks. You can set the user of an entry to the
nobody user so that if the network server
security is breached the possible damage is minimized.
Typically this field is set to root though,
because many servers require root privileges in order
to function correctly.
this field is pathname to the actual server program to execute for this entry.
this field comprises the rest of the line and is optional. This field is where you place any command line arguments that you wish to pass to the server daemon program when it is launched.
/etc/inetd.confAs for the /etc/services file all modern distributions will include
a good /etc/inetd.conf file for you to work with. Here, for
completeness is the /etc/inetd.conf file from the
Debian distribution.
# /etc/inetd.conf: see inetd(8) for further informations.
#
# Internet server configuration database
#
#
# Modified for Debian by Peter Tobias <tobias@et-inf.fho-emden.de>
#
# <service_name> <sock_type> <proto> <flags> <user> <server_path> <args>
#
# Internal services
#
#echo stream tcp nowait root internal
#echo dgram udp wait root internal
discard stream tcp nowait root internal
discard dgram udp wait root internal
daytime stream tcp nowait root internal
daytime dgram udp wait root internal
#chargen stream tcp nowait root internal
#chargen dgram udp wait root internal
time stream tcp nowait root internal
time dgram udp wait root internal
#
# These are standard services.
#
telnet stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.telnetd
ftp stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.ftpd
#fsp dgram udp wait root /usr/sbin/tcpd /usr/sbin/in.fspd
#
# Shell, login, exec and talk are BSD protocols.
#
shell stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.rshd
login stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.rlogind
#exec stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.rexecd
talk dgram udp wait root /usr/sbin/tcpd /usr/sbin/in.talkd
ntalk dgram udp wait root /usr/sbin/tcpd /usr/sbin/in.ntalkd
#
# Mail, news and uucp services.
#
smtp stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.smtpd
#nntp stream tcp nowait news /usr/sbin/tcpd /usr/sbin/in.nntpd
#uucp stream tcp nowait uucp /usr/sbin/tcpd /usr/lib/uucp/uucico
#comsat dgram udp wait root /usr/sbin/tcpd /usr/sbin/in.comsat
#
# Pop et al
#
#pop-2 stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.pop2d
#pop-3 stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.pop3d
#
# `cfinger' is for the GNU finger server available for Debian. (NOTE: The
# current implementation of the `finger' daemon allows it to be run as `root'.)
#
#cfinger stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.cfingerd
#finger stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.fingerd
#netstat stream tcp nowait nobody /usr/sbin/tcpd /bin/netstat
#systat stream tcp nowait nobody /usr/sbin/tcpd /bin/ps -auwwx
#
# Tftp service is provided primarily for booting. Most sites
# run this only on machines acting as "boot servers."
#
#tftp dgram udp wait nobody /usr/sbin/tcpd /usr/sbin/in.tftpd
#tftp dgram udp wait nobody /usr/sbin/tcpd /usr/sbin/in.tftpd /boot
#bootps dgram udp wait root /usr/sbin/bootpd bootpd -i -t 120
#
# Kerberos authenticated services (these probably need to be corrected)
#
#klogin stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.rlogind -k
#eklogin stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.rlogind -k -x
#kshell stream tcp nowait root /usr/sbin/tcpd /usr/sbin/in.rshd -k
#
# Services run ONLY on the Kerberos server (these probably need to be corrected)
#
#krbupdate stream tcp nowait root /usr/sbin/tcpd /usr/sbin/registerd
#kpasswd stream tcp nowait root /usr/sbin/tcpd /usr/sbin/kpasswdd
#
# RPC based services
#
#mountd/1 dgram rpc/udp wait root /usr/sbin/tcpd /usr/sbin/rpc.mountd
#rstatd/1-3 dgram rpc/udp wait root /usr/sbin/tcpd /usr/sbin/rpc.rstatd
#rusersd/2-3 dgram rpc/udp wait root /usr/sbin/tcpd /usr/sbin/rpc.rusersd
#walld/1 dgram rpc/udp wait root /usr/sbin/tcpd /usr/sbin/rpc.rwalld
#
# End of inetd.conf.
ident stream tcp nowait nobody /usr/sbin/identd identd -i
There are a number of miscellaneous files relating to network configuration under linux that you might be interested in. You may never have to modify these files, but it is worth describing them so you know what they contain and what they are for.
