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 <title>An In-Depth Discussion of Virtual Host Matching - Apache HTTP Server</title>
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 <p class="apache">Apache HTTP Server Version 2.3</p>
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 <a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs-project/">Documentation</a> &gt; <a href="../">Version 2.3</a> &gt; <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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 </div>


     <p>The virtual host code was completely rewritten in
     <strong>Apache 1.3</strong>. This document attempts to explain
     exactly what Apache does when deciding what virtual host to
     serve a hit from. With the help of the new
     <code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code>
     directive virtual host configuration should be a lot easier and
     safer than with versions prior to 1.3.</p>

     <p>If you just want to <cite>make it work</cite> without
     understanding how, here are <a href="examples.html">some
     examples</a>.</p>

 </div>
 <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#configparsing">Config File Parsing</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#hostmatching">Virtual Host Matching</a></li>
 <li><img alt="" src="../images/down.gif" /> <a href="#tips">Tips</a></li>
 </ul></div>
 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="configparsing" id="configparsing">Config File Parsing</a></h2>

     <p>There is a <em>main_server</em> which consists of all the
     definitions appearing outside of
     <code>&lt;VirtualHost&gt;</code> sections. There are virtual
     servers, called <em>vhosts</em>, which are defined by
     <code class="directive"><a href="../mod/core.html#virtualhost">&lt;VirtualHost&gt;</a></code>
     sections.</p>

     <p>The directives
     <code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code>,
     <code class="directive"><a href="../mod/core.html#servername">ServerName</a></code>,
     <code class="directive"><a href="../mod/core.html#serverpath">ServerPath</a></code>,
     and <code class="directive"><a href="../mod/core.html#serveralias">ServerAlias</a></code>
     can appear anywhere within the definition of a server. However,
     each appearance overrides the previous appearance (within that
     server).</p>

     <p>The default value of the <code>Listen</code> field for
     main_server is 80. The main_server has no default
     <code>ServerPath</code>, or <code>ServerAlias</code>. The
     default <code>ServerName</code> is deduced from the server's IP
     address.</p>

     <p>The main_server Listen directive has two functions.  One
     function is to determine the default network port Apache will
     bind to.  The second function is to specify the port number
     which is used in absolute URIs during redirects.</p>

     <p>Unlike the main_server, vhost ports <em>do not</em> affect
     what ports Apache listens for connections on.</p>

     <p>Each address appearing in the <code>VirtualHost</code>
     directive can have an optional port. If the port is unspecified
     it defaults to the value of the main_server's most recent
     <code>Listen</code> statement. The special port <code>*</code>
     indicates a wildcard that matches any port. Collectively the
     entire set of addresses (including multiple <code>A</code>
     record results from DNS lookups) are called the vhost's
     <em>address set</em>.</p>

     <p>Unless a <code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code>
     directive is used for a specific IP address the first vhost
     with that address is treated as an IP-based vhost. The IP
     address can also be the wildcard <code>*</code>.</p>

     <p>If name-based vhosts should be used a
     <code>NameVirtualHost</code> directive <em>must</em> appear
     with the IP address set to be used for the name-based vhosts.
     In other words, you must specify the IP address that holds the
     hostname aliases (CNAMEs) for your name-based vhosts via a
     <code>NameVirtualHost</code> directive in your configuration
     file.</p>

     <p>Multiple <code>NameVirtualHost</code> directives can be used
     each with a set of <code>VirtualHost</code> directives but only
     one <code>NameVirtualHost</code> directive should be used for
     each specific IP:port pair.</p>

     <p>The ordering of <code>NameVirtualHost</code> and
     <code>VirtualHost</code> directives is not important which
     makes the following two examples identical (only the order of
     the <code>VirtualHost</code> directives for <em>one</em>
     address set is important, see below):</p>

