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<title>An In-Depth Discussion of Virtual Host Matching - Apache HTTP Server</title>
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
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<p class="apache">Apache HTTP Server Version 2.2</p>
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<img alt="" src="../images/feather.gif" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.2</a> > <a href="./">Virtual Hosts</a></div><div id="page-content"><div id="preamble"><h1>An In-Depth Discussion of Virtual Host Matching</h1>
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<p><span>Available Languages: </span><a href="../en/vhosts/details.html" title="English"> en </a> |
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<a href="../fr/vhosts/details.html" hreflang="fr" rel="alternate" title="Fran�ais"> fr </a> |
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<a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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<p>The virtual host code was completely rewritten in
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<strong>Apache 1.3</strong>. This document attempts to explain
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exactly what Apache does when deciding what virtual host to
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serve a hit from. With the help of the new
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<code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code>
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directive virtual host configuration should be a lot easier and
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safer than with versions prior to 1.3.</p>
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<p>If you just want to <cite>make it work</cite> without
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understanding how, here are <a href="examples.html">some
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<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#configparsing">Config File Parsing</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#hostmatching">Virtual Host Matching</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#tips">Tips</a></li>
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<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<h2><a name="configparsing" id="configparsing">Config File Parsing</a></h2>
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<p>There is a <em>main_server</em> which consists of all the
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definitions appearing outside of
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<code><VirtualHost></code> sections. There are virtual
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servers, called <em>vhosts</em>, which are defined by
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<code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>
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<code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code>,
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<code class="directive"><a href="../mod/core.html#servername">ServerName</a></code>,
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<code class="directive"><a href="../mod/core.html#serverpath">ServerPath</a></code>,
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and <code class="directive"><a href="../mod/core.html#serveralias">ServerAlias</a></code>
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can appear anywhere within the definition of a server. However,
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each appearance overrides the previous appearance (within that
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<p>The default value of the <code>Listen</code> field for
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main_server is 80. The main_server has no default
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<code>ServerPath</code>, or <code>ServerAlias</code>. The
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default <code>ServerName</code> is deduced from the server's IP
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<p>The main_server Listen directive has two functions. One
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function is to determine the default network port Apache will
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bind to. The second function is to specify the port number
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which is used in absolute URIs during redirects.</p>
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<p>Unlike the main_server, vhost ports <em>do not</em> affect
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what ports Apache listens for connections on.</p>
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<p>Each address appearing in the <code>VirtualHost</code>
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directive can have an optional port. If the port is unspecified
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it defaults to the value of the main_server's most recent
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<code>Listen</code> statement. The special port <code>*</code>
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indicates a wildcard that matches any port. Collectively the
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entire set of addresses (including multiple <code>A</code>
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record results from DNS lookups) are called the vhost's
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<em>address set</em>.</p>
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<p>Unless a <code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code>
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directive is used for a specific IP address the first vhost
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with that address is treated as an IP-based vhost. The IP
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address can also be the wildcard <code>*</code>.</p>
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<p>If name-based vhosts should be used a
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<code>NameVirtualHost</code> directive <em>must</em> appear
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with the IP address set to be used for the name-based vhosts.
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In other words, you must specify the IP address that holds the
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hostname aliases (CNAMEs) for your name-based vhosts via a
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<code>NameVirtualHost</code> directive in your configuration
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<p>Multiple <code>NameVirtualHost</code> directives can be used
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each with a set of <code>VirtualHost</code> directives but only
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one <code>NameVirtualHost</code> directive should be used for
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each specific IP:port pair.</p>
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<p>The ordering of <code>NameVirtualHost</code> and
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<code>VirtualHost</code> directives is not important which
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makes the following two examples identical (only the order of
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the <code>VirtualHost</code> directives for <em>one</em>
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address set is important, see below):</p>
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<td><div class="example"><p><code>
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NameVirtualHost 111.22.33.44<br />
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<VirtualHost 111.22.33.44><br />
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</VirtualHost><br />
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<VirtualHost 111.22.33.44><br />
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</VirtualHost><br />
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NameVirtualHost 111.22.33.55<br />
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<VirtualHost 111.22.33.55><br />
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</VirtualHost><br />
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<VirtualHost 111.22.33.55><br />
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</code></p></div></td>
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<td><div class="example"><p><code>
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<VirtualHost 111.22.33.44><br />
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</VirtualHost><br />
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<VirtualHost 111.22.33.55><br />
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</VirtualHost><br />
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<VirtualHost 111.22.33.44><br />
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</VirtualHost><br />
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<VirtualHost 111.22.33.55><br />
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</VirtualHost><br />
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NameVirtualHost 111.22.33.44<br />
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NameVirtualHost 111.22.33.55<br />
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</code></p></div></td>
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<p>(To aid the readability of your configuration you should
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prefer the left variant.)</p>
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<p>After parsing the <code>VirtualHost</code> directive, the
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vhost server is given a default <code>Listen</code> equal to the
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port assigned to the first name in its <code>VirtualHost</code>
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<p>The complete list of names in the <code>VirtualHost</code>
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directive are treated just like a <code>ServerAlias</code> (but
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are not overridden by any <code>ServerAlias</code> statement)
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if all names resolve to the same address set. Note that
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subsequent <code>Listen</code> statements for this vhost will not
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affect the ports assigned in the address set.</p>
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<p>During initialization a list for each IP address is
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generated and inserted into an hash table. If the IP address is
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used in a <code>NameVirtualHost</code> directive the list
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contains all name-based vhosts for the given IP address. If
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there are no vhosts defined for that address the
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<code>NameVirtualHost</code> directive is ignored and an error
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is logged. For an IP-based vhost the list in the hash table is
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<p>Due to a fast hashing function the overhead of hashing an IP
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address during a request is minimal and almost not existent.
