When Apache starts, it connects to some port and address on the local machine and waits for incoming requests. By default, it listens to all addresses on the machine, and to the port as specified by the Port directive in the server configuration. However, it can be told to listen to more the one port, or to listen to 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.
There are two directives used to restrict or specify which addresses and ports Apache listens to.
Syntax: BindAddress [ * | IP-address | hostname ]
Context: server config
Makes the server bind to just the specified address. If the argument is * (an asterisk), the server binds to all interfaces currently marked as up on the server. The port bound to is set with the Port directive. Only one BindAddress should be used.
Syntax: Listen [ port | IP-address:port ]
Context: server config
Listen can be used instead of BindAddress and Port. It tells the server to accept incoming requests (to listen) on the specified port or address-and-port combination. If the first format is used, with a port number only, the server listens on the given port on all interfaces marked as up, instead of the port given by the Port directive. 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 to. 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 220.127.116.11:80 Listen 18.104.22.168:8000
BindAddress and Listen do not implement Virtual Hosts. They tell the main Apache daemon process what addresses and ports to bind and listen on. 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:
Note that if the <VirtualHost> is set for an address and port that the server is not listening to, it cannot be accessed.
See also the documentation on Virtual Hosts, BindAddress directive, Port directive, DNS Issues and <VirtualHost> section.