![[APACHE DOCUMENTATION]](../images/sub.gif)
This document describes how to efficiently serve an arbitrary number
of virtual hosts with Apache 1.3. Some familiarity with
mod_rewrite is
useful.
The techniques described here are of interest if your
httpd.conf contains hundreds of
<VirtualHost> sections that are substantially the
same, for example:
NameVirtualHost 111.22.33.44 <VirtualHost 111.22.33.44> ServerName www.customer-1.com DocumentRoot /www/hosts/www.customer-1.com/docs ScriptAlias /cgi-bin/ /www/hosts/www.customer-1.com/cgi-bin </VirtualHost> <VirtualHost 111.22.33.44> ServerName www.customer-2.com DocumentRoot /www/hosts/www.customer-2.com/docs ScriptAlias /cgi-bin/ /www/hosts/www.customer-2.com/cgi-bin </VirtualHost> # blah blah blah <VirtualHost 111.22.33.44> ServerName www.customer-N.com DocumentRoot /www/hosts/www.customer-N.com/docs ScriptAlias /cgi-bin/ /www/hosts/www.customer-N.com/cgi-bin </VirtualHost>
The basic idea is to replace all of the static
<VirtualHost> configuration with a mechanism that
works it out dynamically. This has a number of advantages:
The main disadvantage is that you cannot have a different log file
for each server; however if you have very many virtual hosts then
doing this is dubious anyway because it eats file descriptors. It's
better to log to a pipe or a fifo and arrange for the process at the
other end to distribute the logs (and perhaps accumulate statistics,
etc.). A LogFormat directive that includes
%{SERVER_NAME}e for the virtual host makes it easy to do this.
All of the dynamic virtual hosts will either be configured as part
of the main server configuration, or within a
<VirtualHost> section. For a simple (very uniform)
setup, <VirtualHost> sections aren't needed at all.
A couple of things need to be `faked' to make the dynamic virtual
host look like a normal one. The most important is the server name
(configured with ServerName and available to CGIs via the
SERVER_NAME environment variable). The way it is
determined is controlled by the UseCanonicalName
directive: with UseCanonicalName off the server name
comes from the contents of the Host: header in the
request. If there is no Host: header then the value
configured with ServerName is used instead.
The other one is the document root (configured with
DocumentRoot and available to CGIs via the
DOCUMENT_ROOT environment variable). This is used by the
core module when mapping URIs to filenames, but in the context of
dynamic virtual hosting its value only matters if any CGIs or SSI
documents make use of the DOCUMENT_ROOT environment
variable. This is an Apache extension to the CGI specification and as
such shouldn't really be relied upon, especially because this
technique breaks it: there isn't currently a way of setting
DOCUMENT_ROOT dynamically.
The meat of the mechanism works via Apache's URI-to-filename
translation API phase. This is used by a number of modules:
mod_rewrite,
mod_alias,
mod_userdir,
and the core module.
In the default configuration these modules are called in that order
and given a chance to say that they know what the filename is. Most of
these modules do it in a fairly simple fashion (e.g. the core module
concatenates the document root and the URI) except for
mod_rewrite, which provides enough functionality to do
all sorts of sick and twisted things (like dynamic virtual hosting).
Note that because of the order in which the modules are called, using
a mod_rewrite configuration that matches any URI means
that the other modules (particularly mod_alias) will
cease to function. The examples below show how to deal with this.
The dynamic virtual hosting idea is very simple: use the server name as well as the URI to determine the corresponding filename.
This extract from httpd.conf implements the virtual
host arrangement outlined in the Motivation
section above, but in a generic fashion.
The first half shows some other configuration options that are
needed to make the mod_rewrite part work as expected; the
second half uses mod_rewrite to do the actual work. Some
care is taken to do a per-dynamic-virtual-host equivalent of
ScriptAlias.
# dynamic ServerName
UseCanonicalName Off
# splittable logs
LogFormat "%{SERVER_NAME}e %h %l %u %t \"%r\" %s %b" vcommon
CustomLog logs/access_log vcommon
<Directory /www/hosts>
# ExecCGI is needed here because we can't force
# CGI execution in the way that ScriptAlias does
Options FollowSymLinks ExecCGI
</Directory>
# now for the hard bit
RewriteEngine On
# a ServerName derived from a Host: header may be any case at all
RewriteMap lowercase int:tolower
## deal with normal documents first:
# allow Alias /icons/ to work - repeat for other aliases
RewriteCond %{REQUEST_URI} !^/icons/
# allow CGIs to work
RewriteCond %{REQUEST_URI} !^/cgi-bin/
# do the magic
RewriteRule ^/(.*)$ /www/hosts/${lowercase:%{SERVER_NAME}}/docs/$1
## and now deal with CGIs - we have to force a MIME type
RewriteCond %{REQUEST_URI} ^/cgi-bin/
RewriteRule ^/(.*)$ /www/hosts/${lowercase:%{SERVER_NAME}}/cgi-bin/$1 [T=application/x-httpd-cgi]
