Perl Cookbook

Setting Up a UDP Server

Problem

You want to write a UDP server.

Solution

First bind to the port the server is to be contacted on. With IO::Socket, this is easily accomplished:

use IO::Socket;
$server = IO::Socket::INET->new(LocalPort => $server_port,
                                Proto     => "udp")
    or die "Couldn't be a udp server on port $server_port : $@\n";

Then, go into a loop receiving messages:

while ($him = $server->recv($datagram, $MAX_TO_READ, $flags)) {
    # do something
}

Discussion

Life with UDP is much simpler than life with TCP. Instead of accepting client connections one at a time and committing yourself to a long-term relationship, take messages from clients as they come in. The recv function returns the address of the sender, which you must then decode.

Example 17.2 is a small UDP-based server that just sits around waiting for messages. Every time a message comes in, we find out who sent it and send them a message based on the previous message, and then save the new message.

Example 17-2. udpqotd

#!/usr/bin/perl -w
# udpqotd - UDP message server use strict; use IO::Socket; my($sock, $oldmsg, $newmsg, $hisaddr, $hishost, $MAXLEN, $PORTNO); $MAXLEN = 1024; $PORTNO = 5151; $sock = IO::Socket::INET->new(LocalPort => $PORTNO, Proto => 'udp') or die "socket: $@"; print "Awaiting UDP messages on port $PORTNO\n"; $oldmsg = "This is the starting message."; while ($sock->recv($newmsg, $MAXLEN)) { my($port, $ipaddr) = sockaddr_in($sock->peername); $hishost = gethostbyaddr($ipaddr, AF_INET); print "Client $hishost ...