Linux Network Administrator Guide, Second Edition

networks file like this:[30]

# /etc/networks for the Virtual Brewery
brew-net 172.16.1.0
wine-net 172.16.2.0

Interface Configuration for IP

After setting up your hardware as explained in Chapter 4, Configuring the Serial Hardware, you have to make these devices known to the kernel networking software. A couple of commands are used to configure the network interfaces and initialize the routing table. These tasks are usually performed from the network initialization script each time you boot the system. The basic tools for this process are called ifconfig (where 'if' stands for interface) and route.

ifconfig is used to make an interface accessible to the kernel networking layer. This involves the assignment of an IP address and other parameters, and activation of the interface, also known as 'bringing up' the interface. Being active here means that the kernel will send and receive IP datagrams through the interface. The simplest way to invoke it is with:

ifconfig interface ip-address

This command assigns ip-address to interface and activates it. All other parameters are set to default values. For instance, the default network mask is derived from the network class of the IP address, such as 255.255.0.0 for a class B address. ifconfig is described in detail in the section 'All About ifconfig'.

route allows you to add or remove routes from the kernel routing table. It can be invoked as:

route [add|del] [-net|-host] target [if]

The add and del arguments determine whether to add or delete the route to target. The -net and - host arguments tell the route command whether the target is a network or a host (a host is assumed if you don't specify). The if argument is again optional, and allows you to specify to which network interface the route should be directed - the Linux kernel makes a sensible guess if you don't supply this information. This topic will be explained in more detail in succeeding sections.

The Loopback Interface

The very first interface to be activated is the loopback interface:

# ifconfig lo 127.0.0.1

Occasionally, you will see the dummy hostname localhost being used instead of the IP address. ifconfig will look up the name in the hosts file, where an entry should declare it as the hostname for 127.0.0.1:

# Sample /etc/hosts entry for localhost
localhost 127.0.0.1

To view the configuration of an interface, you invoke ifconfig, giving it only the interface name as argument:

$ ifconfig lo
lo Link encap:Local Loopback
   inet addr:127.0.0.1 Mask:255.0.0.0
   UP LOOPBACK RUNNING MTU:3924 Metric:1
   RX packets:0 errors:0 dropped:0 overruns:0 frame:0

   TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
   Collisions:0

As you can see, the loopback interface has been assigned a netmask of 255.0.0.0, since 127.0.0.1 is a class A address.

Now you can almost start playing with your mini-network. What is still missing is an entry in the routing table that tells IP that it may use this interface as a route to destination 127.0.0.1. This is accomplished by using:

# route add 127.0.0.1

Again, you can use localhost instead of the IP address, provided you've entered it into your /etc/hosts.

Next, you should check that everything works fine, for example by using ping. ping is the networking equivalent of a sonar device.[31] The command is used to verify that a given address is actually reachable, and to measure the delay that occurs when sending a datagram to it and back again. The time required for this process is often referred to as the 'round-trip time':

# ping localhost
PING localhost (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=0.4 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=255 time=0.4 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=255 time=0.4 ms
^C
 -- localhost ping statistics -- 3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.4/0.4/0.4 ms
#

When you invoke ping as shown here, it will continue emitting packets forever, unless interrupted by the user. The ^C marks the place where we pressed Ctrl-C.

The previous example shows that packets for 127.0.0.1 are properly delivered and a reply is returned to ping almost instantaneously. This shows that you have successfully set up your first network interface.

If the output you get from ping does not resemble that shown in the previous example, you are in trouble. Check any errors if they indicate that some file hasn't been installed properly. Check that the ifconfig and route binaries you use are compatible with the kernel release you run, and above all, that the kernel has been compiled with networking enabled (you see this from the presence of the /proc/net directory). If you get an error message saying 'Network unreachable,' you probably got the route command wrong. Make sure you use the same address you gave to ifconfig.

The steps previously described are enough to use networking applications on a standalone host. After adding the lines mentioned earlier to your network initialization script and making sure it will be executed at boot time, you may reboot your machine and try out various applications. For instance, telnet localhost should establish a telnet connection to your host, giving you a login: prompt.

However, the loopback interface is useful not only as an example in networking books, or as a test bed during development, but is actually used by some applications during normal operation.[32] Therefore, you always have to configure it, regardless of whether your machine is attached to a network or not.