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<title>An In-Depth Discussion of Virtual Host Matching - Apache HTTP Server Version 2.4</title>
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<p>This document attempts to explain
    exactly what Apache HTTP Server does when deciding what virtual host to
    serve a request from.</p>

<p>Most users should read about <a href="name-based.html#namevip">
    Name-based vs. IP-based Virtual Hosts</a> to decide which type they
    want to use, then read more about <a href="name-based.html">name-based</a>
    or <a href="ip-based.html">IP-based</a> virtualhosts, and then see
    <a href="examples.html">some examples</a>.</p>

<p>If you want to understand all the details, then you can
    come back to this page.</p>

</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="../../images/down.gif"> <a href="#configparsing">Configuration File</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><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></ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../../images/up.gif"></a></div>
<div class="section">
<h2 id="configparsing">Configuration File <a title="Permanent link" href="#configparsing" class="permalink">&para;</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.</p>

<p>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>Each <code>VirtualHost</code> directive includes one
    or more addresses and optional ports.</p>

<p>Hostnames can be used in place of IP addresses in a virtual
    host definition, but they are resolved at startup and if any name
    resolutions fail, those virtual host definitions are ignored.
    This is, therefore, not recommended.</p>

<p>The address can be specified as
    <code>*</code>, which will match a request if no
    other vhost has the explicit address on which the request was
    received. </p>

<p>The address appearing in the <code>VirtualHost</code>
    directive can have an optional port. If the port is unspecified,
    it is treated as a wildcard port, which can also be indicated
    explicitly using <code>*</code>.
    The wildcard port matches any port.</p>

<p>(Port numbers specified in the <code>VirtualHost</code> directive do
    not influence what port numbers Apache will listen on, they only control
    which <code>VirtualHost</code> will be selected to handle a request.
    Use the <code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code> directive to
    control the addresses and ports on which the server listens.)
    </p>

<p>Collectively the
    entire set of addresses (including multiple
    results from DNS lookups) are called the vhost's
    <em>address set</em>.</p>

<p>Apache automatically discriminates on the
    basis of the HTTP <code>Host</code> header supplied by the client
    whenever the most specific match for an IP address and port combination
    is listed in multiple virtual hosts.</p>

<p>The
    <code class="directive"><a href="../mod/core.html#servername">ServerName</a></code> directive
    may appear anywhere within the definition of a server. However,
    each appearance overrides the previous appearance (within that
    server).  If no <code>ServerName</code> is specified, the server
    attempts to deduce it from the server's IP address.</p>

<p>The first name-based vhost in the configuration file for a
    given IP:port pair is significant because it is used for all
    requests received on that address and port for which no other
    vhost for that IP:port pair has a matching ServerName or
    ServerAlias.  It is also used for all SSL connections if the
    server does not support <a class="glossarylink" href="../glossary.html#servernameindication" title="see glossary">Server Name Indication</a>.</p>

<p>The complete list of names in the <code>VirtualHost</code>
    directive are treated just like a (non wildcard) <code>ServerAlias</code>
    (but are not overridden by any <code>ServerAlias</code> statement).</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/mpm_common.html#receivebuffersize">ReceiveBufferSize</a></code>,
      or <code class="directive"><a href="../mod/mpm_common.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 id="hostmatching">Virtual Host Matching <a title="Permanent link" href="#hostmatching" class="permalink">&para;</a></h2>

<p>The server determines which vhost to use for a request in two
    phases: an IP-based match when the connection is established,
    then an optional name-based match when the request is received.</p>

<h3 id="hashtable">Phase 1: IP address and port matching</h3>

<p>When a connection is first received, the server looks up the
    destination IP address and port in its list of
    <code>VirtualHost</code> addresses. This lookup follows a strict
    priority order:</p>

<table class="bordered"><tr class="header"><th>Priority</th><th>Match type</th><th>Example</th></tr>
<tr><td>1</td><td>Exact IP address, exact port</td>
        <td><code>&lt;VirtualHost 10.0.0.1:80&gt;</code></td></tr>
<tr class="odd"><td>2</td><td>Exact IP address, wildcard port</td>
        <td><code>&lt;VirtualHost 10.0.0.1:*&gt;</code></td></tr>
<tr><td>3</td><td>Wildcard address (<code>*</code>), exact port</td>
        <td><code>&lt;VirtualHost *:80&gt;</code></td></tr>
<tr class="odd"><td>4</td><td>Wildcard address, wildcard port</td>
        <td><code>&lt;VirtualHost *:*&gt;</code></td></tr>
<tr><td>5</td><td>Main server</td>
        <td>(no matching <code>VirtualHost</code>)</td></tr>
</table>

