4J

I have been looking at packet sniffing for various applications for a while now. Problem is that my needs are more "real time/in-line" than "post capture processing" (ala libpcap and friends).

So here are some notes on my progress. Consider this part 1.

Since I am using Linux exclusively, I had another look at Netfilter. I discovered basically two tools I could use:

• libipq, and
• libnetfilter_queue

The later seems to be a replacement for libipq, but I could not find a lot of documentation and implementation notes, so I stick with libipq for now.

Since I am more confident in Perl, I was delighted to find a Perl interface to libipq - perlipq.

On Debian based systems, ensure you have the following packages installed:

• iptables
• libiptables-ipv4-ipqueue-perl

If you also want to start looking inside the packets, you might need something like NetPacket::IP.

Here is a simple test you can do to see the interface in action. First create a basic firewall script (fw.sh) with the following:

    #!/bin/sh

    iptables -F
    iptables -A INPUT -j ACCEPT
    iptables -A OUTPUT -j ACCEPT
    iptables -t mangle -F
    iptables -t mangle -A OUTPUT -p tcp -j QUEUE

Next, create a Perl script (test.pl):

      #!/usr/bin/perl
      package passer;
      use strict;
      $^W = 1;

      use IPTables::IPv4::IPQueue qw(:constants);
      use NetPacket::IP;

      sub main
      {

              my ($queue, $msg);

              $queue = new IPTables::IPv4::IPQueue() or die IPTables::IPv4::IPQueue->errstr;
              $queue->set_mode( IPQ_COPY_PACKET, 1500 );

              while (1) {

                      $msg    = $queue->get_message() or die IPTables::IPv4::IPQueue->errstr;

                      my $pl  = $msg->payload();
                      my $len = $msg->data_len();

                      eval {

                              my $ip          = NetPacket::IP->decode( $pl );
                              my $ipsrc       = $ip->{src_ip};
                              my $ipdest      = $ip->{dest_ip};
                              print " > from $ipsrc to $ipdest -=> $len\n";

                      };

                      $queue->set_verdict($msg->packet_id(), NF_ACCEPT) > 0 or die IPTables::IPv4::IPQueue->errstr;

              }
      }

      main();

To start the monitoring, run the initial firewall script, then run the above Perl script.

To stop the script, simply press CTRL+C. All networking should also stop as the script no longer passes the accept back.

To reset the firewall script, run:

       # iptables -t mangle -F ; iptables -F

That's it.

I still have a number of things I want to do, including:

• Use NetPacket::TCP to get SRC and DST ports
• Load test (how is network performance influenced?)
• During output, I only saw packets leaving the firewall (SRC was the FW, and DST was the client, although the client initiated the session - weird). I need to investigate this, as I am no iptables expert :-)
• Do it in C? (http://www.imchris.org/projects/libipq.html)

Well, I will hopefully post more updates in the not too distant future. Until then, happy computing :-)

Additional Resources:

• http://search.cpan.org/~atrak/NetPacket-0.04/NetPacket/IP.pm