Port / UDP
3478STUN / TURN
STUN discovers a client's public address for NAT traversal, and TURN relays media when a direct path cannot be established.
If it is internet-facing: Port 3478 is routine where it is expected. Its risk lives in the service configuration behind it, not the port itself.
Exposure
Internet-facing
Transport
Cleartext
Risk if public
Low
Exposure verdict
Port 3478 normally answers from the internet, but the transport is not confidential by default. Enforce the encrypted variant before the traffic leaves your network.
Traffic crosses the network readable and modifiable. Anything sensitive on this port needs a tunnel, a VPN, or a different protocol.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other messaging & media ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- TURN relays consume real bandwidth, so an open relay without credentials is an expensive gift.
- STUN binding requests reveal the client's public address mapping, which is the point of the protocol.
Recommended firewall rule
Allow it, and harden the service
This port is expected to answer from anywhere, so the firewall is not where its security comes from.
ufw
sudo ufw allow 3478/udpiptables
sudo iptables -A INPUT -p udp --dport 3478 -j ACCEPTCheck what is really there
Listening locally
sudo ss -ulpn 'sport = :3478'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iUDP:3478macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 3478 <host>A version probe confirms whether STUN / TURN is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 3478The quick yes/no when you only need to know whether a path exists through the firewall.