[TUTORIAL] Xonotic Server behind OPNSense (and pfSense)

When NATting behind a OPNSense (or pfSense) firewall, setting up a port-forward is enough to let people manually connect to your Xonotic server when they know the IP and the port. However, the server is not showing up in the in-game server list or dpmaster.deathmask.net. This can be solved.
The description below is based on OPNSense, but it can probably be applied to pfSense too, because they share a lot. Some settings may be in a slightly different place or have a slightly different names though.

First: the port-forward:

• Go to Firewall - NAT - Port Forward
  
• Click +Add
  
• Interface WAN
  
• Protocol UDP
  
• Destination WAN address
  
• Destination port range (other) [port of Xonotic Server] (from en to)
  
• Redirect target IP Single Host or Network [IP of Xonotic Server]
  
• Redirect target port (other) [port of Xonotic Server]
  

Second: make outbound NAT rule:
This is to disable source port rewriting. Xonotic (and other Quak'ish games don't like it)
Read  more about it here.

• Go to Firewall-NAT-Outbound
  
• Click  on Hybrid outbound NAT rule generation
  
• Click +Add
  
• Interface WAN
  
• Protocol UDP
  
• Source Address Single host or Network [IP of Xonotic Server]/32
  
• Source port (other) [port of Xonotic Server]
  
• Destination address any
  
• Destination port any
  
• Enable Static-port
  

Especially the last is very important!
I hope this helps some people.
I'm leaving this here because it took me ages to find this out.