 <table><tr>
 <td><div class="example"><p><code>
   NameVirtualHost 111.22.33.44<br />
   &lt;VirtualHost 111.22.33.44&gt;<br />
   # server A<br />
   ...<br />
   &lt;/VirtualHost&gt;<br />
   &lt;VirtualHost 111.22.33.44&gt;<br />
   # server B<br />
   ...<br />
   &lt;/VirtualHost&gt;<br />
   <br />
   NameVirtualHost 111.22.33.55<br />
   &lt;VirtualHost 111.22.33.55&gt;<br />
   # server C<br />
   ...<br />
   &lt;/VirtualHost&gt;<br />
   &lt;VirtualHost 111.22.33.55&gt;<br />
   # server D<br />
   ...<br />
   &lt;/VirtualHost&gt;
 </code></p></div></td>
 <td><div class="example"><p><code>
   &lt;VirtualHost 111.22.33.44&gt;<br />
   # server A<br />
   &lt;/VirtualHost&gt;<br />
   &lt;VirtualHost 111.22.33.55&gt;<br />
   # server C<br />
   ...<br />
   &lt;/VirtualHost&gt;<br />
   &lt;VirtualHost 111.22.33.44&gt;<br />
   # server B<br />
   ...<br />
   &lt;/VirtualHost&gt;<br />
   &lt;VirtualHost 111.22.33.55&gt;<br />
   # server D<br />
   ...<br />
   &lt;/VirtualHost&gt;<br />
   <br />
   NameVirtualHost 111.22.33.44<br />
   NameVirtualHost 111.22.33.55<br />
   <br />
 </code></p></div></td>
 </tr></table>


     <p>(To aid the readability of your configuration you should
     prefer the left variant.)</p>

     <p>After parsing the <code>VirtualHost</code> directive, the
     vhost server is given a default <code>Listen</code> equal to the
     port assigned to the first name in its <code>VirtualHost</code>
     directive.</p>

     <p>The complete list of names in the <code>VirtualHost</code>
     directive are treated just like a <code>ServerAlias</code> (but
     are not overridden by any <code>ServerAlias</code> statement)
     if all names resolve to the same address set. Note that
     subsequent <code>Listen</code> statements for this vhost will not
     affect the ports assigned in the address set.</p>

     <p>During initialization a list for each IP address is
     generated and inserted into an hash table. If the IP address is
     used in a <code>NameVirtualHost</code> directive the list
     contains all name-based vhosts for the given IP address. If
     there are no vhosts defined for that address the
     <code>NameVirtualHost</code> directive is ignored and an error
     is logged. For an IP-based vhost the list in the hash table is
     empty.</p>

     <p>Due to a fast hashing function the overhead of hashing an IP
     address during a request is minimal and almost not existent.
     Additionally the table is optimized for IP addresses which vary
     in the last octet.</p>

     <p>For every vhost various default values are set. In
     particular:</p>

     <ol>
       <li>If a vhost has no <code class="directive"><a href="../mod/core.html#serveradmin">ServerAdmin</a></code>,
       <code class="directive"><a href="../mod/core.html#resourceconfig">ResourceConfig</a></code>,
       <code class="directive"><a href="../mod/core.html#accessconfig">AccessConfig</a></code>,
       <code class="directive"><a href="../mod/core.html#timeout">Timeout</a></code>,
       <code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code>,
       <code class="directive"><a href="../mod/core.html#keepalive">KeepAlive</a></code>,
       <code class="directive"><a href="../mod/core.html#maxkeepaliverequests">MaxKeepAliveRequests</a></code>,
       <code class="directive"><a href="../mod/core.html#receivebuffersize">ReceiveBufferSize</a></code>,
       or <code class="directive"><a href="../mod/core.html#sendbuffersize">SendBufferSize</a></code>
       directive then the respective value is inherited from the
       main_server. (That is, inherited from whatever the final
       setting of that value is in the main_server.)</li>

       <li>The "lookup defaults" that define the default directory
       permissions for a vhost are merged with those of the
       main_server. This includes any per-directory configuration
       information for any module.</li>

       <li>The per-server configs for each module from the
       main_server are merged into the vhost server.</li>
     </ol>

     <p>Essentially, the main_server is treated as "defaults" or a
     "base" on which to build each vhost. But the positioning of
     these main_server definitions in the config file is largely
     irrelevant -- the entire config of the main_server has been
     parsed when this final merging occurs. So even if a main_server
     definition appears after a vhost definition it might affect the
     vhost definition.</p>

