10.1. Networking Overview

10.1.1. Networking protocols

Linux supports many different networking protocols. We list only the most important: TCP/IP

The Transport Control Protocol and the Internet Protocol are the two most popular ways of communicating on the Internet. A lot of applications, such as your browser and E-mail program, are built on top of this protocol suite.

Very simply put, IP provides a solution for sending packets of information from one machine to another, while TCP ensures that the packets are arranged in streams, so that packets from different applications don't get mixed up, and that the packets are sent and received in the correct order.

The Internet was originally developed three decades ago for the United States Department of Defense (DoD), mainly for the purpose of interconnecting different-brand computers. Another reason for the development of TCP/IP was to provide a reliable data transport system over an unreliable network.

TCP/IP networking has been present in Linux since its beginnings. It has been implemented from scratch. It is one of the most robust, fast and reliable implementations and is one of the key factors of the success of Linux. Linux and networking are made for each other, in so much that not connecting your Linux system to the network may result in slow startup and other troubles. Even if you don't use any network connections to other computers, networking protocols are used for internal system and application communications. Linux expects to be networked.

A good starting point for learning more about TCP and IP is in the following documents:

  • man 7 ip: Describes the IPv4 protocol implementation on Linux (version 4 currently being the most wide-spread edition of the IP protocol).

  • man 7 tcp: Implementation of the TCP protocol.

  • RFC793, RFC1122, RFC2001 for TCP, and RFC791, RFC1122 and RFC1112 for IP.

    The Request For Comments documents contain the descriptions of networking standards, protocols, applications and implementation. These documents are managed by the Internet Engineering Task Force, an international community concerned with the smooth operation of the Internet and the evolution and development of the Internet architecture.

    Your ISP usually has an RFC archive available, or you can browse the RFCs via http://www.ietf.org/rfc.html. TCP/IPv6

Nobody expected the Internet to grow as fast as it does. IP proved to have quite some disadvantages when a really large number of computers is in a network, the most important being the availability of unique addresses to assign to each machine participating. Thus, IP version 6 was deviced to meet the needs of today's Internet.

Unfortunately, not all applications and services support IPv6, yet. A migration is currently being set in motion in many environments that can benefit from an upgrade to IPv6. For some applications, the old protocol is still used, for applications that have been reworked the new version is already active. So when checking your network configuration, sometimes it might be a bit confusing since all kinds of measures can be taken to hide one protocol from the other so as the two don't mix up connections.

More information can be found in the following documents:

  • man 7 ipv6: the Linux IPv6 protocol implementation.

  • RFC1883 describing the IPv6 protocol. PPP, SLIP, PLIP, PPPOE

The Linux kernel has built-in support for PPP (Point-to-Point-Protocol), SLIP (Serial Line IP) and PLIP (Parallel Line IP). PPP is the most popular way individual users access their ISP (Internet Service Provider), although in densely populated areas it is often being replaced by PPPOE, PPP over Ethernet, the protocol used in cable modem connections.

Most Linux distributions provide easy-to-use tools for setting up an Internet connection. The only thing you basically need is a username and password to connect to your Internet Service Provider (ISP), and a telephone number in the case of PPP. These data are entered in the graphical configuration tool, which will likely also allow for starting and stopping the connection to your provider. ISDN

The Linux kernel has built-in ISDN capabilities. Isdn4linux controls ISDN PC cards and can emulate a modem with the Hayes command set ("AT" commands). The possibilities range from simply using a terminal program to full connection to the Internet.

Check your system documentation. AppleTalk

Appletalk is the name of Apple's internetworking stack. It allows a peer-to-peer network model which provides basic functionality such as file and printer sharing. Each machine can simultaneously act as a client and a server, and the software and hardware necessary are included with every Apple computer.

Linux provides full AppleTalk networking. Netatalk is a kernel-level implementation of the AppleTalk Protocol Suite, originally for BSD-derived systems. It includes support for routing AppleTalk, serving UNIX and AFS file systems using AppleShare and serving UNIX printers and accessing AppleTalk printers. SMB/NMB

For compatibility with MS Windows environments, the Samba suite, including support for the NMB and SMB protocols, can be installed on any UNIX-like system. The Server Message Block protocol (also called Session Message Block, NetBIOS or LanManager protocol) is used on MS Windows 3.11, NT, 95/98, 2K and XP to share disks and printers.

The basic functions of the Samba suite are: sharing Linux drives with Windows machines, accessing SMB shares from Linux machines, sharing Linux printers with Windows machines and sharing Windows printers with Linux machines.

Most Linux distributions provide a samba package, which does most of the server setup and starts up smbd, the Samba server, and nmbd, the netbios name server, at boot time by default. Samba can be configured graphically, via a web interface or via the command line and text configuration files. More information can be found at the following locations:

  • man smb.conf: describes the format of the main Samba configuration file.

