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
-2+
The penultimate and all preceding parts: www.butterthlies
1+ and -1+
The same as 0: sales.butterthlies.com
If N or M is greater than the
number of parts available, a single underscore is interpolated.