Port / UDP
138NetBIOS datagram
NetBIOS datagram service carries connectionless browsing and messaging traffic in legacy Windows networks.
If it is internet-facing: Port 138 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Trusted network only
Transport
Cleartext
Risk if public
Elevated
Exposure verdict
Port 138 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
Traffic crosses the network readable and modifiable. Anything sensitive on this port needs a tunnel, a VPN, or a different protocol.
Assigned by IANA through a review process. On Unix systems a process needs root or the CAP_NET_BIND_SERVICE capability to bind here, which is why containers so often remap these to a higher port.
Grouped with the other network services ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
1 note- Part of the legacy NetBIOS stack; disable it wherever SMB over TCP on 445 is available.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 138 proto udpiptables
sudo iptables -A INPUT -p udp -s 10.0.0.0/8 --dport 138 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 138 -j DROPIf you do need it open to everyone
sudo ufw allow 138/udpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -ulpn 'sport = :138'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:138macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 138 <host>A version probe confirms whether NetBIOS datagram is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 138The quick yes/no when you only need to know whether a path exists through the firewall.