     <p>If the main_server has no <code>ServerName</code> at this
     point, then the hostname of the machine that <code class="program"><a href="../programs/httpd.html">httpd</a></code>
     is running on is used instead. We will call the <em>main_server address
     set</em> those IP addresses returned by a DNS lookup on the
     <code>ServerName</code> of the main_server.</p>

     <p>For any undefined <code>ServerName</code> fields, a
     name-based vhost defaults to the address given first in the
     <code>VirtualHost</code> statement defining the vhost.</p>

     <p>Any vhost that includes the magic <code>_default_</code>
     wildcard is given the same <code>ServerName</code> as the
     main_server.</p>

 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="hostmatching" id="hostmatching">Virtual Host Matching</a></h2>

     <p>The server determines which vhost to use for a request as
     follows:</p>

     <h3><a name="hashtable" id="hashtable">Hash table lookup</a></h3>

     <p>When the connection is first made by a client, the IP
     address to which the client connected is looked up in the
     internal IP hash table.</p>

     <p>If the lookup fails (the IP address wasn't found) the
     request is served from the <code>_default_</code> vhost if
     there is such a vhost for the port to which the client sent the
     request. If there is no matching <code>_default_</code> vhost
     the request is served from the main_server.</p>

     <p>If the IP address is not found in the hash table then the
     match against the port number may also result in an entry
     corresponding to a <code>NameVirtualHost *</code>, which is
     subsequently handled like other name-based vhosts.</p>

     <p>If the lookup succeeded (a corresponding list for the IP
     address was found) the next step is to decide if we have to
     deal with an IP-based or a name-base vhost.</p>


     <h3><a name="ipbased" id="ipbased">IP-based vhost</a></h3>

     <p>If the entry we found has an empty name list then we have
     found an IP-based vhost, no further actions are performed and
     the request is served from that vhost.</p>


     <h3><a name="namebased" id="namebased">Name-based vhost</a></h3>

     <p>If the entry corresponds to a name-based vhost the name list
     contains one or more vhost structures. This list contains the
     vhosts in the same order as the <code>VirtualHost</code>
     directives appear in the config file.</p>

     <p>The first vhost on this list (the first vhost in the config
     file with the specified IP address) has the highest priority
     and catches any request to an unknown server name or a request
     without a <code>Host:</code> header field.</p>

     <p>If the client provided a <code>Host:</code> header field the
     list is searched for a matching vhost and the first hit on a
     <code>ServerName</code> or <code>ServerAlias</code> is taken
     and the request is served from that vhost. A <code>Host:</code>
     header field can contain a port number, but Apache always
     matches against the real port to which the client sent the
     request.</p>

     <p>If the client submitted a HTTP/1.0 request without
     <code>Host:</code> header field we don't know to what server
     the client tried to connect and any existing
     <code>ServerPath</code> is matched against the URI from the
     request. The first matching path on the list is used and the
     request is served from that vhost.</p>

     <p>If no matching vhost could be found the request is served
     from the first vhost with a matching port number that is on the
     list for the IP to which the client connected (as already
     mentioned before).</p>


     <h3><a name="persistent" id="persistent">Persistent connections</a></h3>

     <p>The IP lookup described above is only done <em>once</em> for a
     particular TCP/IP session while the name lookup is done on
     <em>every</em> request during a KeepAlive/persistent
     connection. In other words a client may request pages from
     different name-based vhosts during a single persistent
     connection.</p>


     <h3><a name="absoluteURI" id="absoluteURI">Absolute URI</a></h3>

     <p>If the URI from the request is an absolute URI, and its
     hostname and port match the main server or one of the
     configured virtual hosts <em>and</em> match the address and
     port to which the client sent the request, then the
     scheme/hostname/port prefix is stripped off and the remaining
     relative URI is served by the corresponding main server or
     virtual host. If it does not match, then the URI remains
     untouched and the request is taken to be a proxy request.</p>