/etc/protocolsThe /etc/protocols file is a database that maps protocol id numbers
against protocol names. This is used by programmers to allow them to
specify protocols by name in their programs and also by some programs
such as tcpdump to allow them to display names instead of numbers
in their output. The general syntax of the file is:
protocolname number aliases
The /etc/protocols file supplied with the
Debian distribution is as follows:
# /etc/protocols:
# $Id: NET3-4-HOWTO.sgml,v 1.2 2000/07/19 15:33:03 gferg dead $
#
# Internet (IP) protocols
#
# from: @(#)protocols 5.1 (Berkeley) 4/17/89
#
# Updated for NetBSD based on RFC 1340, Assigned Numbers (July 1992).
ip 0 IP # internet protocol, pseudo protocol number
icmp 1 ICMP # internet control message protocol
igmp 2 IGMP # Internet Group Management
ggp 3 GGP # gateway-gateway protocol
ipencap 4 IP-ENCAP # IP encapsulated in IP (officially ``IP'')
st 5 ST # ST datagram mode
tcp 6 TCP # transmission control protocol
egp 8 EGP # exterior gateway protocol
pup 12 PUP # PARC universal packet protocol
udp 17 UDP # user datagram protocol
hmp 20 HMP # host monitoring protocol
xns-idp 22 XNS-IDP # Xerox NS IDP
rdp 27 RDP # "reliable datagram" protocol
iso-tp4 29 ISO-TP4 # ISO Transport Protocol class 4
xtp 36 XTP # Xpress Tranfer Protocol
ddp 37 DDP # Datagram Delivery Protocol
idpr-cmtp 39 IDPR-CMTP # IDPR Control Message Transport
rspf 73 RSPF # Radio Shortest Path First.
vmtp 81 VMTP # Versatile Message Transport
ospf 89 OSPFIGP # Open Shortest Path First IGP
ipip 94 IPIP # Yet Another IP encapsulation
encap 98 ENCAP # Yet Another IP encapsulation
/etc/networksThe /etc/networks file has a similar function to that of the
/etc/hosts file. It provides a simple database of network names
against network addresses. Its format differs in that there may be
only two fields per line and that the fields are coded as:
networkname networkaddress
loopnet 127.0.0.0
localnet 192.168.0.0
amprnet 44.0.0.0
When you use commands like the route command, if a destination is
a network and that network has an entry in the /etc/networks file
then the route command will display that network name instead of its
address.
Let me start this section by warning you that securing your machine and network against malicious attack is a complex art. I do not consider myself an expert in this field at all and while the following mechanisms I describe will help, if you are serious about security then I recommend you do some research of your own into the subject. There are many good references on the Internet relating to the subject, including the Security-HOWTO
An important rule of thumb is:
`Don't run servers you don't intend to use'.
Many distributions come configured with all sorts of services configured and
automatically started. To ensure even a minimum level of safety you should go
through your /etc/inetd.conf file and comment out (place a `#' at
the start of the line) any entries for services you don't intend to use.
Good candidates are services such as: shell, login, exec,
uucp, ftp and informational services such as finger,
netstat and systat.
There are all sorts of security and access control mechanisms, I'll describe the most elementary of them.
The /etc/ftpusers file is a simple mechanism that allows you to
deny certain users from logging into your machine via ftp. The
/etc/ftpusers file is read by the ftp daemon program (ftpd) when
an incoming ftp connection is received. The file is a simple list of those
users who are disallowed from logging in. It might looks something like:
# /etc/ftpusers - users not allowed to login via ftp
root
uucp
bin
mail
The /etc/securetty file allows you to specify which tty devices
root is allowed to login on. The /etc/securetty file is read
by the login program (usually /bin/login). Its format is a list of
the tty devices names allowed, on all others root login is disallowed:
# /etc/securetty - tty's on which root is allowed to login
tty1
tty2
tty3
tty4
The tcpd program you will have seen listed in the same
/etc/inetd.conf provides logging and access control mechanisms to
services it is configured to protect.
When it is invoked by the inetd program it reads two files containing access rules and either allows or denies access to the server it is protecting accordingly.
It will search the rules files until the first match is found. If no match is
found then it assumes that access should be allowed to anyone. The files it
searches in sequence are: /etc/hosts.allow, /etc/hosts.deny.
I'll describe each of these in turn. For a complete description of this
facility you should refer to the appropriate man pages
(hosts_access(5) is a good starting point).
The /etc/hosts.allow file is a configuration file of the
/usr/sbin/tcpd program. The hosts.allow file contains
rules describing which hosts are allowed access to a service on
your machine.