  • The Samba section of the RedHat Support Resources on Printing and File Services.

  • The Samba Project Documentation (or check your local samba.org mirror) contains an easy to read installation and testing guide, which also explains how to configure your Samba server as a Primary Domain Controller.

  • The Samba website contains more documentation, including a troubleshooting guide. Miscellaneous protocols

Linux also has support for Amateur Radio, WAN internetworking (X25, Frame Relay, ATM), InfraRed and other wireless connections, but since these protocols usually require special hardware, we won't discuss them in this document.

10.1.2. Network configuration and information Configuration of local network interfaces

All the big, userfriendly Linux distributions come with various graphical tools, allowing for easy setup of the computer in a local network or for connecting it to an Internet Service Provider. These tools can be started up from the command line or from a menu:

  • RedHat Linux comes with redhat-config-network, which has both a graphical and a text mode interface.

  • Suse's YAST or YAST2 is an all-in-one configuration tool.

  • Mandrake comes with a Network and Internet Configuration Wizard, which is preferably started up from Mandrake's Control Center.

Your system documentation provides plenty of advice and information about availability and use of tools.

Information you'll need to provide:

  • For connecting to the local network, for instance with your home computers, or at work: hostname, domainname and IP address. If you want to set up your own network, best do some more reading first. At work, this information is likely to be given to your computer automatically when you boot it up. When in doubt, it is better not to specify any information than making it up.

  • For connecting to the Internet: username and password for your ISP, telephone number when using a modem. Your ISP usually automatically assigns you a hostname, IP address and all the other things necessary for your Internet applications to work. Network configuration files

The graphical helper tools edit a specific set of network configuration files, using a couple of basic commands. The exact names of the configuration files and their location in the file system is largely dependent on your Linux distribution and version. However, a couple of network configuration files are common on all UNIX systems:

  • /etc/hosts: always contains the localhost IP address,, which is used for interprocess communication. Never remove this line! Sometimes contains addresses of additional hosts, which can be contacted without using an external naming service such as DNS (the Domain Name Server).

    A sample hosts file for a small home network:

# Do not remove the following line, or various programs
    # that require network functionality will fail.       localhost.localdomain   localhost	tux.mylan.com		tux	winxp.mylan.com		winxp

    Read more in man hosts.

  • /etc/resolv.conf: configure access to a DNS server. This file contains your domain name and the name server(s) to contact:

search mylan.com

    Read more in the resolv.conf man page.

  • /etc/nsswitch.conf: defines the order in which to contact different name services. For Internet use, it is important that dns shows up in the "hosts" line:

[bob@tux ~] grep hosts /etc/nsswitch.conf
    hosts:	files dns

    This instructs your computer to look up hostnames and IP addresses first in the /etc/hosts file, and to contact the DNS server if a given host does not occur in the local hosts file. Other possible name services to contact are LDAP, NIS and NIS+.

    More in man nsswitch.conf. Network configuration commands

The distribution-specific scripts and graphical tools use ip (or ifconfig and route on older systems) to configure the kernel's networking configuration.

The ip command is used for assigning IP addresses to interfaces, for setting up routes to the Internet and to other networks, for displaying TCP/IP configurations etcetera.

The following commands show IP address and routing information:

benny@home benny> ip addr show
1: lo: <LOOPBACK,UP> mtu 16436 qdisc noqueue 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet brd scope host lo
    inet6 ::1/128 scope host 
2: eth0: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 100
    link/ether 00:50:bf:7e:54:9a brd ff:ff:ff:ff:ff:ff
    inet brd scope global eth0
    inet6 fe80::250:bfff:fe7e:549a/10 scope link 

benny@home benny> ip route show dev eth0  scope link dev lo  scope link 
default via dev eth0 

On your laptop which you usually connect to the company network using the onboard Ethernet connection, but which you are now to configure for dial-in at home or in a hotel, you might need to activate the PCMCIA card. This is done using the cardctl control utility. However, a good distribution should provide PCMCIA support in the network configuration tools, preventing users from having to execute PCMCIA commands manually.

Further discussion of network configuration is out of the scope of this document. Your primary source for extra information is the man pages for the services you want to set up. Additional reading:

  • The Modem-HOWTO: Help with selecting, connecting, configuring, trouble-shooting, and understanding analog modems for a PC.

  • LDP HOWTO Index, section 4.4: categorized list of HOWTOs about general networking, protocols, dial-up, DNS, VPNs, bridging, routinfg, security and more.

  • The /usr/share/doc/iproute-<version>ip-cref.ps file, viewable with for instance gv. Network interface names

On a Linux machine, the device name lo or the local loop is linked with the internal address. The computer will have a hard time making your applications work if this device is not present; it is always there, even on computers which are not networked.

