where is the right entry point for matching a tracked related connection?

[Matthias Dettling <m-dettling@gmx.de>]

Hello developers,

I am searching the right place in my linux kernel source (v 2.4.26) for 
extending the iptables command to match in a packet "RELATED" to a 
specific connection, informations that were tracked before.

In detail I want to use this for a FTP-connection.
My aim is to open a FTP control-connection on port 21 in passive mode. 
In the so opened channel, the port for the real data transfer is 
negotiated. This negotiated connection is tracked by the 
ftp-helper-module and is allowed, because it corresponds to the related 
connection on port 21 (FW-Rule: "iptables -A INPUT -p tcp --sport 1024: 
--dport 1024: -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT").

But there is no possibility to test wheter the related connection is 
really on port 21.
So I want to modify the matching module "conntrack" (ipt_conntrack.c) 
written by Marc Boucher to do so.

The thing i should know for doing this, is how i can get access to the 
information of a tracked "RELATED" connection.
After reading the hacking-howto i thought that access is gained through 
the pointer "nfct" of the "sk_buff" structure, but with this i can't 
find anywhere port informations of the originating packet (of the 
related connection).
Can somebody help me?

best regards

M. Dettling