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+<?xml version="1.0" encoding="ISO-8859-1"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ This file is generated from xml source: DO NOT EDIT
+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+ -->
+<title>An In-Depth Discussion of Virtual Host Matching - Apache HTTP Server</title>
+<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
+<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
+<link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
+<link href="../images/favicon.ico" rel="shortcut icon" /></head>
+<body id="manual-page"><div id="page-header">
+<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>
+<p class="apache">Apache HTTP Server Version 2.0</p>
+<img alt="" src="../images/feather.gif" /></div>
+<div class="up"><a href="./"><img title="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
+<div id="path">
+<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/">Documentation</a> &gt; <a href="../">Version 2.0</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>
+<div class="toplang">
+<p><span>Available Languages: </span><a href="../en/vhosts/details.html" title="English">&nbsp;en&nbsp;</a> |
+<a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
+<a href="../tr/vhosts/details.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</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>&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#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="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean">&nbsp;ko&nbsp;</a> |
+<a href="../tr/vhosts/details.html" hreflang="tr" rel="alternate" title="Türkçe">&nbsp;tr&nbsp;</a></p>
+</div><div id="footer">
+<p class="apache">Copyright 2009 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>
+<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>
+</body></html> \ No newline at end of file