 <h3><a name="observations" id="observations">Observations</a></h3>

     <ul>
       <li>A name-based vhost can never interfere with an IP-base
       vhost and vice versa. IP-based vhosts can only be reached
       through an IP address of its own address set and never
       through any other address. The same applies to name-based
       vhosts, they can only be reached through an IP address of the
       corresponding address set which must be defined with a
       <code>NameVirtualHost</code> directive.</li>

       <li><code>ServerAlias</code> and <code>ServerPath</code>
       checks are never performed for an IP-based vhost.</li>

       <li>The order of name-/IP-based, the <code>_default_</code>
       vhost and the <code>NameVirtualHost</code> directive within
       the config file is not important. Only the ordering of
       name-based vhosts for a specific address set is significant.
       The one name-based vhosts that comes first in the
       configuration file has the highest priority for its
       corresponding address set.</li>

       <li>For security reasons the port number given in a
       <code>Host:</code> header field is never used during the
       matching process. Apache always uses the real port to which
       the client sent the request.</li>

       <li>If a <code>ServerPath</code> directive exists which is a
       prefix of another <code>ServerPath</code> directive that
       appears later in the configuration file, then the former will
       always be matched and the latter will never be matched. (That
       is assuming that no <code>Host:</code> header field was
       available to disambiguate the two.)</li>

       <li>If two IP-based vhosts have an address in common, the
       vhost appearing first in the config file is always matched.
       Such a thing might happen inadvertently. The server will give
       a warning in the error logfile when it detects this.</li>

       <li>A <code>_default_</code> vhost catches a request only if
       there is no other vhost with a matching IP address
       <em>and</em> a matching port number for the request. The
       request is only caught if the port number to which the client
       sent the request matches the port number of your
       <code>_default_</code> vhost which is your standard
       <code>Listen</code> by default. A wildcard port can be
       specified (<em>i.e.</em>, <code>_default_:*</code>) to catch
       requests to any available port. This also applies to
       <code>NameVirtualHost *</code> vhosts.</li>

       <li>The main_server is only used to serve a request if the IP
       address and port number to which the client connected is
       unspecified and does not match any other vhost (including a
       <code>_default_</code> vhost). In other words the main_server
       only catches a request for an unspecified address/port
       combination (unless there is a <code>_default_</code> vhost
       which matches that port).</li>

       <li>A <code>_default_</code> vhost or the main_server is
       <em>never</em> matched for a request with an unknown or
       missing <code>Host:</code> header field if the client
       connected to an address (and port) which is used for
       name-based vhosts, <em>e.g.</em>, in a
       <code>NameVirtualHost</code> directive.</li>

       <li>You should never specify DNS names in
       <code>VirtualHost</code> directives because it will force
       your server to rely on DNS to boot. Furthermore it poses a
       security threat if you do not control the DNS for all the
       domains listed. There's <a href="../dns-caveats.html">more
       information</a> available on this and the next two
       topics.</li>

       <li><code>ServerName</code> should always be set for each
       vhost. Otherwise A DNS lookup is required for each
       vhost.</li>
       </ul>


 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
 <div class="section">
 <h2><a name="tips" id="tips">Tips</a></h2>

     <p>In addition to the tips on the <a href="../dns-caveats.html#tips">DNS Issues</a> page, here are
     some further tips:</p>

     <ul>
       <li>Place all main_server definitions before any
       <code>VirtualHost</code> definitions. (This is to aid the
       readability of the configuration -- the post-config merging
       process makes it non-obvious that definitions mixed in around
       virtual hosts might affect all virtual hosts.)</li>

       <li>Group corresponding <code>NameVirtualHost</code> and
       <code>VirtualHost</code> definitions in your configuration to
       ensure better readability.</li>

       <li>Avoid <code>ServerPaths</code> which are prefixes of
       other <code>ServerPaths</code>. If you cannot avoid this then
       you have to ensure that the longer (more specific) prefix
       vhost appears earlier in the configuration file than the
       shorter (less specific) prefix (<em>i.e.</em>, "ServerPath
       /abc" should appear after "ServerPath /abc/def").</li>
     </ul>

