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Port 27015 is the default port for Source engine game servers such as Counter-Strike 2 and Garry's Mod and uses UDP and TCP. This tool shows you the port details and generates suitable firewall commands for your server.
Built by Justin Schuessler
Try the commands on your own VPS, with root access and your own firewall.
Add multiple ports one at a time, up to 12.
Game Server · Registered port · OSI layer 7
Game port for Source servers such as Counter-Strike 2 and Garry’s Mod: TCP for RCON, UDP for gameplay. Many Steam games use it as a query port.
IANA: commonly used, but unofficial (unassigned)
Firewall commands
Open port
sudo ufw allow 27015/tcp
sudo ufw allow 27015/udpFor Ubuntu and Debian. ufw creates the rule for both IPv4 and IPv6. If ufw is disabled, allow SSH first (sudo ufw allow 22/tcp), then enable it with sudo ufw enable, or you'll lock yourself out.
On a ZAP-Hosting VPS with an active network firewall, you also need to allow the port in the Customer Panel.
Port 27015 explained
Game traffic between players and the server runs over UDP on port 27015. The server also responds to Steam queries over UDP on the same port so it appears in the server browser with its name, map, and player count. RCON uses TCP on port 27015 and lets you control the server remotely. Many other Steam games also use 27015 as a query port, even if their game traffic uses another port.
Port 27015 was not assigned to Valve by IANA; Valve chose it unofficially. It dates back to the GoldSrc and Source engine era and has remained in use for Counter-Strike, Team Fortress, Garry's Mod, and Counter-Strike 2. SourceTV and GOTV also use port 27020 over UDP by default for spectators. If you run multiple servers on one machine, simply increment the ports—for example, 27015, 27016, and so on.
For players to join and the server to appear in the server browser, UDP port 27015 must be open. You only need TCP on port 27015 for RCON, which you shouldn't make available to everyone. Anyone who guesses the rcon_password can kick players, change maps and run commands. Choose a long, random RCON password and, if possible, allow TCP 27015 only from your own IP. Only open GOTV port 27020 if you actually want to allow spectators.
More ports
These ports are often part of the same setup. Click one to see its explanation and firewall commands.
View all well-known ports and the complete IANA list
Port list source: IANA Service Name and Transport Protocol Port Number Registry, as of October 22, 2026. iana.org
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FAQ
Quick answers about port 27015.
Both, but for different purposes. Game traffic and Steam queries use UDP. RCON, which provides remote server control, uses TCP.
For players, sudo ufw allow 27015/udp is enough. For RCON, allow TCP only from your own IP, for example with sudo ufw allow from 203.0.113.10 to any port 27015 proto tcp. Then use the port checker to verify that the server is reachable.
Port 27020 over UDP is the default port for SourceTV and GOTV. It lets spectators join an ongoing game with a delay. If you do not use this feature, keep the port closed.
Often, UDP 27015 isn't allowed through the firewall, or the server is listening on a different IP or port. Check the UDP firewall rule and the port startup parameter. For many games, a missing or invalid Game Server Login Token also prevents the server from being publicly listed.
Any more questions?
Contact SupportWith a ZAP-Hosting VPS, you have root access, can set up services and firewall rules yourself, and can immediately see which ports are open.
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