IP Aliasing

In some cases, you may want to assign a single Linux system that has only one network interface to two or more IP addresses. For example, you may want to run different Web sites that can be accessed with separate IP addresses on this same system. In effect, you are setting up an alias for your system, another address by which it can be accessed. In fact, you are assigning two IP addresses to the same network interface—for example, assigning a single Ethernet card two IP addresses. This procedure, referred to as IP aliasing, is used to set up multiple IP-based virtual hosts for Internet servers. This method enables you to run several Web servers on the same machine using a single interface (or more than one on each of several interfaces). See Chapters 21 and 22 for FTP and Web server information about virtual hosts, and Chapter 34 for Domain Name Service configuration.

Setting up an IP alias is a simple matter of configuring a network interface on your system to listen for the added IP address. Your system needs to know what IP addresses it should listen for and on what network interface. You set up IP aliases using the ifconfig and route commands, or a network administrative tool.

To add another address to the same interface, you need to qualify the interface by adding a colon and a number. For example, if you are adding another IP address to the first Ethernet card (eth0), you would add a :0 to its interface name, eth0:0. The following example shows the ifconfig and route commands for the Ethernet interface 192.168.1.2 and two IP aliases added to it: 192.168.1.100 and 192.168.1.101. To add yet another IP address to this same interface, you would use eth0:1, incrementing the qualifier, and so on. The first ifconfig command assigns the main IP address, 192.168.1.2, to the first Ethernet device, eth0. Then, two other IP addresses are assigned to that same device. In the first route command, the network route is set up for the Ethernet device, and then routes are set up for each IP interface. The interfaces for the two aliases are indicated with eth0:0 and eth0:1:

ifconfig eth0 192.168.1.2
ifconfig eth0:0 192.168.1.100
ifconfig eth0:1 192.168.1.101
route add -net 192.168.1.0 dev eth0
route add -host 192.168.1.2 dev eth0
route add -host 192.168.1.100 dev eth0:0
route add -host 192.168.1.101 dev eth0:1

IP aliasing must be supported by the kernel before you can use it. If your kernel does not support it, you may have to rebuild the kernel (including IP aliasing support), or use loadable modules to add IP aliasing.