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. However, it needs to be told to listen to specific ports, 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.
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 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 8000To 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
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.