UT2004 was development by Epic Games.
It is still available at GOG and Steam, unless you are shopping from a German IP. Due to legal issues the game is not even displayed in the search results in Germany.
You should plan a few minutes for the installation. Even from SSD with a reasonably up-to-date PC it takes a while. On older computers from DVD to disk it can take up to 45 minutes…
After installing, you should install the UT 2004 patch and the bonus pack. Then you have at least the standard maps. But since not everyone has the same understanding of »standard« maps, there is also a map pack that you can unpack with 7 Zip. The maps belong to
\UT2004\Maps. With the Unreal Anthology installation this is in an extra directory. For those of you who didn't install the game from the Anthology DVD, but maybe have it from the UT 2004 DVD, it might also need the crack. Then you can avoid the DVD while playing. Without a valid CD-Key nothing works anyways. UT2000 is also available at gog.com. But not anymore from IPs in Germany.
The Mobile Infanterie server is available on the standard port:
Ports 7777 and 7778 (UDP) must be forwarded for a UT 2004 server. As with UT 1, 7777 joins the server and 7778 connects for querying. The server can then also be reached directly via the Internet.
When starting a server, you should set your game stats value to false (
?gamestats=true). As Epic's stats servers are not running any longer, it does not make any sense to require players to use the stats with a pseudo account for this feature.
It might occur, that UT 2004 reports that it cannot connect to other computers because the UT2004 executable is not cleared in the Windows Firewall. This also happens also when actually not using the Windows firewall (e. g. using a third party firewall). There is a way around this problem:
After restarting UT2004 you should be good to go.