The file format is quite simple:
# /etc/hosts.allow
#
# <service list>: <host list> [: command]
service list is a comma delimited list of
server names that this rule applies to. Example
server names are: ftpd, telnetd and
fingerd.
host list is a comma delimited list of host
names. You may also use IP addresses here. You may
additionally specify hostnames or addresses using
wildcard characters to match groups of hosts. Examples
include: gw.vk2ktj.ampr.org to match a specific
host, .uts.edu.au to match any hostname
ending in that string, 44. to match any IP
address commencing with those digits. There are some
special tokens to simplify configuration, some of
these are: ALL matches every host, LOCAL
matches any host whose name does not contain a
`.' ie is in the same domain as your machine and
PARANOID matches any host whose name does not
match its address (name spoofing). There is one last
token that is also useful. The EXCEPT token
allows you to provide a list with exceptions. This
will be covered in an example later.
command is an optional parameter. This
parameter is the full pathname of a command that would
be executed everytime this rule is matched. It could
for example run a command that would attempt to
identify who is logged onto the connecting host, or to
generate a mail message or some other warning to a
system administrator that someone is attempting to
connect. There are a number of expansions that may be
included, some common examples are: %h expands to
the name of the connecting host or address if it
doesn't have a name, %d the daemon name being
called.
An example:
# /etc/hosts.allow
#
# Allow mail to anyone
in.smtpd: ALL
# All telnet and ftp to only hosts within my domain and my host at home.
telnetd, ftpd: LOCAL, myhost.athome.org.au
# Allow finger to anyone but keep a record of who they are.
fingerd: ALL: (finger @%h | mail -s "finger from %h" root)
The /etc/hosts.deny file is a configuration file of the
/usr/sbin/tcpd program. The hosts.deny file contains
rules describing which hosts are disallowed access to a service on
your machine.
A simple sample would look something like this:
# /etc/hosts.deny
#
# Disallow all hosts with suspect hostnames
ALL: PARANOID
#
# Disallow all hosts.
ALL: ALL
The PARANOID entry is really redundant because the other entry traps
everything in any case. Either of these entry would make a reasonable default
depending on your particular requirement.
Having an ALL: ALL default in the /etc/hosts.deny and then
specifically enabling on those services and hosts that you want in the
/etc/hosts.allow file is the safest configuration.
The hosts.equiv file is used to grant certain hosts and users access
rights to accounts on your machine without having to supply a password. This
is useful in a secure environment where you control all machines, but is a
security hazard otherwise. Your machine is only as secure as the least secure
of the trusted hosts. To maximize security, don't use this mechanism and
encourage your users not to use the .rhosts file as well.
Many sites will be interested in running an anonymous ftp server to
allow other people to upload and download files without requiring a specific
userid. If you decide to offer this facility make sure you configure the
ftp daemon properly for anonymous access. Most man pages for
ftpd(8) describe in some length how to go about this. You should
always ensure that you follow these instructions. An important tip is to
not use a copy of your /etc/passwd file in the anonymous account
/etc directory, make sure you strip out all account details except
those that you must have, otherwise you will be vulnerable to brute force
password cracking techniques.
Not allowing datagrams to even reach your machine or servers is an excellent means of security. This is covered in depth in the Firewall-HOWTO, and (more concisely) in a later section of this document.
Here are some other, potentially religious suggestions for you to consider.
despite its popularity the sendmail daemon appears with frightening regularity on security warning announcements. Its up to you, but I choose not to run it.
be wary of these. There are all sorts of possible exploits for these services. It is difficult finding an option to services like NFS, but if you configure them, make sure you are careful with who you allow mount rights to.
This section covers information specific to Ethernet and IP. These subsections have been grouped together because I think they are the most interesting ones in the formerly-called ``Technology Specific'' Section. Anyone with a LAN should be able to benefit from these goodies.
Ethernet device names are `eth0', `eth1', `eth2' etc. The first
card detected by the kernel is assigned `eth0' and the rest are assigned
sequentially in the order they are detected.
By default, the Linux kernel only probes for one Ethernet device, you need to pass command line arguments to the kernel in order to force detection of furter boards.
To learn how to make your ethernet card(s) working under Linux you should refer to the Ethernet-HOWTO.
Once you have your kernel properly built to support your ethernet card then configuration of the card is easy.
Typically you would use something like (which most distributions already do for you, if you configured them to support your ethernet):
root# ifconfig eth0 192.168.0.1 netmask 255.255.255.0 up
root# route add -net 192.168.0.0 netmask 255.255.255.0 eth0
Most of the ethernet drivers were developed by Donald Becker,
becker@CESDIS.gsfc.nasa.gov.
