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

  <title>Binding to Addresses and Ports</title>

  <summary>
    <p>Configuring Apache HTTP Server 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 httpd 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
    <a href="vhosts/">Virtual Host</a> feature, which determines how
    <code>httpd</code> 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 port(s) 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>
    <highlight language="config">
Listen 80
Listen 8000
    </highlight>
    </example>

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

    <example>
    <highlight language="config">
Listen 192.0.2.1:80
Listen 192.0.2.5:8000
    </highlight>
    </example>

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

    <example>
    <highlight language="config">
      Listen [2001:db8::a00:20ff:fea7:ccea]:80
    </highlight>
    </example>

    <note type="warning"><p>Overlapping <directive
    module="mpm_common">Listen</directive> directives will result in a
    fatal error which will prevent the server from starting up.</p>

    <example>
      (48)Address already in use: make_sock: could not bind to address [::]:80
    </example>

    <p>See <a 
    href="http://wiki.apache.org/httpd/CouldNotBindToAddress">the
    discussion in the wiki</a> for further troubleshooting tips.</p>

</note>

  </section>

  <section id="reload">
    <title>Changing Listen configuration on restart</title>

    <p>When httpd is restarted, special consideration must be made for
    changes to <directive module="mpm_common">Listen</directive> directives.  During a restart, httpd keeps ports
    bound (as in the original configuration) to avoid generating
    "Connection refused" errors for any new attempts to connect to the
    server. If changes are made to the set of <directive module="mpm_common">Listen</directive> directives used
    which conflict with the old configuration, configuration will fail
    and the server will terminate.</p>

    <p>For example, changing from configuration:</p>
    
    <example>
    <highlight language="config">
      Listen 127.0.0.1:80
    </highlight>
    </example>

    <p>to the following may fail, because binding to port 80 across
    all addresses conflicts with binding to port 80 on just
    127.0.0.1.</p>
    
    <example>
    <highlight language="config">
      Listen 80
    </highlight>
    </example>

    <p>To have such configuration changes take effect, it is necessary
    to stop and then start the server.</p>
    
  </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 httpd to allocate IPv6 sockets, and to handle requests sent
    over IPv6.</p>

    <p>One complicating factor for httpd 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 httpd.</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 <code>httpd</code> 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 httpd was
    built.</p>

    <p>If you want httpd 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>
    <highlight language="config">
Listen 0.0.0.0:80
Listen 192.0.2.1:80
    </highlight>
    </example>

    <p>If your platform supports it and you want httpd 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="protocol">
    <title>Specifying the protocol with Listen</title>
    <p>The optional second <var>protocol</var> argument of
       <directive module="mpm_common">Listen</directive>
       is not required for most
       configurations. If not specified, <code>https</code> is the default for
       port 443 and <code>http</code> the default for all other ports.  The
       protocol is used to determine which module should handle a request, and
       to apply protocol specific optimizations with the
       <directive module="core">AcceptFilter</directive> directive.</p>

    <p>You only need to set the protocol if you are running on non-standard
       ports.  For example, running an <code>https</code> site on port 8443:</p>

    <example>
    <highlight language="config">
      Listen 192.170.2.1:8443 https
    </highlight>
    </example>
  </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>

