[FreeNX-kNX] Logging user traffic

[mir]

Hi to all
Till now, My Linux box was a gateway to internet for small net ~20
hosts. I want to change it,  I've started Freenx server on Debian Etch
and I plan to cut direct connection from local hosts to internet
(security reason). On gateway Linux box I implemented net-acct  to log
all traffic from local host to internet hosts. 

My problem to start Freenx server for my network users and cut direct
connection is: How to identify outgoing connection per user, similar way
net-acct do.

First Idea was to patch Linux kernel on box running Freenx server.
Patched kernel should assign for outgoing connections tcp/udp ports
depending on user (owner of this connection). This way I could simply
recognize who made any logged connection. There is also patch for 2.4
kernel http://www.ex-parrot.com/~pdw/user-port-hack/ 
But for kernel 2.6, I found nothing. Maybe someone could adopt this
patch for kernel 2.6
Maybe someone has different idea, how to log user traffic from host
running Freenx server ?

To change all computers in local network to be only terminals for freenx
server I think user connection logging problem should be solved.

Mirek