The EQL device name is `eql'. With the standard kernel source you may have
only one EQL device per machine. EQL provides a means of utilizing multiple
point to point lines such as PPP, slip or plip as a single logical link to
carry tcp/ip. Often it is cheaper to use multiple lower speed lines than to
have one high speed line installed.
Kernel Compile Options:
Network device support --->
[*] Network device support
<*> EQL (serial line load balancing) support
To support this mechanism the machine at the other end of the lines must also support EQL. Linux, Livingstone Portmasters and newer dial-in servers support compatible facilities.
To configure EQL you will need the eql tools which are available from: metalab.unc.edu.
Configuration is fairly straightforward. You start by configuring the eql interface. The eql interface is just like any other network device. You configure the IP address and mtu using the ifconfig utility, so something like:
root# ifconfig eql 192.168.10.1 mtu 1006
Next you need to manually initiate each of the lines you will use. These may be any combination of point to point network devices. How you initiate the connections will depend on what sort of link they are, refer to the appropriate sections for further information.
Lastly you need to associate the serial link with the EQL device, this is called `enslaving' and is done with the eql_enslave command as shown:
root# eql_enslave eql sl0 28800
root# eql_enslave eql ppp0 14400
The `estimated speed' parameter you supply eql_enslave doesn't do anything directly. It is used by the EQL driver to determine what share of the datagrams that device should receive, so you can fine tune the balancing of the lines by playing with this value.
To disassociate a line from an EQL device you use the eql_emancipate command as shown:
root# eql_emancipate eql sl0
You add routing as you would for any other point to point link, except your
routes should refer to the eql device rather than the actual serial
devices themselves, typically you would use:
root# route add default eql
The EQL driver was developed by Simon Janes, simon@ncm.com.
The IP accounting features of the Linux kernel allow you to collect and analyze some network usage data. The data collected comprises the number of packets and the number of bytes accumulated since the figures were last reset. You may specify a variety of rules to categorize the figures to suit whatever purpose you may have. This option has been removed in kernel 2.1.102, because the old ipfwadm-based firewalling was replaced by ``ipfwchains''.
Kernel Compile Options:
Networking options --->
[*] IP: accounting
After you have compiled and installed the kernel you need to use the ipfwadm command to configure IP accounting. There are many different ways of breaking down the accounting information that you might choose. I've picked a simple example of what might be useful to use, you should read the ipfwadm man page for more information.
Scenario: You have a ethernet network that is linked to the internet via a PPP link. On the ethernet you have a machine that offers a number of services and that you are interested in knowing how much traffic is generated by each of ftp and world wide web traffic, as well as total tcp and udp traffic.
You might use a command set that looks like the following, which is shown as a shell script:
#!/bin/sh
#
# Flush the accounting rules
ipfwadm -A -f
#
# Set shortcuts
localnet=44.136.8.96/29
any=0/0
# Add rules for local ethernet segment
ipfwadm -A in -a -P tcp -D $localnet ftp-data
ipfwadm -A out -a -P tcp -S $localnet ftp-data
ipfwadm -A in -a -P tcp -D $localnet www
ipfwadm -A out -a -P tcp -S $localnet www
ipfwadm -A in -a -P tcp -D $localnet
ipfwadm -A out -a -P tcp -S $localnet
ipfwadm -A in -a -P udp -D $localnet
ipfwadm -A out -a -P udp -S $localnet
#
# Rules for default
ipfwadm -A in -a -P tcp -D $any ftp-data
ipfwadm -A out -a -P tcp -S $any ftp-data
ipfwadm -A in -a -P tcp -D $any www
ipfwadm -A out -a -P tcp -S $any www
ipfwadm -A in -a -P tcp -D $any
ipfwadm -A out -a -P tcp -S $any
ipfwadm -A in -a -P udp -D $any
ipfwadm -A out -a -P udp -S $any
#
# List the rules
ipfwadm -A -l -n
#
The names ``ftp-data'' and ``www'' refer to lines in
/etc/services. The last command lists each of the Accounting
rules and displays the collected totals.
