A while back I got really into monitoring things on my firewall. I have a windowmaker client that can connect to this. The server below is useful for all kinds of things though. I'll post a perl client too (if there's any interest). You can just telnet this, which is kinda fun. :)

#!/usr/bin/perl

use IO::Socket;
$port = 7654;

## This could really do anything you want...
sub print_stuff {
    open in, "/proc/net/dev";
    while(<in>) {
        print $client "$_" if /eth[01]:/ or /ppp[01]:/;
    }
    close in;
}

## Run the farqin thing:
$bn = `basename $0`; `killall -v -q -1 $bn`;

if (fork) {
    print "Forkin away...\n";
    exit 0;
} else {
    $0 = "$bn [$port]";
    &open_it;
    &listen_to_it;
}

sub open_it {
    $sock = new IO::Socket::INET (
        LocalPort  => "$port",
        Proto     => tcp,
        Reuse     =>   1,
        Listen    =>   1
    ) or die "Could not create socket: $!\n";
}

sub listen_to_it {
    while($client = $sock->accept) {
        &print_stuff;
        shutdown $client, 2;
    }
}
[download]

UPDATE (12/14/06): these days I would do this a bit differently, if I'd do it at all... the following is untested.

use strict;

my $server = new MyMonitor( 8000 );
   run $server;

exit 0;

package MyMonitor;

use strict;
use Net::Server;

use base qw(Net::Server);

1;

sub run {
    my $this = shift;

    $this->SUPER::run( port => $this->{port}, proto => $this->{proto} 
+);
}

sub new {
    my $class = shift;
    my $this  = bless {}, $class;

    $this->{port}  = shift || 3000;
    $this->{proto} = shift || 'tcp';

    return $this;
}

sub process_request {
    my $this = shift;
    my $prop = $this->{server};

    open IN, "/proc/net/dev" or die "problem opening procnetdev: $!";
    if( $prop->{udp_true} ){
        $prop->{client}->send($_, 0) while <IN>;
        close IN;
        return;
    }
    
    print $_ while <IN>;
    close IN;
}
[download]