 </div></div>
 <div class="bottomlang">
 <p><span>Available Languages: </span><a href="../en/vhosts/details.html" title="English">&nbsp;en&nbsp;</a> |
 <a href="../fr/vhosts/details.html" hreflang="fr" rel="alternate" title="Français">&nbsp;fr&nbsp;</a> |
 <a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a></p>
 </div><div id="footer">
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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></div>
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	<p class="apache">Apache HTTP Server Version 2.3</p>
	<img alt="" src="../images/feather.gif" /></div>
	<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
	<div id="path">
	<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.3</a> > <a href="./">Virtual Hosts</a></div><div id="page-content"><div id="preamble"><h1>An In-Depth Discussion of Virtual Host Matching</h1>
	<div class="toplang">
	<p><span>Available Languages: </span><a href="../en/vhosts/details.html" title="English"> en </a> \|
	<a href="../fr/vhosts/details.html" hreflang="fr" rel="alternate" title="Français"> fr </a> \|
	<a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
	</div>


	<p>The virtual host code was completely rewritten in
	<strong>Apache 1.3</strong>. This document attempts to explain
	exactly what Apache does when deciding what virtual host to
	serve a hit from. With the help of the new
	<code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code>
	directive virtual host configuration should be a lot easier and
	safer than with versions prior to 1.3.</p>

	<p>If you just want to <cite>make it work</cite> without
	understanding how, here are <a href="examples.html">some
	examples</a>.</p>

	</div>
	<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#configparsing">Config File Parsing</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#hostmatching">Virtual Host Matching</a></li>
	<li><img alt="" src="../images/down.gif" /> <a href="#tips">Tips</a></li>
	</ul></div>
	<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="configparsing" id="configparsing">Config File Parsing</a></h2>

	<p>There is a <em>main_server</em> which consists of all the
	definitions appearing outside of
	<code><VirtualHost></code> sections. There are virtual
	servers, called <em>vhosts</em>, which are defined by
	<code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>
	sections.</p>

	<p>The directives
	<code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code>,
	<code class="directive"><a href="../mod/core.html#servername">ServerName</a></code>,
	<code class="directive"><a href="../mod/core.html#serverpath">ServerPath</a></code>,
	and <code class="directive"><a href="../mod/core.html#serveralias">ServerAlias</a></code>
	can appear anywhere within the definition of a server. However,
	each appearance overrides the previous appearance (within that
	server).</p>

	<p>The default value of the <code>Listen</code> field for
	main_server is 80. The main_server has no default
	<code>ServerPath</code>, or <code>ServerAlias</code>. The
	default <code>ServerName</code> is deduced from the server's IP
	address.</p>

	<p>The main_server Listen directive has two functions. One
	function is to determine the default network port Apache will
	bind to. The second function is to specify the port number
	which is used in absolute URIs during redirects.</p>

	<p>Unlike the main_server, vhost ports <em>do not</em> affect
	what ports Apache listens for connections on.</p>

	<p>Each address appearing in the <code>VirtualHost</code>
	directive can have an optional port. If the port is unspecified
	it defaults to the value of the main_server's most recent
	<code>Listen</code> statement. The special port <code>*</code>
	indicates a wildcard that matches any port. Collectively the
	entire set of addresses (including multiple <code>A</code>
	record results from DNS lookups) are called the vhost's
	<em>address set</em>.</p>

	<p>Unless a <code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code>
	directive is used for a specific IP address the first vhost
	with that address is treated as an IP-based vhost. The IP
	address can also be the wildcard <code>*</code>.</p>

	<p>If name-based vhosts should be used a
	<code>NameVirtualHost</code> directive <em>must</em> appear
	with the IP address set to be used for the name-based vhosts.
	In other words, you must specify the IP address that holds the
	hostname aliases (CNAMEs) for your name-based vhosts via a
	<code>NameVirtualHost</code> directive in your configuration
	file.</p>

	<p>Multiple <code>NameVirtualHost</code> directives can be used
	each with a set of <code>VirtualHost</code> directives but only
	one <code>NameVirtualHost</code> directive should be used for
	each specific IP:port pair.</p>

	<p>The ordering of <code>NameVirtualHost</code> and
	<code>VirtualHost</code> directives is not important which
	makes the following two examples identical (only the order of
	the <code>VirtualHost</code> directives for <em>one</em>
	address set is important, see below):</p>

