This document describes how to efficiently serve an arbitrary number of virtual hosts with the Apache HTTP Server. A separate document discusses using
mod_rewrite to create dynamic mass virtual hosts.
The techniques described here are of interest if your
httpd.conf contains many
<VirtualHost> sections that are substantially the same, for example:
<VirtualHost 126.96.36.199> ServerName customer-1.example.com DocumentRoot "/www/hosts/customer-1.example.com/docs" ScriptAlias "/cgi-bin/" "/www/hosts/customer-1.example.com/cgi-bin" </VirtualHost> <VirtualHost 188.8.131.52> ServerName customer-2.example.com DocumentRoot "/www/hosts/customer-2.example.com/docs" ScriptAlias "/cgi-bin/" "/www/hosts/customer-2.example.com/cgi-bin" </VirtualHost> <VirtualHost 184.108.40.206> ServerName customer-N.example.com DocumentRoot "/www/hosts/customer-N.example.com/docs" ScriptAlias "/cgi-bin/" "/www/hosts/customer-N.example.com/cgi-bin" </VirtualHost>
We wish to replace these multiple
<VirtualHost> blocks with a mechanism that works them 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 many virtual hosts, doing this can be a bad idea anyway, because of the number of file descriptors needed. It is better to log to a pipe or a fifo, and arrange for the process at the other end to split up the log files into one per virtual host. One example of such a process can be found in the split-logfile utility.
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 used here is based on automatically inserting this information into the pathname of the file that is used to satisfy the request. This can be most easily done by using
mod_vhost_alias with Apache httpd. Alternatively, mod_rewrite can be used.
Both of these modules are disabled by default; you must enable one of them when configuring and building Apache httpd if you want to use this technique.
A couple of things need to be determined from the request in order to make the dynamic virtual host look like a normal one. The most important is the server name, which is used by the server 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 is taken from the contents of the
Host: header in the request. With
UseCanonicalName DNS, it is taken 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 httpd 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 determine is the document root (configured with
DocumentRoot and available to CGI scripts via the
DOCUMENT_ROOT environment variable). In a normal configuration, this is used by the core module when mapping URIs to filenames, but when the server is configured to do dynamic virtual hosting, that job must be taken over by another module (either
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.
# get the server name from the Host: header UseCanonicalName Off # this log format can be split per-virtual-host based on the first field # using the split-logfile utility. 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. The variable
%0 references the requested servername, as indicated in the
mod_vhost_alias documentation for more usage examples.
This is an adjustment of the above system, tailored for an ISP's web hosting server. Using
%2, we can select substrings of the server name to use in the filename so that, for example, the documents for
www.user.example.com are found in
/home/user/www. It uses a single
cgi-bin directory instead of one per virtual host.
UseCanonicalName Off LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon CustomLog logs/access_log vcommon # include part of the server name in the filenames VirtualDocumentRoot "/home/%2/www" # single cgi-bin directory ScriptAlias "/cgi-bin/" "/www/std-cgi/"
There are examples of more complicated
VirtualDocumentRoot settings in the
With more complicated setups, you can use httpd's normal
<VirtualHost> directives to control the scope of the various virtual hosting configurations. For example, you could have one IP address for general customers' homepages, and another for commercial customers, with the following setup. This can be combined with conventional
<VirtualHost> configuration sections, as shown below.
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 220.127.116.11> ServerName www.commercial.example.com CustomLog "logs/access_log.commercial" vcommon VirtualDocumentRoot "/www/commercial/%0/docs" VirtualScriptAlias "/www/commercial/%0/cgi-bin" </VirtualHost> <VirtualHost 18.104.22.168> ServerName www.homepages.example.com CustomLog "logs/access_log.homepages" vcommon VirtualDocumentRoot "/www/homepages/%0/docs" ScriptAlias "/cgi-bin/" "/www/std-cgi/" </VirtualHost>
If the first VirtualHost block does not include a
ServerName directive, the reverse DNS of the relevant IP will be used instead. If this is not the server name you wish to use, a bogus entry (eg.
ServerName none.example.com) can be added to get around this behaviour.
The configuration changes suggested to turn the first example into an IP-based virtual hosting setup result in a rather inefficient setup. A new DNS lookup is required for every request. To avoid this overhead, the filesystem can be arranged to correspond to the IP addresses, instead of to the host names, thereby negating the need for a DNS lookup. Logging will also have to be adjusted to fit this system.
# 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"
Mass virtual hosting may also be accomplished using
mod_rewrite, either using simple
RewriteRule directives, or using more complicated techniques such as storing the vhost definitions externally and accessing them via
RewriteMap. These techniques are discussed in the rewrite documentation.
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Licensed under the Apache License, Version 2.0.