An important point to note when analyzing IP accounting is that totals for all rules that match will be incremented so that to obtain differential figures you need to perform appropriate maths. For example if I wanted to know how much data was not ftp nor www I would substract the individual totals from the rule that matches all ports.
root# ipfwadm -A -l -n
IP accounting rules
pkts bytes dir prot source destination ports
0 0 in tcp 0.0.0.0/0 44.136.8.96/29 * -> 20
0 0 out tcp 44.136.8.96/29 0.0.0.0/0 20 -> *
10 1166 in tcp 0.0.0.0/0 44.136.8.96/29 * -> 80
10 572 out tcp 44.136.8.96/29 0.0.0.0/0 80 -> *
252 10943 in tcp 0.0.0.0/0 44.136.8.96/29 * -> *
231 18831 out tcp 44.136.8.96/29 0.0.0.0/0 * -> *
0 0 in udp 0.0.0.0/0 44.136.8.96/29 * -> *
0 0 out udp 44.136.8.96/29 0.0.0.0/0 * -> *
0 0 in tcp 0.0.0.0/0 0.0.0.0/0 * -> 20
0 0 out tcp 0.0.0.0/0 0.0.0.0/0 20 -> *
10 1166 in tcp 0.0.0.0/0 0.0.0.0/0 * -> 80
10 572 out tcp 0.0.0.0/0 0.0.0.0/0 80 -> *
253 10983 in tcp 0.0.0.0/0 0.0.0.0/0 * -> *
231 18831 out tcp 0.0.0.0/0 0.0.0.0/0 * -> *
0 0 in udp 0.0.0.0/0 0.0.0.0/0 * -> *
0 0 out udp 0.0.0.0/0 0.0.0.0/0 * -> *
The new accounting code is accessed via ``IP Firewall Chains''.
See
the IP chains home page for more information. Among other
things, you'll now need to use ipchains instead of ipfwadm
to configure your filters. (From Documentation/Changes in the
latest kernel sources).
There are some applications where being able to configure multiple IP addresses to a single network device is useful. Internet Service Providers often use this facility to provide a `customized' to their World Wide Web and ftp offerings for their customers. You can refer to the ``IP-Alias mini-HOWTO'' for more information than you find here.
Kernel Compile Options:
Networking options --->
....
[*] Network aliasing
....
<*> IP: aliasing support
After compiling and installing your kernel with IP_Alias support
configuration is very simple. The aliases are added to virtual network
devices associated with the actual network device. A simple naming
convention applies to these devices being <devname>:<virtual
dev num>, e.g. eth0:0, ppp0:10 etc. Note that the the
ifname:number device can only be configured after the main
interface has been set up.
For example, assume you have an ethernet network that supports two different IP subnetworks simultaneously and you wish your machine to have direct access to both, you could use something like:
root# ifconfig eth0 192.168.1.1 netmask 255.255.255.0 up
root# route add -net 192.168.1.0 netmask 255.255.255.0 eth0
root# ifconfig eth0:0 192.168.10.1 netmask 255.255.255.0 up
root# route add -net 192.168.10.0 netmask 255.255.255.0 eth0:0
To delete an alias you simply add a `-' to the end of its name and refer
to it and is as simple as:
root# ifconfig eth0:0- 0
All routes associated with that alias will also be deleted automatically.
IP Firewall and Firewalling issues are covered in more depth in the Firewall-HOWTO. IP Firewalling allows you to secure your machine against unauthorized network access by filtering or allowing datagrams from or to IP addresses that you nominate. There are three different classes of rules, incoming filtering, outgoing filtering and forwarding filtering. Incoming rules are applied to datagrams that are received by a network device. Outgoing rules are applied to datagrams that are to be transmitted by a network device. Forwarding rules are applied to datagrams that are received and are not for this machine, ie datagrams that would be routed.
Kernel Compile Options:
Networking options --->
[*] Network firewalls
....
[*] IP: forwarding/gatewaying
....
[*] IP: firewalling
[ ] IP: firewall packet logging
Configuration of the IP firewall rules is performed using the ipfwadm command. As I mentioned earlier, security is not something I am expert at, so while I will present an example you can use, you should do your own research and develop your own rules if security is important to you.
Probably the most common use of IP firewall is when you are using your linux machine as a router and firewall gateway to protect your local network from unauthorized access from outside your network.
The following configuration is based on a contribution from Arnt Gulbrandsen,
<agulbra@troll.no>.
The example describes the configuration of the firewall rules on the Linux firewall/router machine illustrated in this diagram:
- -
\ | 172.16.37.0
\ | /255.255.255.0
\ --------- |
| 172.16.174.30 | Linux | |
NET =================| f/w |------| ..37.19
| PPP | router| | --------
/ --------- |--| Mail |
/ | | /DNS |
/ | --------
- -
The following commands would normally be placed in an rc file
so that they were automatically started each time the system
boots. For maximum security they would be performed after the network
interfaces are configured, but before the interfaces are actually
brought up to prevent anyone gaining access while the firewall machine
is rebooting.
