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Linux Network Administrators Guide


5.7.1. The Loopback Interface

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


 # ifconfig lo 

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 :


 # Sample /etc/hosts entry for localhost

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: Mask:
 RX packets:0 errors:0 dropped:0 overruns:0 frame:0
 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0

As you can see, the loopback interface has been assigned a netmask of , since 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 . This is accomplished by using:


 # route add 

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.[1] 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 ( 56 data bytes
 64 bytes from icmp_seq=0 ttl=255 time=0.4 ms
 64 bytes from icmp_seq=1 ttl=255 time=0.4 ms
 64 bytes from icmp_seq=2 ttl=255 time=0.4 ms
 --- 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 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.[2] Therefore, you always have to configure it, regardless of whether your machine is attached to a network or not.

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