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- XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
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-<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" />
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-<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>
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