#!/bin/sh
# Flush the 'Forwarding' rules table
# Change the default policy to 'accept'
#
/sbin/ipfwadm -F -f
/sbin/ipfwadm -F -p accept
#
# .. and for 'Incoming'
#
/sbin/ipfwadm -I -f
/sbin/ipfwadm -I -p accept
# First off, seal off the PPP interface
# I'd love to use '-a deny' instead of '-a reject -y' but then it
# would be impossible to originate connections on that interface too.
# The -o causes all rejected datagrams to be logged. This trades
# disk space against knowledge of an attack of configuration error.
#
/sbin/ipfwadm -I -a reject -y -o -P tcp -S 0/0 -D 172.16.174.30
# Throw away certain kinds of obviously forged packets right away:
# Nothing should come from multicast/anycast/broadcast addresses
#
/sbin/ipfwadm -F -a deny -o -S 224.0/3 -D 172.16.37.0/24
#
# and nothing coming from the loopback network should ever be
# seen on a wire
#
/sbin/ipfwadm -F -a deny -o -S 127.0/8 -D 172.16.37.0/24
# accept incoming SMTP and DNS connections, but only
# to the Mail/Name Server
#
/sbin/ipfwadm -F -a accept -P tcp -S 0/0 -D 172.16.37.19 25 53
#
# DNS uses UDP as well as TCP, so allow that too
# for questions to our name server
#
/sbin/ipfwadm -F -a accept -P udp -S 0/0 -D 172.16.37.19 53
#
# but not "answers" coming to dangerous ports like NFS and
# Larry McVoy's NFS extension. If you run squid, add its port here.
#
/sbin/ipfwadm -F -a deny -o -P udp -S 0/0 53 \
-D 172.16.37.0/24 2049 2050
# answers to other user ports are okay
#
/sbin/ipfwadm -F -a accept -P udp -S 0/0 53 \
-D 172.16.37.0/24 53 1024:65535
# Reject incoming connections to identd
# We use 'reject' here so that the connecting host is told
# straight away not to bother continuing, otherwise we'd experience
# delays while ident timed out.
#
/sbin/ipfwadm -F -a reject -o -P tcp -S 0/0 -D 172.16.37.0/24 113
# Accept some common service connections from the 192.168.64 and
# 192.168.65 networks, they are friends that we trust.
#
/sbin/ipfwadm -F -a accept -P tcp -S 192.168.64.0/23 \
-D 172.16.37.0/24 20:23
# accept and pass through anything originating inside
#
/sbin/ipfwadm -F -a accept -P tcp -S 172.16.37.0/24 -D 0/0
# deny most other incoming TCP connections and log them
# (append 1:1023 if you have problems with ftp not working)
#
/sbin/ipfwadm -F -a deny -o -y -P tcp -S 0/0 -D 172.16.37.0/24
# ... for UDP too
#
/sbin/ipfwadm -F -a deny -o -P udp -S 0/0 -D 172.16.37.0/24
Good firewall configurations are a little tricky. This example should be a reasonable starting point for you. The ipfwadm manual page offers some assistance in how to use the tool. If you intend to configure a firewall, be sure to ask around and get as much advice from sources you consider reliable and get someone to test/sanity check your configuration from the outside.
The new firewalling code is accessed via ``IP Firewall Chains''.
See
the IP chanins home page for more information. Among other
things, you'll now need to use ipchains instead of ipfwadm
to configure your filters. (From Documentation/Changes in the
latest kernel sources).
We are aware that this is a sorely out of date statement and we are currently working on getting this section more current. You can expect a newer version in August of 1999.
Why would you want to encapsulate IP datagrams within IP datagrams? It must seem an odd thing to do if you've never seen an application of it before. Ok, here are a couple of common places where it is used: Mobile-IP and IP-Multicast. What is perhaps the most widely spread use of it though is also the least well known, Amateur Radio.
Kernel Compile Options:
Networking options --->
[*] TCP/IP networking
[*] IP: forwarding/gatewaying
....
<*> IP: tunneling
IP tunnel devices are called `tunl0', `tunl1' etc.
"But why ?". Ok, ok. Conventional IP routing rules mandate that an IP network comprises a network address and a network mask. This produces a series of contiguous addresses that may all be routed via a single routing entry. This is very convenient, but it means that you may only use any particular IP address while you are connected to the particular piece of network to which it belongs. In most instances this is ok, but if you are a mobile netizen then you may not be able to stay connected to the one place all the time. IP/IP encapsulation (IP tunneling) allows you to overcome this restriction by allowing datagrams destined for your IP address to be wrapped up and redirected to another IP address. If you know that you're going to be operating from some other IP network for some time you can set up a machine on your home network to accept datagrams to your IP address and redirect them to the address that you will actually be using temporarily.
