Soulseek is a Peer-to-Peer protocol. Here is an attempt at documenting the protocol.
Alternative clients such as PySoulSeek, Nicotine+ and SolarSeek encourage clients to listen on the first available TCP port in the range 2234 to 2239. However, the official client picks a random port.
l7-filter uses the soulseek pattern. This matches "Pierce Firewall" messages and downloads (so it is effective for bandwidth limiting), but does not match the login and other chatter. It has been tested against Linux clients.