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348 lines
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<title>An In-Depth Discussion of Virtual Host Matching - Apache HTTP Server Version 2.4</title>
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<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
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<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
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</script>
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<link href="../images/favicon.ico" rel="shortcut icon" /></head>
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<body id="manual-page"><div id="page-header">
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<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/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 2.4</p>
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<img alt="" src="../images/feather.png" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<div id="path">
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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 2.4</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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<div class="toplang">
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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="Français"> fr </a> |
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<a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
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<a href="../tr/vhosts/details.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
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</div>
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<p>This document attempts to explain
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exactly what Apache HTTP Server does when deciding what virtual host to
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serve a request from.</p>
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<p>Most users should read about <a href="name-based.html#namevip">
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Name-based vs. IP-based Virtual Hosts</a> to decide which type they
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want to use, then read more about <a href="name-based.html">name-based</a>
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or <a href="ip-based.html">IP-based</a> virtualhosts, and then see
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<a href="examples.html">some examples</a>.</p>
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<p>If you want to understand all the details, then you can
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come back to this page.</p>
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</div>
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<div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#configparsing">Configuration File</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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</ul><h3>See also</h3><ul class="seealso"><li><a href="ip-based.html">IP-based Virtual Host Support</a></li><li><a href="name-based.html">Name-based Virtual Hosts Support</a></li><li><a href="examples.html">Virtual Host examples for common setups</a></li><li><a href="mass.html">Dynamically configured mass virtual hosting</a></li><li><a href="#comments_section">Comments</a></li></ul></div>
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<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="configparsing" id="configparsing">Configuration File</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.</p>
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<p>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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sections.</p>
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<p>Each <code>VirtualHost</code> directive includes one
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or more addresses and optional ports.</p>
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<p>Hostnames can be used in place of IP addresses in a virtual
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host definition, but they are resolved at startup and if any name
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resolutions fail, those virtual host definitions are ignored.
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This is, therefore, not recommended.</p>
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<p>The address can be specified as
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<code>*</code>, which will match a request if no
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other vhost has the explicit address on which the request was
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received. </p>
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<p>The 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 is treated as a wildcard port, which can also be indicated
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explicitly using <code>*</code>.
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The wildcard port matches any port.</p>
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<p>(Port numbers specified in the <code>VirtualHost</code> directive do
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not influence what port numbers Apache will listen on, they only control
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which <code>VirtualHost</code> will be selected to handle a request.
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Use the <code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code> directive to
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control the addresses and ports on which the server listens.)
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</p>
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<p>Collectively the
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entire set of addresses (including multiple
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results from DNS lookups) are called the vhost's
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<em>address set</em>.</p>
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<p>Apache automatically discriminates on the
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basis of the HTTP <code>Host</code> header supplied by the client
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whenever the most specific match for an IP address and port combination
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is listed in multiple virtual hosts.</p>
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<p>The
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<code class="directive"><a href="../mod/core.html#servername">ServerName</a></code> directive
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may 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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server). If no <code>ServerName</code> is specified, the server
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attempts to deduce it from the server's IP address.</p>
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<p>The first name-based vhost in the configuration file for a
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given IP:port pair is significant because it is used for all
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requests received on that address and port for which no other
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vhost for that IP:port pair has a matching ServerName or
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ServerAlias. It is also used for all SSL connections if the
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server does not support <a class="glossarylink" href="../glossary.html#servernameindication" title="see glossary">Server Name Indication</a>.</p>
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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 (non wildcard) <code>ServerAlias</code>
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(but are not overridden by any <code>ServerAlias</code> statement).</p>
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<p>For every vhost various default values are set. In
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particular:</p>
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<ol>
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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#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/mpm_common.html#receivebuffersize">ReceiveBufferSize</a></code>,
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or <code class="directive"><a href="../mod/mpm_common.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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</ol>
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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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main server.</p>
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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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follows:</p>
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<h3><a name="hashtable" id="hashtable">IP address lookup</a></h3>
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<p>When the connection is first received on some address and port,
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the server looks for all the <code>VirtualHost</code> definitions
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that have the same IP address and port.</p>
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<p>If there are no exact matches for the address and port, then
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wildcard (<code>*</code>) matches are considered.</p>
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<p>If no matches are found, the request is served by the
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main server.</p>
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<p>If there are <code>VirtualHost</code> definitions for
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the IP address, the next step is to decide if we have to
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deal with an IP-based or a name-based vhost.</p>
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<h3><a name="ipbased" id="ipbased">IP-based vhost</a></h3>
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<p>If there is exactly one <code>VirtualHost</code> directive
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listing the IP address and port combination that was determined
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to be the best match, no further actions are performed and
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the request is served from the matching vhost.</p>
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<h3><a name="namebased" id="namebased">Name-based vhost</a></h3>
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<p>If there are multiple <code>VirtualHost</code> directives listing
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the IP address and port combination that was determined to be the
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best match, the "list" in the remaining steps refers to the list of vhosts
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that matched, in the order they were in the configuration file.</p>
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<p>If the connection is using SSL, the server supports <a class="glossarylink" href="../glossary.html#servernameindication" title="see glossary">Server Name Indication</a>, and
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the SSL client handshake includes the TLS extension with the
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requested hostname, then that hostname is used below just like the
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<code>Host:</code> header would be used on a non-SSL connection.
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Otherwise, the first name-based vhost whose address matched is
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used for SSL connections. This is significant because the
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vhost determines which certificate the server will use for the
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connection.</p>
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<p>If the request contains a <code>Host:</code> header field, the
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list is searched for the first vhost with a matching
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<code>ServerName</code> or <code>ServerAlias</code>, and the
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request is served from that vhost. A <code>Host:</code> header
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field can contain a port number, but Apache always ignores it and
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matches against the real port to which the client sent the
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request.</p>
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<p>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 (such as a HTTP/1.0
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request).</p>
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<h3><a name="persistent" id="persistent">Persistent connections</a></h3>
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<p>The <em>IP lookup</em> described above is only done <em>once</em> for a
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particular TCP/IP session while the <em>name lookup</em> 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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connection.</p>
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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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<ul>
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<li>Name-based virtual hosting is a process applied after
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the server has selected the best matching IP-based virtual
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host.</li>
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<li>If you don't care what IP address the client has connected to, use a
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"*" as the address of every virtual host, and name-based virtual hosting
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is applied across all configured virtual hosts.</li>
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<li><code>ServerName</code> and <code>ServerAlias</code>
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checks are never performed for an IP-based vhost.</li>
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<li>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>Any port in the <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 two vhosts have an address in common, those common addresses
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act as name-based virtual hosts implicitly. This is new behavior as of
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2.3.11.</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
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does not match any vhost (including a
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<code>*</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>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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topics.</li>
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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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vhost.</li>
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</ul>
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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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<ul>
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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
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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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</ul>
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</div></div>
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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="../fr/vhosts/details.html" hreflang="fr" rel="alternate" title="Français"> fr </a> |
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<a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean"> ko </a> |
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<a href="../tr/vhosts/details.html" hreflang="tr" rel="alternate" title="Türkçe"> tr </a></p>
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</div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Libera.chat, or sent to our <a href="https://httpd.apache.org/lists.html">mailing lists</a>.</div>
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<p class="apache">Copyright 2024 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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