<div class="note"><h3>Important</h3>
    <p>The server uses the <em>first match found</em> following this
    priority order. Once a match is found at a given priority level,
    no lower-priority levels are considered &mdash; even if a
    lower-priority vhost has a <code>ServerName</code> that matches the
    request's <code>Host</code> header. Name-based matching (Phase 2)
    only occurs among vhosts that tied at the same priority level.</p>
    </div>

<p>If there are <code>VirtualHost</code> definitions for
    the IP address, the next step is to decide if we have to
    deal with an IP-based or a name-based vhost.</p>

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

<p>If Phase 1 produces exactly one matching
    <code>VirtualHost</code>, the request is served directly from that
    vhost with no further matching.</p>

<h3 id="namebased">Phase 2: Name-based matching</h3>

<p>If Phase 1 produces multiple <code>VirtualHost</code>
    definitions at the same priority level, the server performs
    name-based matching among those vhosts using the
    <code>Host:</code> header from the request (or the SNI hostname
    for SSL connections).</p>

<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
    the SSL client handshake includes the TLS extension with the
    requested hostname, then that hostname is used below just like the
    <code>Host:</code> header would be used on a non-SSL connection.
    Otherwise, the first name-based vhost whose address matched is
    used for SSL connections.  This is significant because the
    vhost determines which certificate the server will use for the
    connection.</p>

<p>The matching vhosts are searched in the order they appear in
    the configuration file:</p>

<ol>
    <li>The <code class="directive"><a href="../mod/core.html#servername">ServerName</a></code> and
    <code class="directive"><a href="../mod/core.html#serveralias">ServerAlias</a></code> of each vhost
    are compared against the hostname from the request.  The first
    match wins.</li>

<li>If no <code>ServerName</code> or <code>ServerAlias</code>
    matches, the <em>first vhost in the list</em> is used.  This is
    the <em>default name-based vhost</em> for that address and port
    combination.</li>
    </ol>

<p>A <code>Host:</code> header field can contain a port number,
    but Apache always ignores it and matches against the real port to
    which the client sent the request.</p>

<p>If the request has no <code>Host:</code> header (such as a
    HTTP/1.0 request), the first matching vhost is used. If a
    <code class="directive"><a href="../mod/core.html#serverpath">ServerPath</a></code> is configured for
    any of the matching vhosts and the request URL matches that path,
    the request is served from that vhost instead.  This is a legacy
    mechanism for HTTP/1.0 clients; see the
    <a href="examples.html#serverpath">ServerPath example</a> for
    details.</p>

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

<p>The IP-based lookup (Phase 1) is performed only <em>once</em>
    for a particular TCP/IP session, while the name-based lookup
    (Phase 2) is performed 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 id="absoluteURI">Absolute URI</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 id="observations">Observations</h3>

<ul>
      <li>Name-based virtual hosting is a process applied <em>after</em>
      the server has narrowed the candidate list via IP-based matching.</li>

<li>If you don't care what IP address the client has connected to, use a
      "<code>*</code>" as the address of every virtual host, and name-based virtual hosting
      is applied across all configured virtual hosts.</li>

<li><code>ServerName</code> and <code>ServerAlias</code>
      checks are never performed for an IP-based vhost (one where
      there is only a single vhost for that IP:port).</li>

<li>Only the ordering of
      name-based vhosts for a specific address set is significant.
      The one name-based vhost that comes first in the
      configuration file has the highest priority for its
      corresponding address set.</li>

<li>Any port in the <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 two vhosts have an address in common, those common addresses
      act as name-based virtual hosts implicitly.  This is new behavior as of
      2.3.11.</li>

<li>The main server is only used to serve a request if the IP
      address and port number to which the client connected
      does not match any vhost (including a
      <code>*</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>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 id="tips">Tips <a title="Permanent link" href="#tips" class="permalink">&para;</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>
    </ul>

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