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Additionally the table is optimized for IP addresses which vary
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in the last octet.</p>
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<p>For every vhost various default values are set. In
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<li>If a vhost has no <code class="directive"><a href="../mod/core.html#serveradmin">ServerAdmin</a></code>,
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<code class="directive"><a href="../mod/core.html#resourceconfig">ResourceConfig</a></code>,
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<code class="directive"><a href="../mod/core.html#accessconfig">AccessConfig</a></code>,
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<code class="directive"><a href="../mod/core.html#timeout">Timeout</a></code>,
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<code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code>,
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<code class="directive"><a href="../mod/core.html#keepalive">KeepAlive</a></code>,
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<code class="directive"><a href="../mod/core.html#maxkeepaliverequests">MaxKeepAliveRequests</a></code>,
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<code class="directive"><a href="../mod/core.html#receivebuffersize">ReceiveBufferSize</a></code>,
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or <code class="directive"><a href="../mod/core.html#sendbuffersize">SendBufferSize</a></code>
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directive then the respective value is inherited from the
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main_server. (That is, inherited from whatever the final
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setting of that value is in the main_server.)</li>
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<li>The "lookup defaults" that define the default directory
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permissions for a vhost are merged with those of the
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main_server. This includes any per-directory configuration
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information for any module.</li>
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<li>The per-server configs for each module from the
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main_server are merged into the vhost server.</li>
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<p>Essentially, the main_server is treated as "defaults" or a
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"base" on which to build each vhost. But the positioning of
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these main_server definitions in the config file is largely
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irrelevant -- the entire config of the main_server has been
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parsed when this final merging occurs. So even if a main_server
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definition appears after a vhost definition it might affect the
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vhost definition.</p>
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<p>If the main_server has no <code>ServerName</code> at this
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point, then the hostname of the machine that <code class="program"><a href="../programs/httpd.html">httpd</a></code>
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is running on is used instead. We will call the <em>main_server address
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set</em> those IP addresses returned by a DNS lookup on the
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<code>ServerName</code> of the main_server.</p>
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<p>For any undefined <code>ServerName</code> fields, a
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name-based vhost defaults to the address given first in the
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<code>VirtualHost</code> statement defining the vhost.</p>
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<p>Any vhost that includes the magic <code>_default_</code>
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wildcard is given the same <code>ServerName</code> as the
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="hostmatching" id="hostmatching">Virtual Host Matching</a></h2>
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<p>The server determines which vhost to use for a request as
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<h3><a name="hashtable" id="hashtable">Hash table lookup</a></h3>
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<p>When the connection is first made by a client, the IP
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address to which the client connected is looked up in the
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internal IP hash table.</p>
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<p>If the lookup fails (the IP address wasn't found) the
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request is served from the <code>_default_</code> vhost if
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there is such a vhost for the port to which the client sent the
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request. If there is no matching <code>_default_</code> vhost
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the request is served from the main_server.</p>
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<p>If the IP address is not found in the hash table then the
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match against the port number may also result in an entry
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corresponding to a <code>NameVirtualHost *</code>, which is
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subsequently handled like other name-based vhosts.</p>
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<p>If the lookup succeeded (a corresponding list for the IP
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address was found) the next step is to decide if we have to
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deal with an IP-based or a name-base vhost.</p>
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<h3><a name="ipbased" id="ipbased">IP-based vhost</a></h3>
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<p>If the entry we found has an empty name list then we have
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found an IP-based vhost, no further actions are performed and
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the request is served from that vhost.</p>
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<h3><a name="namebased" id="namebased">Name-based vhost</a></h3>
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<p>If the entry corresponds to a name-based vhost the name list
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contains one or more vhost structures. This list contains the
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vhosts in the same order as the <code>VirtualHost</code>
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directives appear in the config file.</p>
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<p>The first vhost on this list (the first vhost in the config
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file with the specified IP address) has the highest priority
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and catches any request to an unknown server name or a request
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without a <code>Host:</code> header field.</p>
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<p>If the client provided a <code>Host:</code> header field the
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list is searched for a matching vhost and the first hit on a
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<code>ServerName</code> or <code>ServerAlias</code> is taken
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and the request is served from that vhost. A <code>Host:</code>
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header field can contain a port number, but Apache always
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matches against the real port to which the client sent the
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<p>If the client submitted a HTTP/1.0 request without
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<code>Host:</code> header field we don't know to what server
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the client tried to connect and any existing
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<code>ServerPath</code> is matched against the URI from the
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request. The first matching path on the list is used and the
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request is served from that vhost.</p>
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<p>If no matching vhost could be found the request is served
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from the first vhost with a matching port number that is on the
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list for the IP to which the client connected (as already
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mentioned before).</p>
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<h3><a name="persistent" id="persistent">Persistent connections</a></h3>
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<p>The IP lookup described above is only done <em>once</em> for a
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particular TCP/IP session while the name lookup is done on
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<em>every</em> request during a KeepAlive/persistent
