Allowing TCP/IP connection while sharing the internet
 

We have a linux server as our gateway and our dsl connection is shared from this machine. we also have a TCP/IP server and client programs running on the server which want to establish connections to another machine (across the internet). We always get a broken pipe messgae fro such connections with others. When we disable the internet sharing utility, everything works like a charm. Is there something we need to do in the settings to enable TCP/IP connections? I already made the security option as standard. Please help me. :)