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17.16. Writing a Multihomed Server

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.

17.16.2. Solution

Don't bind your server to a particular address. Instead, bind to
INADDR_ANY. Then once you've
accept ed a connection, use
getsockname on the client socket to find out which
address the client 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);
}

17.16.3. Discussion

Whereas
getpeername (as discussed in Recipe 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 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)) {
# ...
}

17.16.4. See Also

The getsockname function in Chapter 29 of
Programming Perl and in
perlfunc(1); the documentation for the standard
Socket and IO::Socket modules; the section on "Sockets" in Chapter 16
of Programming Perl or
perlipc(1)

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17.15. Writing a Multitasking Server with POE		17.17. Making a Daemon Server

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