	<table><tr>
	<td><div class="example"><p><code>
	NameVirtualHost 111.22.33.44<br />
	<VirtualHost 111.22.33.44><br />
	# server A<br />
	...<br />
	</VirtualHost><br />
	<VirtualHost 111.22.33.44><br />
	# server B<br />
	...<br />
	</VirtualHost><br />
	<br />
	NameVirtualHost 111.22.33.55<br />
	<VirtualHost 111.22.33.55><br />
	# server C<br />
	...<br />
	</VirtualHost><br />
	<VirtualHost 111.22.33.55><br />
	# server D<br />
	...<br />
	</VirtualHost>
	</code></p></div></td>
	<td><div class="example"><p><code>
	<VirtualHost 111.22.33.44><br />
	# server A<br />
	</VirtualHost><br />
	<VirtualHost 111.22.33.55><br />
	# server C<br />
	...<br />
	</VirtualHost><br />
	<VirtualHost 111.22.33.44><br />
	# server B<br />
	...<br />
	</VirtualHost><br />
	<VirtualHost 111.22.33.55><br />
	# server D<br />
	...<br />
	</VirtualHost><br />
	<br />
	NameVirtualHost 111.22.33.44<br />
	NameVirtualHost 111.22.33.55<br />
	<br />
	</code></p></div></td>
	</tr></table>


	<p>(To aid the readability of your configuration you should
	prefer the left variant.)</p>

	<p>After parsing the <code>VirtualHost</code> directive, the
	vhost server is given a default <code>Listen</code> equal to the
	port assigned to the first name in its <code>VirtualHost</code>
	directive.</p>

	<p>The complete list of names in the <code>VirtualHost</code>
	directive are treated just like a <code>ServerAlias</code> (but
	are not overridden by any <code>ServerAlias</code> statement)
	if all names resolve to the same address set. Note that
	subsequent <code>Listen</code> statements for this vhost will not
	affect the ports assigned in the address set.</p>

	<p>During initialization a list for each IP address is
	generated and inserted into an hash table. If the IP address is
	used in a <code>NameVirtualHost</code> directive the list
	contains all name-based vhosts for the given IP address. If
	there are no vhosts defined for that address the
	<code>NameVirtualHost</code> directive is ignored and an error
	is logged. For an IP-based vhost the list in the hash table is
	empty.</p>

	<p>Due to a fast hashing function the overhead of hashing an IP
	address during a request is minimal and almost not existent.
	Additionally the table is optimized for IP addresses which vary
	in the last octet.</p>

	<p>For every vhost various default values are set. In
	particular:</p>

	<ol>
	<li>If a vhost has no <code class="directive"><a href="../mod/core.html#serveradmin">ServerAdmin</a></code>,
	<code class="directive"><a href="../mod/core.html#resourceconfig">ResourceConfig</a></code>,
	<code class="directive"><a href="../mod/core.html#accessconfig">AccessConfig</a></code>,
	<code class="directive"><a href="../mod/core.html#timeout">Timeout</a></code>,
	<code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code>,
	<code class="directive"><a href="../mod/core.html#keepalive">KeepAlive</a></code>,
	<code class="directive"><a href="../mod/core.html#maxkeepaliverequests">MaxKeepAliveRequests</a></code>,
	<code class="directive"><a href="../mod/core.html#receivebuffersize">ReceiveBufferSize</a></code>,
	or <code class="directive"><a href="../mod/core.html#sendbuffersize">SendBufferSize</a></code>
	directive then the respective value is inherited from the
	main_server. (That is, inherited from whatever the final
	setting of that value is in the main_server.)</li>

	<li>The "lookup defaults" that define the default directory
	permissions for a vhost are merged with those of the
	main_server. This includes any per-directory configuration
	information for any module.</li>

	<li>The per-server configs for each module from the
	main_server are merged into the vhost server.</li>
	</ol>

