Binding - Apache HTTP Server
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Binding

Configuring Apache to listen on specific addresses and ports.

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Overview

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 needs to be told to listen on specific ports, or to listen on only selected addresses, or a combination. This is often combined with the Virtual Host feature which determines how Apache responds to different IP addresses, hostnames and ports.

The Listen directive tells the server to accept incoming requests only on the specified port or address-and-port combinations. If only a port number is specified in the Listen 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 Listen 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.

For example, to make the server accept connections on both port 80 and port 8000, use:

Listen 80
Listen 8000

To make the server accept connections on two specified interfaces and port numbers, use

Listen 192.170.2.1:80
Listen 192.170.2.5:8000

IPv6 addresses must be surrounded in square brackets, as in the following example:

Listen [fe80::a00:20ff:fea7:ccea]:80

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Special IPv6 Considerations

A growing number of platforms implement IPv6, and APR supports IPv6 on most of these platforms, allowing Apache to allocate IPv6 sockets and handle requests which were sent over IPv6.

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. But even on systems where it is disallowed by default, a special configure parameter can change this behavior for Apache.

If you want Apache to handle IPv4 and IPv6 connections with a minimum of sockets, which requires using IPv4-mapped IPv6 addresses, specify the --enable-v4-mapped configure option and use generic Listen directives like the following:

Listen 80

With --enable-v4-mapped, the Listen directives in the default configuration file created by Apache will use this form. --enable-v4-mapped is the default on all platforms but FreeBSD, NetBSD, and OpenBSD, so this is probably how your Apache was built.

If you want Apache to handle IPv4 connections only, regardless of what your platform and APR will support, specify an IPv4 address on all Listen directives, as in the following examples:

Listen 0.0.0.0:80
Listen 192.170.2.1:80

If you want Apache to handle IPv4 and IPv6 connections on separate sockets (i.e., to disable IPv4-mapped addresses), specify the --disable-v4-mapped configure option and use specific Listen directives like the following:

Listen [::]:80
Listen 0.0.0.0:80

With --disable-v4-mapped, the Listen directives in the default configuration file created by Apache will use this form. --disable-v4-mapped is the default on FreeBSD, NetBSD, and OpenBSD.

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How This Works With Virtual Hosts

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

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