The port 138/UDP…. is associated with…. NetBIOS Datagram Service…., used by some historical NetBIOS mechanisms to exchange datagrams on a local network. It appears mainly in old Windows environments or compatibility and should not be confused with the NetBIOS name service of port 137….
What is Port 138 for?…
The port 138/UDP…. transports the NetBIOS Datagram Service, historically used for sessionless communications and some Windows legacy ads. It completes port 137 for NetBIOS names and port 139 for sessions. These streams are often linked to local broadcast and should not be assimilated to modern SMB sharing on 445….
The presence of 138 is especially relevant when an old service still depends explicitly on NetBIOS overTTIP…
Transport Protocol
UDP
Safety precautions
Should we open this port in the firewall?
Limit UDP 138 to the necessary legacy segments and avoid any Internet exposure. The NetBIOS should not be unnecessarily extended between networks. If the applications have migrated to DNS and SMB direct, Update dependencies before gradually removing NetBIOS….
How do I check this port?
Diagnosis is done better by observing the datagrams than by a connection test. Check NetBIOS activation on the interfaces, then capture UDP 138 during the reproduction of the problem in order to identify source, destination and possible hil…
- Confirm that the the application is using NetBIOS Datagram….
- Check 137 for name resolution if it also fails….
- Check the limits of,2O between VLAN
- Do not confuse this stream with SMB direct on Tictim 445….
FAQ — port 138 / NetBIOS Datagram….
Is Port 138 used for modern SMB sharing?…
Not directly. Modern SMB can use TPC 445 without NetBIOS; 138 belongs to the NetBIOS legacy operation….
Why is port 138 difficult to test?….
It uses UDP and application datagrams; the lack of response to a port pseudo-test is not enough. A capture during a real exchange is more than theexposure….
Which ports are linked to NetBIOS?….
Ports 137, 138 and 139 have complementary roles for NetBIOS names, datagrams and sessions…