udp_hello_listener.pl

#!/usr/bin/perl -w

## This script listens for a UDP message on a port containing the text "hello".
## The listener will respond back to the client with its own "hello" string.
## Clients can send messages to this listener using netcat.  The following works:
##   echo hello | nc -4u -w1 <listener_hostname> <listener_port>
## Substitute the hostname and port for where this listener is running from.
## Keep in mind that ports less than 1024 will require the listener to run as root.
##
## The listener does not yet take any command line options.
## Update $PORT for the port the udp server will listen on.

use strict;
use IO::Socket;
use Sys::Hostname;

my $localhost = hostname();
my $PORT = 1162;

my $sock = IO::Socket::INET->new(LocalPort => $PORT, Proto => 'udp') or die "socket: $@";

my $message;
while ($sock->recv($message, 1024)) {
    my($port, $ipaddr) = sockaddr_in($sock->peername);
    my $remotehost = gethostbyaddr($ipaddr, AF_INET);

    my $datestring = localtime();
    if ($message =~ /hello/) {
	chomp $message;
        print "[$datestring] $remotehost: $message\n";
	$sock->send("HELLO $remotehost from $localhost\n");
	next;
    }
}

die "recv: $!";