#!/usr/bin/perl

use strict;
use Socket;

socket S, PF_INET, SOCK_RAW, 6;

my $result;
while (my $sender = recv(S, $result, 2000, 0)) {
    print "got a ", length($result), " byte TCP packet...\n";
    if (length($result) < 20) {
        print "  ...which is too small to be an IP packet\n";
    } else {
        printf "  from %d.%d.%d.%d to %d.%d.%d.%d\n",
            map { unpack("C", substr($result, $_, 1)) } (12..19);
        if (length($result) < 40) {
            print "  ...and is too small to be a TCP packet\n";
        } else {
            printf "  from port %d to port %d\n",
                map { unpack("n", substr($result, $_, 2)) } (20, 22);
        }
    }
}