Apache: The Definitive Guide, 3rd Edition

Dynamically Configured Virtual Hosting

An even neater method of managing Virtual Hosting is provided by mod_vhost_alias, which lets you define a single boilerplate configuration and then fills in the details at service time from the IP address and or the Host header in the HTTP request.

All the directives in this module interpolate a string into a pathname. The interpolated string (called the “name”) may be either the server name (see the UseCanonicalName directive for details on how this is determined) or the IP address of the virtual host on the server in dotted-quad format (xxx.xxx.xxx.xxx).

The interpolation is controlled by a mantra, %<code-letter>, which is replaced by some value you supply in the Config file. It’s not unlike the controls for logging — see Chapter 10.

These are the possible formats:

%%

Insert a literal %.

%p

Insert the port number of the virtual host.

%N.M

Insert (part of ) the name. N and M are numbers, used to specify substrings of the name. N selects from the dot-separated components of the name, and M selects characters within whatever N has selected. M is optional and defaults to zero if it isn’t present. The dot must be present if and only if M is present. If we are trying to parse sales.butterthlies.com, the interpretation of N is as follows:

0

The whole name: sales.butterthlies.com

1

The first part: sales

2

The second part: butterthlies

-1

The last part: com

-2

The penultimate part: butterthlies

2+

The second and all subsequent parts: butterthlies.com ...