nope, it uses the same technique as the client (or just about any other bit of internet software that doesnt expect you to specify an outgoing port) and uses the next free port as specifed by the operating system.
If your world is a standard server, rather than a gateway, that should theoretically be it. When the client joins the galaxy, the galaxy server tells the client where the worlds currently are and ding. If it doesnt work, then you've probably got some firewally/nat thing interfering and only accepting messages from a connection you've established explicitly - (i..e it will let the galaxy server talk to the world coz the world initiated the connection to the galaxy) - in which case, for now, you'll probably need to adjust your firewall settings or revert to a fixed, specific port which you've forwarded manually.
yeh i think this is the part in the thread where fooli asks what the point of a firewall is if it doesn't block ports
k. so we're back to explicitly forwarding and recieving. i've set the router to have outgoing ports forwarded to the appropriate machine, using the same port rather than a random number, so i guess we'll sit on that and see how we do