Killing Floor server ports and NAT
If your server works for you but your friends cannot join, this is almost always a networking problem: NAT, firewall, or port forwarding.
This page focuses on the mindset and the checklist that solves most cases.
The simple model (what must be true)
For a friend on the internet to join your server:
- Your server must listen on a fixed port (not random).
- Your router must forward that port to the host machine (if you host at home).
- Your firewall must allow inbound traffic on that port.
- Your public IP must be reachable (some ISPs use CGNAT).
Step-by-step checklist
1) Pick a fixed server port
Do not leave ports random. Use a fixed game port in server config.
If you do not know where to set it, start with the dedicated server guide:
2) Allow it in firewall (host machine)
On Windows, the first run often triggers a firewall prompt. If you missed it:
- Allow the server process in Windows Firewall (Private/Public as needed).
- If you use a third-party firewall, add a rule for inbound UDP on your server port.
3) Forward the port on your router (home hosting)
Create a port forward rule:
- External port: your chosen server port
- Internal IP: the LAN IP of the host machine
- Internal port: same port
- Protocol: usually UDP (depends on server config)
Important:
- Make the host LAN IP static (DHCP reservation), or your forwarding will break later.
4) Verify your public reachability (CGNAT check)
If you configured forwarding and firewall, but nobody can join, your ISP might use CGNAT.
Typical signs:
- You have no real public IPv4 address on your router.
- Port forwarding seems to do nothing.
In that case, options are:
- host on a VPS with a public IP
- ask ISP for a public IP option
- use an existing community server instead
Troubleshooting (fast)
Symptom: friends can join on LAN, but not from the internet
Almost always port forwarding or CGNAT.
Symptom: one friend can join, another cannot
Could be their network restrictions, firewall, or mismatched connection method. Try:
- Have them try a different network (mobile hotspot) as a test.
Symptom: server is visible in browser, but join fails
Visibility does not always mean join is possible. Check firewall rules and make sure your server is actually listening.
Mini scenario (realistic)
A server host forwards the port but forgets to reserve a static LAN IP. It works for a day, then breaks after a router reboot because the host gets a new IP. Fixing DHCP reservation makes it stable.