Port 138 · NetBIOS Datagram

Port 138 is NetBIOS Datagram, UDP.

NetBIOS datagram service — used by Windows browser elections and some messaging.

Palo Alto

Create the service object for this port, then allow the App-ID on it. Restricting the rule to the port's service object keeps the rule working on a non-default port and stops the App-ID matching on other ports. Replace the placeholders with your own rule name and zones.

set service udp-138 protocol udp port 138

set rulebase security rules <rule-name> from <from-zone> to <to-zone> source any destination any application netbios-dgm service udp-138 action allow

Risk and exposure

Rated medium risk. Restrict this port to the sources that need it and keep the service patched; avoid exposing it to the whole internet unless the service is meant to be public.

Check it

• Linux ss -ulnp | grep :138
• nmap nmap -sU -p 138 <host>

Related ports

• 42 WINS
• 137 NetBIOS Name Service
• 139 NetBIOS Session
• 19 Chargen
• 23 Telnet
• 110 POP3

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