The first ethernet device, eth0 in the case of a standard network interface card, points to your local LAN IP address. Normal client machines only have one network interface card. Routers, connecting networks together, have one network device for each network they serve.

If you use a modem to connect to the Internet, your network device will probably be named ppp0. This is normally also the case for connections using a cable modem. Your host configuration

Apart from the ip command for displaying the network configuration, there's the common netstat command which has a lot of options and is generally useful on any UNIX system. Use the -i option to display information about network interfaces:

bob:~> netstat -i
Kernel Interface table
eth0 1500   0 58459   0      0      0  63865     0      0      0 BMRU
lo  16436   0 24060   0      0      0  24060     0      0      0 LRU

Routing information can be displayed with the -nr option to the netstat command:

bob:~> netstat -nr
Kernel IP routing table
Destination  Gateway      Genmask       Flags MSS Window irtt Iface U      40 0         0 eth0     U      40 0         0 lo       UG     40 0         0 eth0

This is a typical client machine in an IP network. It only has one network device, eth0. The lo interface is the local loop. Other hosts

An impressive amount of tools is focused on network management and remote administration of Linux machines. Your local Linux software mirror will offer plenty of those. It would lead us too far to discuss them in this document, so please refer to the program-specific documentation.

We will only discuss some common UNIX/Linux text tools in this section.

To display information on hosts or domains, use the host command:

[emmy@pc10 emmy]$ host www.eunet.be
www.eunet.be. has address

[emmy@pc10 emmy]$ host -t any eunet.be
eunet.be. SOA dns.eunet.be. hostmaster.Belgium.EU.net. 
  2002021300 28800 7200 604800 86400
eunet.be. mail is handled by 50 pophost.eunet.be.
eunet.be. name server ns.EU.net.
eunet.be. name server dns.eunet.be.

Similar information can be displayed using the dig command, which gives additional information about how records are stored in the name server.

To check if a host is alive, use ping. If your system is configured to send more than one packet, interrupt ping with the Ctrl+C key combination:

[emmy@pc10 emmy]$ ping a.host.be 
PING a.host.be ( from 56(84) bytes of data.
64 bytes from a.host.be( ttl=244 time=99.977msec
--- a.host.be ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/mdev = 99.977/99.977/99.977/0.000 ms

To check the route that packets follow to a network host, use the traceroute command:

[emmy@pc10 emmy]$ /usr/sbin/traceroute www.eunet.be
traceroute to www.eunet.be(,30 hops max,38b packets
1 blob (  
       0.297ms  0.257ms  0.174ms
2 adsl-65.myprovider.be ( 
       12.120ms 13.058ms 13.009ms
3 (  
       13.845ms 14.308ms 12.756ms
4 gigabitethernet2-2.intl2.gam.brussels.skynet.be (
       13.123ms 13.164ms 12.527ms
5 pecbru2.car.belbone.be ( 
       16.336ms 13.889ms 13.028ms
6 ser-2-1-110-ias-be-vil-ar01.kpnbelgium.be (
       14.602ms 15.546ms 15.959ms
7 unknown-195-207-939.eunet.be ( 
       16.514ms 17.661ms 18.889ms
8 S0-1-0.Leuven.Belgium.EU.net ( 
       22.714ms 19.193ms 18.432ms
9 dukat.Belgium.EU.net ( 22.758ms * 25.263ms

Specific domain name information can be queried using the whois command, as is explained by many whois servers, like the one below:

[emmy@pc10 emmy]$ whois cnn.com

Whois Server Version 1.3

	$<--snap server message-->

   Domain Name: CNN.COM
   Whois Server: whois.networksolutions.com
   Referral URL: http://www.networksolutions.com
   Name Server: TWDNS-01.NS.AOL.COM
   Name Server: TWDNS-02.NS.AOL.COM
   Name Server: TWDNS-03.NS.AOL.COM
   Name Server: TWDNS-04.NS.AOL.COM
   Updated Date: 12-mar-2002
>>> Last update of whois database: Fri, 5 Apr 2002 05:04:55 EST <<<

The Registry database contains ONLY .COM, .NET, .ORG, .EDU domains
and Registrars.


        $<--snap server message-->

Turner Broadcasting (CNN-DOM)
   1 CNN Center
   Atlanta, GA 30303

   Domain Name: CNN.COM

   Administrative Contact:
        $<--snap contactinfo-->   
   Technical Contact:
        $<--snap contactinfo-->   
   Billing Contact:
	$<--snap contactinfo-->
   Record last updated on 12-Mar-2002.
   Record expires on 23-Sep-2009.
   Record created on 22-Sep-1993.
   Database last updated on 4-Apr-2002 20:10:00 EST.

   Domain servers in listed order:


For other domain names than .com, .net, .org and .edu, specify the whois server, such as this one for .be domains:

whois domain.be@whois.dns.be