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Perl Cookbook by Nathan Torkington, Tom Christiansen

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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.

Solution

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 Section 17.7) 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 ...

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