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<title>Binding</title>

<summary>

<p>Configuring Apache HTTPD 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

Virtual Host feature, which determines how httpd 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 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 httpd 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>

Listen 0.0.0.0:80<br />

Listen 192.0.2.1:80

</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="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>