192.168.1/24 192.168.2/24
- -
| ppp0 = ppp0 = |
| aaa.bbb.ccc.ddd fff.ggg.hhh.iii |
| |
| /-----\ /-----\ |
| | | // | | |
|---| A |------//---------| B |---|
| | | // | | |
| \-----/ \-----/ |
| |
- -
Linux router `A' would be configured with a script like the following:
#!/bin/sh
PATH=/sbin:/usr/sbin
mask=255.255.255.0
remotegw=fff.ggg.hhh.iii
#
# Ethernet configuration
ifconfig eth0 192.168.1.1 netmask $mask up
route add -net 192.168.1.0 netmask $mask eth0
#
# ppp0 configuration (start ppp link, set default route)
pppd
route add default ppp0
#
# Tunnel device configuration
ifconfig tunl0 192.168.1.1 up
route add -net 192.168.2.0 netmask $mask gw $remotegw tunl0
Linux router `B' would be configured with a similar script:
#!/bin/sh
PATH=/sbin:/usr/sbin
mask=255.255.255.0
remotegw=aaa.bbb.ccc.ddd
#
# Ethernet configuration
ifconfig eth0 192.168.2.1 netmask $mask up
route add -net 192.168.2.0 netmask $mask eth0
#
# ppp0 configuration (start ppp link, set default route)
pppd
route add default ppp0
#
# Tunnel device configuration
ifconfig tunl0 192.168.2.1 up
route add -net 192.168.1.0 netmask $mask gw $remotegw tunl0
The command:
route add -net 192.168.1.0 netmask $mask gw $remotegw tunl0
192.168.1.0/24 inside an
IPIP encap datagram with a destination address of aaa.bbb.ccc.ddd'.
Note that the configurations are reciprocated at either end. The tunnel device
uses the `gw' in the route as the destination of the IP datagram
in which it will place the datagram it has received to route. That machine
must know how to decapsulate IPIP datagrams, that is, it must also be
configured with a tunnel device.
It doesn't have to be a whole network you route. You could for example route
just a single IP address. In that instance you might configure the tunl
device on the `remote' machine with its home IP address and at the A end just
use a host route (and Proxy Arp) rather than a network route via the tunnel
device. Let's redraw and modify our configuration appropriately. Now we
have just host `B' which to want to act and behave as if it is both
fully connected to the Internet and also part of the remote network supported
by host `A':
192.168.1/24
-
| ppp0 = ppp0 =
| aaa.bbb.ccc.ddd fff.ggg.hhh.iii
|
| /-----\ /-----\
| | | // | |
|---| A |------//---------| B |
| | | // | |
| \-----/ \-----/
| also: 192.168.1.12
-
Linux router `A' would be configured with:
#!/bin/sh
PATH=/sbin:/usr/sbin
mask=255.255.255.0
remotegw=fff.ggg.hhh.iii
#
# Ethernet configuration
ifconfig eth0 192.168.1.1 netmask $mask up
route add -net 192.168.1.0 netmask $mask eth0
#
# ppp0 configuration (start ppp link, set default route)
pppd
route add default ppp0
#
# Tunnel device configuration
ifconfig tunl0 192.168.1.1 up
route add -host 192.168.1.12 gw $remotegw tunl0
#
# Proxy ARP for the remote host
arp -s 192.168.1.12 xx:xx:xx:xx:xx:xx pub
Linux host `B' would be configured with:
#!/bin/sh
PATH=/sbin:/usr/sbin
mask=255.255.255.0
remotegw=aaa.bbb.ccc.ddd
#
# ppp0 configuration (start ppp link, set default route)
pppd
route add default ppp0
#
# Tunnel device configuration
ifconfig tunl0 192.168.1.12 up
route add -net 192.168.1.0 netmask $mask gw $remotegwtunl0
This sort of configuration is more typical of a Mobile-IP application. Where a single host wants to roam around the Internet and maintain a single usable IP address the whole time. You should refer to the Mobile-IP section for more information on how that is handled in practice.
Many people have a simple dialup account to connect to the Internet. Nearly everybody using this sort of configuration is allocated a single IP address by the Internet Service Provider. This is normally enough to allow only one host full access to the network. IP Masquerade is a clever trick that enables you to have many machines make use of that one IP address, by causing the other hosts to look like, hence the term masquerade, the machine supporting the dialup connection. There is a small caveat and that is that the masquerade function nearly always works only in one direction, that is the masqueraded hosts can make calls out, but they cannot accept or receive network connections from remote hosts. This means that some network services do not work such as talk and others such as ftp must be configured to operate in passive (PASV) mode to operate. Fortunately the most common network services such as telnet, World Wide Web and irc do work just fine.
