ICQ over Linux firewall with ipchains

Hello, We have set up a Linux firewall with kernel 2.2.16 and ipchains. While everthing else works fine, we have problems to establish a client-client connection between iqc2k clients (i.e. for chat requests). Is there a known solution for this? We already looked at the icq module for ipchains, but that does not work for icq2k. Setting up socks instead of ipchains is also not an option in our opinion. Any help is very much appreciated!!! Thanx in advance, Charlie & Dirk