	<p>Essentially, the main_server is treated as "defaults" or a
	"base" on which to build each vhost. But the positioning of
	these main_server definitions in the config file is largely
	irrelevant -- the entire config of the main_server has been
	parsed when this final merging occurs. So even if a main_server
	definition appears after a vhost definition it might affect the
	vhost definition.</p>

	<p>If the main_server has no <code>ServerName</code> at this
	point, then the hostname of the machine that <code class="program"><a href="../programs/httpd.html">httpd</a></code>
	is running on is used instead. We will call the <em>main_server address
	set</em> those IP addresses returned by a DNS lookup on the
	<code>ServerName</code> of the main_server.</p>

	<p>For any undefined <code>ServerName</code> fields, a
	name-based vhost defaults to the address given first in the
	<code>VirtualHost</code> statement defining the vhost.</p>

	<p>Any vhost that includes the magic <code>_default_</code>
	wildcard is given the same <code>ServerName</code> as the
	main_server.</p>

	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="hostmatching" id="hostmatching">Virtual Host Matching</a></h2>

	<p>The server determines which vhost to use for a request as
	follows:</p>

	<h3><a name="hashtable" id="hashtable">Hash table lookup</a></h3>

	<p>When the connection is first made by a client, the IP
	address to which the client connected is looked up in the
	internal IP hash table.</p>

	<p>If the lookup fails (the IP address wasn't found) the
	request is served from the <code>_default_</code> vhost if
	there is such a vhost for the port to which the client sent the
	request. If there is no matching <code>_default_</code> vhost
	the request is served from the main_server.</p>

	<p>If the IP address is not found in the hash table then the
	match against the port number may also result in an entry
	corresponding to a <code>NameVirtualHost *</code>, which is
	subsequently handled like other name-based vhosts.</p>

	<p>If the lookup succeeded (a corresponding list for the IP
	address was found) the next step is to decide if we have to
	deal with an IP-based or a name-base vhost.</p>



	<h3><a name="ipbased" id="ipbased">IP-based vhost</a></h3>

	<p>If the entry we found has an empty name list then we have
	found an IP-based vhost, no further actions are performed and
	the request is served from that vhost.</p>



	<h3><a name="namebased" id="namebased">Name-based vhost</a></h3>

	<p>If the entry corresponds to a name-based vhost the name list
	contains one or more vhost structures. This list contains the
	vhosts in the same order as the <code>VirtualHost</code>
	directives appear in the config file.</p>

	<p>The first vhost on this list (the first vhost in the config
	file with the specified IP address) has the highest priority
	and catches any request to an unknown server name or a request
	without a <code>Host:</code> header field.</p>

	<p>If the client provided a <code>Host:</code> header field the
	list is searched for a matching vhost and the first hit on a
	<code>ServerName</code> or <code>ServerAlias</code> is taken
	and the request is served from that vhost. A <code>Host:</code>
	header field can contain a port number, but Apache always
	matches against the real port to which the client sent the
	request.</p>

	<p>If the client submitted a HTTP/1.0 request without
	<code>Host:</code> header field we don't know to what server
	the client tried to connect and any existing
	<code>ServerPath</code> is matched against the URI from the
	request. The first matching path on the list is used and the
	request is served from that vhost.</p>

	<p>If no matching vhost could be found the request is served
	from the first vhost with a matching port number that is on the
	list for the IP to which the client connected (as already
	mentioned before).</p>



	<h3><a name="persistent" id="persistent">Persistent connections</a></h3>

	<p>The IP lookup described above is only done <em>once</em> for a
	particular TCP/IP session while the name lookup is done on
	<em>every</em> request during a KeepAlive/persistent
	connection. In other words a client may request pages from
	different name-based vhosts during a single persistent
	connection.</p>



	<h3><a name="absoluteURI" id="absoluteURI">Absolute URI</a></h3>

	<p>If the URI from the request is an absolute URI, and its
	hostname and port match the main server or one of the
	configured virtual hosts <em>and</em> match the address and
	port to which the client sent the request, then the
	scheme/hostname/port prefix is stripped off and the remaining
	relative URI is served by the corresponding main server or
	virtual host. If it does not match, then the URI remains
	untouched and the request is taken to be a proxy request.</p>