Kernel Compile Options:
Code maturity level options --->
[*] Prompt for development and/or incomplete code/drivers
Networking options --->
[*] Network firewalls
....
[*] TCP/IP networking
[*] IP: forwarding/gatewaying
....
[*] IP: masquerading (EXPERIMENTAL)
Normally you have your linux machine supporting a slip or PPP dialup line just as it would if it were a standalone machine. Additionally it would have another network device configured, perhaps an ethernet, configured with one of the reserved network addresses. The hosts to be masqueraded would be on this second network. Each of these hosts would have the IP address of the ethernet port of the linux machine set as their default gateway or router.
A typical configuration might look something like this:
- -
\ | 192.168.1.0
\ | /255.255.255.0
\ --------- |
| | Linux | .1.1 |
NET =================| masq |------|
| PPP/slip | router| | --------
/ --------- |--| host |
/ | | |
/ | --------
- -
The most relevant commands for this configuration are:
# Network route for ethernet
route add -net 192.168.1.0 netmask 255.255.255.0 eth0
#
# Default route to the rest of the internet.
route add default ppp0
#
# Cause all hosts on the 192.168.1/24 network to be masqueraded.
ipfwadm -F -a m -S 192.168.1.0/24 -D 0.0.0.0/0
Masquerading with IPCHAINS
This is similar to using IPFWADM but the command structure has changed:
# Network route for ethernet
route add -net 192.168.1.0 netmask 255.255.255.0 eth0
#
# Default route to the rest of the internet.
route add default ppp0
#
# Cause all hosts on the 192.168.1/24 network to be masqueraded.
ipchains -A forward -s 192.168.1.0/24 -j MASQ
You can get more information on the Linux IP Masquerade feature from the IP Masquerade Resource Page. Also, a very detailed document about masquesrading is the ``IP-Masquerade mini-HOWTO'' (which also intructs to configure other OS's to run with a Linux masquerade server).
IP transparent proxy is a feature that enables you to redirect servers or services destined for another machine to those services on this machine. Typically this would be useful where you have a linux machine as a router and also provides a proxy server. You would redirect all connections destined for that service remotely to the local proxy server.
Kernel Compile Options:
Code maturity level options --->
[*] Prompt for development and/or incomplete code/drivers
Networking options --->
[*] Network firewalls
....
[*] TCP/IP networking
....
[*] IP: firewalling
....
[*] IP: transparent proxy support (EXPERIMENTAL)
Configuration of the transparent proxy feature is performed using the ipfwadm command
An example that might be useful is as follows:
root# ipfwadm -I -a accept -D 0/0 telnet -r 2323
This example will cause any connection attempts to port telnet
(23) on any host to be redirected to port 2323 on this host. If you
run a service on that port, you could forward telnet connections, log
them or do whatever fits your need.
A more interesting example is redirecting all http traffic
through a local cache. However, the protocol used by proxy servers is
different from native http: where a client connects to
www.server.com:80 and asks for /path/page, when it
connects to the local cache it contacts proxy.local.domain:8080
and asks for www.server.com/path/page.
To filter an http request through the local proxy, you need to
adapt the protocol by inserting a small server, called
transproxy (you can find it on the world wide web). You can choose
to run transproxy on port 8081, and issue this command:
root# ipfwadm -I -a accept -D 0/0 80 -r 8081
transproxy program, then, will receive all connections meant
to reach external servers and will pass them to the local proxy
after fixing protocol differences.
Just when you thought you were beginning to understand IP networking the rules get changed! IPv6 is the shorthand notation for version 6 of the Internet Protocol. IPv6 was developed primarily to overcome the concerns in the Internet community that there would soon be a shortage of IP addresses to allocate. IPv6 addresses are 16 bytes long (128 bits). IPv6 incorporates a number of other changes, mostly simplifications, that will make IPv6 networks more managable than IPv4 networks.
Linux already has a working, but not complete, IPv6 implementation in
the 2.2.* series kernels.
If you wish to experiment with this next generation Internet technology, or have a requirement for it, then you should read the IPv6-FAQ which is available from www.terra.net.
The term "IP mobility" describes the ability of a host that is able to move its network connection from one point on the Internet to another without changing its IP address or losing connectivity. Usually when an IP host changes its point of connectivity it must also change its IP address. IP Mobility overcomes this problem by allocating a fixed IP