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<manualpage metafile="bind.xml.meta">

  <title>Binding</title>

  <summary>
    <p>Configuring Apache to listen on specific addresses and ports.</p>
  </summary>

  <seealso><a href="vhosts/">Virtual Hosts</a></seealso>
  <seealso><a href="dns-caveats.html">DNS Issues</a></seealso>

  <section id="overview">
    <title>Overview</title>

    <related>
      <modulelist>
        <module>core</module>
        <module>mpm_common</module>
      </modulelist>
      <directivelist>
        <directive module="core" type="section">VirtualHost</directive>
        <directive module="mpm_common">Listen</directive>
      </directivelist>
    </related>


    <p>When Apache starts, it binds to some port and address on
    the local machine and waits for incoming requests. By default,
    it listens to all addresses on the machine.  However, it may need to
    be told to listen on specific ports, or only on selected 
    addresses, or a combination of both. This is often combined with the 
    Virtual Host feature, which determines how Apache responds to 
    different IP addresses, hostnames and ports.</p>

    <p>The <directive module="mpm_common">Listen</directive>
    directive tells the server to accept
    incoming requests only on the specified ports or
    address-and-port combinations. If only a port number is
    specified in the <directive module="mpm_common">Listen</directive>
    directive, the server
    listens to the given port on all interfaces. If an IP address
    is given as well as a port, the server will listen on the given
    port and interface. Multiple <directive 
    module="mpm_common">Listen</directive> directives may be used to
    specify a number of addresses and ports to listen on. The
    server will respond to requests from any of the listed
    addresses and ports.</p>

    <p>For example, to make the server accept connections on both
    port 80 and port 8000, on all interfaces, use:</p>

    <example>
      Listen 80<br />
      Listen 8000
    </example>

    <p>To make the server accept connections on port 80 for one interface,
       and port 8000 on another, use</p>

    <example>
      Listen 192.0.2.1:80<br />
      Listen 192.0.2.5:8000
    </example>

    <p>IPv6 addresses must be enclosed in square brackets, as in the
    following example:</p>

    <example>
      Listen [2001:db8::a00:20ff:fea7:ccea]:80
    </example>
  </section>

  <section id="ipv6">
    <title>Special IPv6 Considerations</title>

    <p>A growing number of platforms implement IPv6, and
    <glossary>APR</glossary> supports IPv6 on most of these platforms,
    allowing Apache to allocate IPv6 sockets, and to handle requests sent 
    over IPv6.</p>

    <p>One complicating factor for Apache administrators is whether or
    not an IPv6 socket can handle both IPv4 connections and IPv6 
    connections.  Handling IPv4 connections with an IPv6 socket uses 
    IPv4-mapped IPv6 addresses, which are allowed by default on most 
    platforms, but are disallowed by default on FreeBSD, NetBSD, and 
    OpenBSD, in order to match the system-wide policy on those
    platforms. On systems where it is disallowed by default, a 
    special <program>configure</program> parameter can change this behavior
    for Apache.</p>

    <p>On the other hand, on some platforms, such as Linux and Tru64, the 
    <strong>only</strong> way to handle both IPv6 and IPv4 is to use 
    mapped addresses. If you want Apache to handle IPv4 and IPv6 connections 
    with a minimum of sockets, which requires using IPv4-mapped IPv6 
    addresses, specify the <code>--enable-v4-mapped</code> <program>
    configure</program> option.</p>

    <p><code>--enable-v4-mapped</code> is the default on all platforms except 
    FreeBSD, NetBSD, and OpenBSD, so this is probably how your Apache was 
    built.</p>

    <p>If you want Apache to handle IPv4 connections only, regardless of 
    what your platform and APR will support, specify an IPv4 address on all 
    <directive module="mpm_common">Listen</directive> directives, as in the
    following examples:</p>

    <example>
      Listen 0.0.0.0:80<br />
      Listen 192.0.2.1:80
    </example>

    <p>If your platform supports it and you want Apache to handle IPv4 and 
    IPv6 connections on separate sockets (i.e., to disable IPv4-mapped 
    addresses), specify the <code>--disable-v4-mapped</code> <program>
    configure</program> option. <code>--disable-v4-mapped</code> is the
    default on FreeBSD, NetBSD, and OpenBSD.</p>
  </section>

  <section id="virtualhost">
    <title>How This Works With Virtual Hosts</title>

    <p> The <directive
    module="mpm_common">Listen</directive> directive does not implement 
    Virtual Hosts - it only tells the
    main server what addresses and ports to listen on. If no
    <directive module="core" type="section">VirtualHost</directive>
    directives are used, the server will behave
    in the same way for all accepted requests. However,
    <directive module="core" type="section">VirtualHost</directive>
    can be used to specify a different behavior
    for one or more of the addresses or ports. To implement a
    VirtualHost, the server must first be told to listen to the
    address and port to be used. Then a
    <directive module="core" type="section">VirtualHost</directive> section
    should be created for the specified address and port to set the
    behavior of this virtual host. Note that if the
    <directive module="core" type="section">VirtualHost</directive>
    is set for an address and port that the
    server is not listening to, it cannot be accessed.</p>
  </section>
</manualpage>

