I want to rewrite TCP/IP streams. Ettercap's etterfilter command lets you perform simple live replacements of TCP/IP data based on fixed strings or regexes. Example:

 if (ip.proto == TCP && tcp.dst == 80) {
    if (search(DATA.data, "gzip")) {
       replace("gzip", "    ");
       msg("whited out gzip\n");
    }
 }

 if (ip.proto == TCP && tcp.dst == 80) {
    if (search(DATA.data, "deflate")) {
       replace("deflate", "       "); 
       msg("whited out deflate\n");
    }
 } 

http://ettercap.sourceforge.net/forum/viewtopic.php?t=2833

I would like to rewrite streams based on my own filter program instead of just simple string replacements.

Anyone have an idea of how to do this? Is there anything other than Ettercap that can do live replacement like this, maybe as a plugin to a VPN software or something?

The rewriting should occur at the transport layer (Layer 4) as it does in this example, instead of a lower layer packet-based approach.

Thanks!