	<h3><a name="observations" id="observations">Observations</a></h3>

	<ul>
	<li>A name-based vhost can never interfere with an IP-base
	vhost and vice versa. IP-based vhosts can only be reached
	through an IP address of its own address set and never
	through any other address. The same applies to name-based
	vhosts, they can only be reached through an IP address of the
	corresponding address set which must be defined with a
	<code>NameVirtualHost</code> directive.</li>

	<li><code>ServerAlias</code> and <code>ServerPath</code>
	checks are never performed for an IP-based vhost.</li>

	<li>The order of name-/IP-based, the <code>_default_</code>
	vhost and the <code>NameVirtualHost</code> directive within
	the config file is not important. Only the ordering of
	name-based vhosts for a specific address set is significant.
	The one name-based vhosts that comes first in the
	configuration file has the highest priority for its
	corresponding address set.</li>

	<li>For security reasons the port number given in a
	<code>Host:</code> header field is never used during the
	matching process. Apache always uses the real port to which
	the client sent the request.</li>

	<li>If a <code>ServerPath</code> directive exists which is a
	prefix of another <code>ServerPath</code> directive that
	appears later in the configuration file, then the former will
	always be matched and the latter will never be matched. (That
	is assuming that no <code>Host:</code> header field was
	available to disambiguate the two.)</li>

	<li>If two IP-based vhosts have an address in common, the
	vhost appearing first in the config file is always matched.
	Such a thing might happen inadvertently. The server will give
	a warning in the error logfile when it detects this.</li>

	<li>A <code>_default_</code> vhost catches a request only if
	there is no other vhost with a matching IP address
	<em>and</em> a matching port number for the request. The
	request is only caught if the port number to which the client
	sent the request matches the port number of your
	<code>_default_</code> vhost which is your standard
	<code>Listen</code> by default. A wildcard port can be
	specified (<em>i.e.</em>, <code>_default_:*</code>) to catch
	requests to any available port. This also applies to
	<code>NameVirtualHost *</code> vhosts.</li>

	<li>The main_server is only used to serve a request if the IP
	address and port number to which the client connected is
	unspecified and does not match any other vhost (including a
	<code>_default_</code> vhost). In other words the main_server
	only catches a request for an unspecified address/port
	combination (unless there is a <code>_default_</code> vhost
	which matches that port).</li>

	<li>A <code>_default_</code> vhost or the main_server is
	<em>never</em> matched for a request with an unknown or
	missing <code>Host:</code> header field if the client
	connected to an address (and port) which is used for
	name-based vhosts, <em>e.g.</em>, in a
	<code>NameVirtualHost</code> directive.</li>

	<li>You should never specify DNS names in
	<code>VirtualHost</code> directives because it will force
	your server to rely on DNS to boot. Furthermore it poses a
	security threat if you do not control the DNS for all the
	domains listed. There's <a href="../dns-caveats.html">more
	information</a> available on this and the next two
	topics.</li>

	<li><code>ServerName</code> should always be set for each
	vhost. Otherwise A DNS lookup is required for each
	vhost.</li>
	</ul>


	</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
	<div class="section">
	<h2><a name="tips" id="tips">Tips</a></h2>

	<p>In addition to the tips on the <a href="../dns-caveats.html#tips">DNS Issues</a> page, here are
	some further tips:</p>

	<ul>
	<li>Place all main_server definitions before any
	<code>VirtualHost</code> definitions. (This is to aid the
	readability of the configuration -- the post-config merging
	process makes it non-obvious that definitions mixed in around
	virtual hosts might affect all virtual hosts.)</li>

	<li>Group corresponding <code>NameVirtualHost</code> and
	<code>VirtualHost</code> definitions in your configuration to
	ensure better readability.</li>

	<li>Avoid <code>ServerPaths</code> which are prefixes of
	other <code>ServerPaths</code>. If you cannot avoid this then
	you have to ensure that the longer (more specific) prefix
	vhost appears earlier in the configuration file than the
	shorter (less specific) prefix (<em>i.e.</em>, "ServerPath
	/abc" should appear after "ServerPath /abc/def").</li>
	</ul>

	</div></div>
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