# that's it!
This is an adjustment of the above system tailored for an ISP's
homepages server. Using slightly more complicated rewriting rules we
can select substrings of the server name to use in the filename so
that e.g. the documents for www.user.isp.com are found in
/home/user/. It uses a single cgi-bin
directory instead of one per virtual host.
RewriteEngine on
RewriteMap lowercase int:tolower
# allow CGIs to work
RewriteCond %{REQUEST_URI} !^/cgi-bin/
# check the hostname is right so that the RewriteRule works
RewriteCond ${lowercase:%{HTTP_HOST}} ^www\.[a-z-]+\.isp\.com$
# concatenate the virtual host name onto the start of the URI
# the [C] means do the next rewrite on the result of this one
RewriteRule ^(.+) ${lowercase:%{HTTP_HOST}}$1 [C]
# now create the real file name
RewriteRule ^www\.([a-z-]+)\.isp\.com/(.*) /home/$1/$2
# define the global CGI directory
ScriptAlias /cgi-bin/ /www/std-cgi/
This arrangement uses a separate configuration file to specify the translation from virtual host to document root. This provides more flexibility but requires more configuration.
The vhost.map file contains something like this:
www.customer-1.com /www/customers/1 www.customer-2.com /www/customers/2 # ... www.customer-N.com /www/customers/N
The http.conf contains this:
RewriteEngine on
RewriteMap lowercase int:tolower
# define the map file
RewriteMap vhost txt:/www/conf/vhost.map
# deal with aliases as above
RewriteCond %{REQUEST_URI} !^/icons/
RewriteCond %{REQUEST_URI} !^/cgi-bin/
RewriteCond ${lowercase:%{SERVER_NAME}} ^(.+)$
# this does the file-based remap
RewriteCond ${vhost:%1} ^(/.*)$
RewriteRule ^/(.*)$ %1/docs/$1
RewriteCond %{REQUEST_URI} ^/cgi-bin/
RewriteCond ${lowercase:%{SERVER_NAME}} ^(.+)$
RewriteCond ${vhost:%1} ^(/.*)$
RewriteRule ^/(.*)$ %1/cgi-bin/$1
With more complicated setups, you can use Apache's normal
<VirtualHost> directives to control the scope of
the various rewrite configurations. For example, you could have one IP
address for homepages customers and another for commercial customers
with the following setup. This can of course be combined with
convential <VirtualHost> configuration
sections.
UseCanonicalName Off
LogFormat "%{SERVER_NAME}e %h %l %u %t \"%r\" %s %b" vcommon
CustomLog logs/access_log vcommon
<Directory /www/commercial>
Options FollowSymLinks ExecCGI
AllowOverride All
</Directory>
<Directory /www/homepages>
Options FollowSymLinks
AllowOverride None
</Directory>
<VirtualHost 111.22.33.44>
ServerName www.commercial.isp.com
RewriteEngine On
RewriteMap lowercase int:tolower
RewriteCond %{REQUEST_URI} !^/icons/
RewriteCond %{REQUEST_URI} !^/cgi-bin/
RewriteRule ^/(.*)$ /www/commercial/${lowercase:%{SERVER_NAME}}/docs/$1
RewriteCond %{REQUEST_URI} ^/cgi-bin/
RewriteRule ^/(.*)$ /www/commercial/${lowercase:%{SERVER_NAME}}/cgi-bin/$1 [T=application/x-httpd-cgi]
</VirtualHost>
<VirtualHost 111.22.33.45>
ServerName www.homepages.isp.com
RewriteEngine on
RewriteMap lowercase int:tolower
RewriteCond %{REQUEST_URI} !^/cgi-bin/
RewriteCond ${lowercase:%{HTTP_HOST}} ^www\.[a-z-]+\.isp\.com$
RewriteRule ^(.+) ${lowercase:%{HTTP_HOST}}$1 [C]
RewriteRule ^www\.([a-z-]+)\.isp\.com/(.*) /www/homepages/$1/$2
ScriptAlias /cgi-bin/ /www/std-cgi/
</VirtualHost>
