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Extensible Markup Language | 2003-04-15 | 16KB | 420 lines

<?xml version='1.0' encoding='UTF-8' ?> <!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd"> <?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?> <manualpage metafile="mass.xml.meta"> <parentdocument href="./">Virtual Hosts</parentdocument> <title>Dynamically configured mass virtual hosting</title> <summary> This document describes how to efficiently serve an arbitrary number of virtual hosts with Apache 1.3.  </summary> <section id="motivation"><title>Motivation</title> The techniques described here are of interest if your <code>httpd.conf</code> contains many <code><VirtualHost></code> sections that are substantially the same, for example: <example> NameVirtualHost 111.22.33.44 <VirtualHost 111.22.33.44> <indent> ServerName www.customer-1.com DocumentRoot /www/hosts/www.customer-1.com/docs ScriptAlias /cgi-bin/ /www/hosts/www.customer-1.com/cgi-bin </indent> </VirtualHost> <VirtualHost 111.22.33.44> <indent> ServerName www.customer-2.com DocumentRoot /www/hosts/www.customer-2.com/docs ScriptAlias /cgi-bin/ /www/hosts/www.customer-2.com/cgi-bin </indent> </VirtualHost> # blah blah blah <VirtualHost 111.22.33.44> <indent> ServerName www.customer-N.com DocumentRoot /www/hosts/www.customer-N.com/docs ScriptAlias /cgi-bin/ /www/hosts/www.customer-N.com/cgi-bin </indent> </VirtualHost> </example> The basic idea is to replace all of the static <code><VirtualHost></code> configuration with a mechanism that works it out dynamically. This has a number of advantages: <ol> <li>Your configuration file is smaller so Apache starts faster and uses less memory.</li> <li>Adding virtual hosts is simply a matter of creating the appropriate directories in the filesystem and entries in the DNS - you don't need to reconfigure or restart Apache.</li> </ol> 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.). </section> <section id="overview"><title>Overview</title> A virtual host is defined by two pieces of information: its IP address, and the contents of the <code>Host:</code> 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 <module>mod_vhost_alias</module>, but if you are using a version of Apache up to 1.3.6 then you must use <module>mod_rewrite</module>. 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 <code>ServerName</code> directive, and it is available to CGIs via the <code>SERVER_NAME</code> environment variable. The actual value used at run time is controlled by the <directive module="core">UseCanonicalName</directive> setting. With <code>UseCanonicalName Off</code> the server name comes from the contents of the <code>Host:</code> header in the request. With <code>UseCanonicalName DNS</code> 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 <code>Host:</code> header or the DNS lookup fails then the value configured with <code>ServerName</code> is used instead. The other thing to `fake' is the document root (configured with <code>DocumentRoot</code> and available to CGIs via the <code>DOCUMENT_ROOT</code> 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 <code>mod_vhost_alias</code> or <code>mod_rewrite</code>) which has a different way of doing the mapping. Neither of these modules is responsible for setting the <code>DOCUMENT_ROOT</code> environment variable so if any CGIs or SSI documents make use of it they will get a misleading value. </section> <section id="simple"><title>Simple dynamic virtual hosts</title> This extract from <code>httpd.conf</code> implements the virtual host arrangement outlined in the <a href="#motivation">Motivation</a> section above, but in a generic fashion using <code>mod_vhost_alias</code>. <example> # 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 </example> This configuration can be changed into an IP-based virtual hosting solution by just turning <code>UseCanonicalName Off</code> into <code>UseCanonicalName DNS</code>. The server name that is inserted into the filename is then derived from the IP address of the virtual host. </section> <section id="homepages"><title>A virtually hosted homepages system</title> 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 <code>www.user.isp.com</code> are found in <code>/home/user/</code>. It uses a single <code>cgi-bin</code> directory instead of one per virtual host. <example> # 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/ </example> There are examples of more complicated <code>VirtualDocumentRoot</code> settings in the <module>mod_vhost_alias</module> documentation. </section> <section id="combinations"><title>Using more than one virtual hosting system on the same server</title> With more complicated setups you can use Apache's normal <code><VirtualHost></code> 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 <code><VirtualHost></code> configuration sections. <example> UseCanonicalName Off LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon <Directory /www/commercial> <indent> Options FollowSymLinks AllowOverride All </indent> </Directory> <Directory /www/homepages> <indent> Options FollowSymLinks AllowOverride None </indent> </Directory> <VirtualHost 111.22.33.44> <indent> ServerName www.commercial.isp.com CustomLog logs/access_log.commercial vcommon VirtualDocumentRoot /www/commercial/%0/docs VirtualScriptAlias /www/commercial/%0/cgi-bin </indent> </VirtualHost> <VirtualHost 111.22.33.45> <indent> ServerName www.homepages.isp.com CustomLog logs/access_log.homepages vcommon VirtualDocumentRoot /www/homepages/%0/docs ScriptAlias /cgi-bin/ /www/std-cgi/ </indent> </VirtualHost> </example> </section> <section id="ipbased"><title>More efficient IP-based virtual hosting</title> After <a href="#simple">the first example</a> 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. <example> # 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 </example> </section> <section id="oldversion"><title>Using older versions of Apache</title> The examples above rely on <code>mod_vhost_alias</code> which appeared after version 1.3.6. If you are using a version of Apache without <code>mod_vhost_alias</code> then you can implement this technique with <code>mod_rewrite</code> 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 <code>%V</code> log format directive; in versions 1.3.0 - 1.3.3 the <code>%v</code> option did what <code>%V</code> does; version 1.3.4 has no equivalent. In all these versions of Apache the <code>UseCanonicalName</code> directive can appear in <code>.htaccess</code> files which means that customers can cause the wrong thing to be logged. Therefore the best thing to do is use the <code>%{Host}i</code> directive which logs the <code>Host:</code> header directly; note that this may include <code>:port</code> on the end which is not the case for <code>%V</code>. </section> <section id="simple.rewrite"><title>Simple dynamic virtual hosts using <code>mod_rewrite</code></title> This extract from <code>httpd.conf</code> does the same thing as <a href="#simple">the first example</a>. The first half is very similar to the corresponding part above but with some changes for backward compatibility and to make the <code>mod_rewrite</code> part work properly; the second half configures <code>mod_rewrite</code> to do the actual work. There are a couple of especially tricky bits: By default, <code>mod_rewrite</code> runs before the other URI translation modules (<code>mod_alias</code> etc.) so if they are used then <code>mod_rewrite</code> must be configured to accommodate them. Also, mome magic must be performed to do a per-dynamic-virtual-host equivalent of <code>ScriptAlias</code>. <example> # 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> <indent> # ExecCGI is needed here because we can't force # CGI execution in the way that ScriptAlias does Options FollowSymLinks ExecCGI </indent> </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! </example> </section> <section id="homepages.rewrite"><title>A homepages system using <code>mod_rewrite</code></title> This does the same thing as <a href="#homepages">the second example</a>. <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/ </example> </section> <section id="xtra-conf"><title>Using a separate virtual host configuration file</title> This arrangement uses more advanced <code>mod_rewrite</code> 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 <code>vhost.map</code> file contains something like this: <example> www.customer-1.com /www/customers/1 www.customer-2.com /www/customers/2 # ... www.customer-N.com /www/customers/N </example> The <code>http.conf</code> contains this: <example> 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 </example> </section> </manualpage>