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connection. In other words a client may request pages from
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different name-based vhosts during a single persistent
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<h3><a name="absoluteURI" id="absoluteURI">Absolute URI</a></h3>
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<p>If the URI from the request is an absolute URI, and its
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hostname and port match the main server or one of the
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configured virtual hosts <em>and</em> match the address and
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port to which the client sent the request, then the
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scheme/hostname/port prefix is stripped off and the remaining
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relative URI is served by the corresponding main server or
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virtual host. If it does not match, then the URI remains
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untouched and the request is taken to be a proxy request.</p>
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<h3><a name="observations" id="observations">Observations</a></h3>
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<li>A name-based vhost can never interfere with an IP-base
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vhost and vice versa. IP-based vhosts can only be reached
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through an IP address of its own address set and never
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through any other address. The same applies to name-based
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vhosts, they can only be reached through an IP address of the
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corresponding address set which must be defined with a
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<code>NameVirtualHost</code> directive.</li>
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<li><code>ServerAlias</code> and <code>ServerPath</code>
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checks are never performed for an IP-based vhost.</li>
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<li>The order of name-/IP-based, the <code>_default_</code>
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vhost and the <code>NameVirtualHost</code> directive within
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the config file is not important. Only the ordering of
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name-based vhosts for a specific address set is significant.
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The one name-based vhosts that comes first in the
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configuration file has the highest priority for its
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corresponding address set.</li>
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<li>For security reasons the port number given in a
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<code>Host:</code> header field is never used during the
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matching process. Apache always uses the real port to which
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the client sent the request.</li>
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<li>If a <code>ServerPath</code> directive exists which is a
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prefix of another <code>ServerPath</code> directive that
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appears later in the configuration file, then the former will
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always be matched and the latter will never be matched. (That
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is assuming that no <code>Host:</code> header field was
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available to disambiguate the two.)</li>
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<li>If two IP-based vhosts have an address in common, the
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vhost appearing first in the config file is always matched.
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Such a thing might happen inadvertently. The server will give
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a warning in the error logfile when it detects this.</li>
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<li>A <code>_default_</code> vhost catches a request only if
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there is no other vhost with a matching IP address
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<em>and</em> a matching port number for the request. The
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request is only caught if the port number to which the client
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sent the request matches the port number of your
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<code>_default_</code> vhost which is your standard
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<code>Listen</code> by default. A wildcard port can be
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specified (<em>i.e.</em>, <code>_default_:*</code>) to catch
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requests to any available port. This also applies to
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<code>NameVirtualHost *</code> vhosts.</li>
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<li>The main_server is only used to serve a request if the IP
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address and port number to which the client connected is
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unspecified and does not match any other vhost (including a
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<code>_default_</code> vhost). In other words the main_server
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only catches a request for an unspecified address/port
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combination (unless there is a <code>_default_</code> vhost
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which matches that port).</li>
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<li>A <code>_default_</code> vhost or the main_server is
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<em>never</em> matched for a request with an unknown or
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missing <code>Host:</code> header field if the client
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connected to an address (and port) which is used for
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name-based vhosts, <em>e.g.</em>, in a
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<code>NameVirtualHost</code> directive.</li>
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<li>You should never specify DNS names in
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<code>VirtualHost</code> directives because it will force
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your server to rely on DNS to boot. Furthermore it poses a
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security threat if you do not control the DNS for all the
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domains listed. There's <a href="../dns-caveats.html">more
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information</a> available on this and the next two
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<li><code>ServerName</code> should always be set for each
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vhost. Otherwise A DNS lookup is required for each
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="tips" id="tips">Tips</a></h2>
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<p>In addition to the tips on the <a href="../dns-caveats.html#tips">DNS Issues</a> page, here are
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some further tips:</p>
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<li>Place all main_server definitions before any
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<code>VirtualHost</code> definitions. (This is to aid the
418
readability of the configuration -- the post-config merging
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process makes it non-obvious that definitions mixed in around
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virtual hosts might affect all virtual hosts.)</li>
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<li>Group corresponding <code>NameVirtualHost</code> and
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<code>VirtualHost</code> definitions in your configuration to
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ensure better readability.</li>
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<li>Avoid <code>ServerPaths</code> which are prefixes of
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other <code>ServerPaths</code>. If you cannot avoid this then
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you have to ensure that the longer (more specific) prefix
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vhost appears earlier in the configuration file than the
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shorter (less specific) prefix (<em>i.e.</em>, "ServerPath
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/abc" should appear after "ServerPath /abc/def").</li>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="../en/vhosts/details.html" title="English"> en </a> |
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<a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
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</div><div id="footer">
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<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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