You only need port 27015 forwarded on UDP to run a server. If you want it to show up on the Steam server list, you need another between 27010 and 27020 forwarded on TCP ( I forget which one excactly ).
You can actually run the server on any port. Just add +port xxxxx to the launch options.
On my CS server, I do ./hlds_run -game cstrike +map de_dust +port 27017 -secure
If you're on windows you can just edit the shortcut and add +port 666 or whatever to it.