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
bound yet another IP address, 192.168.124.1, to the same interface,
simulating a remote server to demonstrate Apache’s
proxy server. The important feature to note here is that the address
192.168.124.1 is on a different IP network from the address
192.168.123.2, even though it shares the same physical network. No
subnet mask was needed in this case, as the error message it
suppressed arose from the fact that 192.168.123.2 and 192.168.123.3
are on the same network.
Unfortunately, each Unix implementation tends to do this slightly differently, so these commands may not work on your system. Check your manuals!
In real life, we do not have much to do with IP addresses. Web sites (and Internet hosts generally) are known by their names, such as www.butterthlies.com or sales.butterthlies.com , which we shall meet later. On the authors’ desktop system, these names both translate into 192.168.123.2. The distinction between them is made by Apache’ Virtual Hosting mechanism — see Chapter 4.