Writing a Multi-Homed Server

Problem

You want to write a server that knows that the machine it runs on has multiple IP addresses, and that it should possibly do different things for each address.

Don't bind your server to a particular address. Instead, bind to INADDR_ANY. Then, once you've accepted a connection, use getsockname on the client socket to find out which address they connected to:

use Socket; socket(SERVER, PF_INET, SOCK_STREAM, getprotobyname('tcp')); setsockopt(SERVER, SOL_SOCKET, SO_REUSEADDR, 1); bind(SERVER, sockaddr_in($server_port, INADDR_ANY)) or die "Binding: $!\n"; # accept loop while (accept(CLIENT, SERVER)) {
 $my_socket_address = getsockname(CLIENT); ($port, $myaddr) = sockaddr_in($my_socket_address);
}

Discussion

Whereas getpeername (as discussed in ) returns the address of the remote end of the socket, getsockname returns the address of the local end. When we've bound to INADDR_ANY, thus accepting connections on any address the machine has, we need to use getsockname to identify which address the client connected to.

If you're using IO::Socket::INET, your code will look like this:

$server = IO::Socket::INET->new(LocalPort => $server_port, Type => SOCK_STREAM, Proto => 'tcp', Listen => 10) or die "Can't create server socket: $@\n"; while ($client = $server->accept()) {
 $my_socket_address = $client->sockname(); ($port, $myaddr) = sockaddr_in($my_socket_address); # ... }

If you don't specify a local port to IO::Socket::INET->new, your socket will be bound to INADDR_ANY.

If you want your server to listen only for a particular virtual host, don't use INADDR_ANY. Instead, bind to a specific host address:

use Socket; $port = 4269; # port to bind to $host = "specific.host.com"; # virtual host to listen on socket(Server, PF_INET, SOCK_STREAM, getprotobyname("tcp")) or die "socket: $!"; bind(Server, sockaddr_in($port, inet_aton($host))) or die "bind: $!"; while ($client_address = accept(Client, Server)) {
 # ... }