This document describes how to efficiently serve an arbitrary number of virtual hosts with Apache 1.3.
mod_rewrite
mod_rewrite
The techniques described here are of interest if your httpd.conf
contains many <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 virtual host; however if you have very many virtual hosts then doing this is dubious anyway because it eats file descriptors. It is better to log to a pipe or a fifo and arrange for the process at the other end to distribute the logs to the customers (it can also accumulate statistics, etc.).
A virtual host is defined by two pieces of information: its IP address, and the contents of the Host:
header in the HTTP request. The dynamic mass virtual hosting technique is based on automatically inserting this information into the pathname of the file that is used to satisfy the request. This is done most easily using mod_vhost_alias
, but if you are using a version of Apache up to 1.3.6 then you must use mod_rewrite
. Both of these modules are disabled by default; you must enable one of them when configuring and building Apache if you want to use this technique.
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 which is used by Apache to generate self-referential URLs, etc. It is configured with the ServerName
directive, and it is available to CGIs via the SERVER_NAME
environment variable. The actual value used at run time is controlled by the UseCanonicalName
setting. With UseCanonicalName Off
the server name comes from the contents of the Host:
header in the request. With UseCanonicalName DNS
it comes from a reverse DNS lookup of the virtual host's IP address. The former setting is used for name-based dynamic virtual hosting, and the latter is used for IP-based hosting. If Apache cannot work out the server name because there is no Host:
header or the DNS lookup fails then the value configured with ServerName
is used instead.
The other thing to `fake' is the document root (configured with DocumentRoot
and available to CGIs via the DOCUMENT_ROOT
environment variable). In a normal configuration this setting is used by the core module when mapping URIs to filenames, but when the server is configured to do dynamic virtual hosting that job is taken over by another module (either mod_vhost_alias
or mod_rewrite
) which has a different way of doing the mapping. Neither of these modules is responsible for setting the DOCUMENT_ROOT
environment variable so if any CGIs or SSI documents make use of it they will get a misleading value.
This extract from httpd.conf
implements the virtual host arrangement outlined in the Motivation section above, but in a generic fashion using mod_vhost_alias
.
# get the server name from the Host: header UseCanonicalName Off # this log format can be split per-virtual-host based on the first field LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon CustomLog logs/access_log vcommon # include the server name in the filenames used to satisfy requests VirtualDocumentRoot /www/hosts/%0/docs VirtualScriptAlias /www/hosts/%0/cgi-bin
This configuration can be changed into an IP-based virtual hosting solution by just turning UseCanonicalName Off
into UseCanonicalName DNS
. The server name that is inserted into the filename is then derived from the IP address of the virtual host.
This is an adjustment of the above system tailored for an ISP's homepages server. Using a slightly more complicated configuration 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.
# all the preliminary stuff is the same as above, then # include part of the server name in the filenames VirtualDocumentRoot /www/hosts/%2/docs # single cgi-bin directory ScriptAlias /cgi-bin/ /www/std-cgi/
There are examples of more complicated VirtualDocumentRoot
settings in the mod_vhost_alias
documentation.
With more complicated setups you can use Apache's normal <VirtualHost>
directives to control the scope of the various virtual hosting 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 conventional <VirtualHost>
configuration sections.
UseCanonicalName Off LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon <Directory /www/commercial> Options FollowSymLinks AllowOverride All </Directory> <Directory /www/homepages> Options FollowSymLinks AllowOverride None </Directory> <VirtualHost 111.22.33.44> ServerName www.commercial.isp.com CustomLog logs/access_log.commercial vcommon VirtualDocumentRoot /www/commercial/%0/docs VirtualScriptAlias /www/commercial/%0/cgi-bin </VirtualHost> <VirtualHost 111.22.33.45> ServerName www.homepages.isp.com CustomLog logs/access_log.homepages vcommon VirtualDocumentRoot /www/homepages/%0/docs ScriptAlias /cgi-bin/ /www/std-cgi/ </VirtualHost>
After the first example I noted that it is easy to turn it into an IP-based virtual hosting setup. Unfortunately that configuration is not very efficient because it requires a DNS lookup for every request. This can be avoided by laying out the filesystem according to the IP addresses themselves rather than the corresponding names and changing the logging similarly. Apache will then usually not need to work out the server name and so incur a DNS lookup.
# get the server name from the reverse DNS of the IP address UseCanonicalName DNS # include the IP address in the logs so they may be split LogFormat "%A %h %l %u %t \"%r\" %s %b" vcommon CustomLog logs/access_log vcommon # include the IP address in the filenames VirtualDocumentRootIP /www/hosts/%0/docs VirtualScriptAliasIP /www/hosts/%0/cgi-bin
The examples above rely on mod_vhost_alias
which appeared after version 1.3.6. If you are using a version of Apache without mod_vhost_alias
then you can implement this technique with mod_rewrite
as illustrated below, but only for Host:-header-based virtual hosts.
In addition there are some things to beware of with logging. Apache 1.3.6 is the first version to include the %V
log format directive; in versions 1.3.0 - 1.3.3 the %v
option did what %V
does; version 1.3.4 has no equivalent. In all these versions of Apache the UseCanonicalName
directive can appear in .htaccess
files which means that customers can cause the wrong thing to be logged. Therefore the best thing to do is use the %{Host}i
directive which logs the Host:
header directly; note that this may include :port
on the end which is not the case for %V
.
mod_rewrite
This extract from httpd.conf
does the same thing as the first example. The first half is very similar to the corresponding part above but with some changes for backward compatibility and to make the mod_rewrite
part work properly; the second half configures mod_rewrite
to do the actual work.
There are a couple of especially tricky bits: By default, mod_rewrite
runs before the other URI translation modules (mod_alias
etc.) so if they are used then mod_rewrite
must be configured to accommodate them. Also, mome magic must be performed to do a per-dynamic-virtual-host equivalent of ScriptAlias
.
# get the server name from the Host: header UseCanonicalName Off # splittable logs LogFormat "%{Host}i %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!
mod_rewrite
This does the same thing as the second example.
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 ${lowercase:%{SERVER_NAME}} ^www\.[a-z-]+\.isp\.com$ # 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:%{SERVER_NAME}}$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 more advanced mod_rewrite
features to get the translation from virtual host to document root from a separate configuration file. This provides more flexibility but requires more complicated 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