Port 27015 Source engine games · TCP and UDP

What runs on port 27015

Source engine dedicated servers (Counter-Strike, Team Fortress 2, Garry's Mod, Left 4 Dead) and Steam's client traffic: UDP 27015 for the game, TCP 27015 for RCON. Also the beginning of the 27015–27030 range Steam uses for matchmaking and voice. Game servers are meant to be public; RCON on TCP should be firewalled to admins.

Software that uses it: SRCDS (Source Dedicated Server) · Steam client · Valheim (2456), Rust (28015) use neighbours

Should port 27015 be open?

Designed to face the internet. This port exists to be reachable from anywhere, so the usual question is not whether to open it but how the service behind it is configured and patched.

• The Steam server browser needs UDP 27015 reachable; sv_region and the A2S query protocol are what the browser uses.

Check port 27015

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :27015'
ss -lunp 'sport = :27015'
sudo lsof -i :27015

# from another machine: is it reachable?
nc -zv host 27015
nmap -p 27015 host
nc -zuv host 27015
sudo nmap -sU -p 27015 host
curl -v telnet://host:27015      # raw TCP connect test, Ctrl-C to leave

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 27015

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 27015 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 27015 proto udp
sudo ufw deny 27015/tcp
sudo ufw deny 27015/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 meta l4proto { tcp, udp } th dport 27015 accept
nft add rule inet filter input meta l4proto { tcp, udp } th dport 27015 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 27015 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 27015 -j DROP
iptables -A INPUT -p udp --dport 27015 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 27015 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=27015 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 27015" with no source is just "open".

Related ports

• 25565 Minecraft

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions