Port 137: NetBIOS Name Service
Port 137 is the NetBIOS name service, an old Windows way of finding computers by name on a local network.
What runs on port 137
Before DNS was universal on LANs, Windows machines announced and looked up names over UDP 137. It still exists for backwards compatibility, alongside 138 (datagrams) and 139 (sessions). Samba implements it on Linux and NAS devices.
On a home network
Windows PCs, NAS boxes running Samba and some routers with file sharing answer on 137. A NetBIOS name query to them returns the machine name, workgroup and sometimes the logged-in user.
If port 137 is reachable from the internet
There is no reason for 137 to be reachable from the internet. It leaks machine and user names and has been used for small reflection attacks. It is also a sign that the device behind it may have 139 and 445 exposed too.
How to check whether it is open on your network
A LAN scan will often show device names learned through NetBIOS; that is normal inside. Confirm your router does not forward UDP 137 and that no PC is in the DMZ.
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