Apache: The Definitive Guide, 3rd Edition

Multiple sites: Unix

It is fortunate that the crucial Unix utility ifconfig , which binds IP addresses to physical interfaces, often allows the binding of multiple IP numbers to a single interface so that people can switch from one IP number to another and maintain service during the transition. This is known as “IP aliasing” and can be used to maintain multiple “virtual” web servers on a single machine.

In practical terms, on many versions of Unix, we run ifconfig to give multiple IP addresses to the same interface. The interface in this context is actually the bit of software — the driver — that handles the physical connection (Ethernet card, serial port, etc.) to the outside. While writing this book, we accessed the practice sites through an Ethernet connection between a Windows 95 machine (the client) and a FreeBSD box (the server) running Apache.

Our environment was very untypical, since the whole thing sat on a desktop with no access to the Web. The FreeBSD box was set up using ifconfig in a script lan_setup, which contained the following lines:

ifconfig ep0 192.168.123.2
ifconfig ep0 192.168.123.3 alias netmask 0xFFFFFFFF
ifconfig ep0 192.168.124.1 alias

The first line binds the IP address 192.168.123.2 to the physical interface ep0. The second binds an alias of 192.168.123.3 to the same interface. We used a subnet mask (netmask 0xFFFFFFFF) to suppress a tedious error message generated by the FreeBSD TCP/IP stack. This address was used to demonstrate virtual hosts. We also ...