Port 137: NetBIOS name
UDP 137 is the port for NetBIOS Name Service. Never on the internet; it leaks host names and workgroup details.
| Service | NetBIOS Name Service |
|---|---|
| Protocol | UDP |
| Exposure risk | High: do not expose to the internet |
| Range | Well-known (0–1023): binding needs root/admin on Unix |
Port 137 is UDP-only. UDP has no handshake, so no online checker can reliably tell open from filtered; test with the service's own client.
What port 137 does
Legacy Windows name resolution on local networks, alongside 138 (datagram) and 139 (session). Modern Windows uses DNS and SMB on 445 directly.
Should port 137 be open?
Never on the internet; it leaks host names and workgroup details.
Find what's listening on port 137
- Linux:
sudo ss -tulpn | grep :137 - macOS:
sudo lsof -nP -i :137 - Windows:
netstat -ano | findstr :137, then look up the PID in Task Manager.
Open or block it in a firewall
- ufw:
sudo ufw allow 137/udp(ordeny) - firewalld:
sudo firewall-cmd --add-port=137/udp --permanent && sudo firewall-cmd --reload - Windows:
New-NetFirewallRule -DisplayName "Port 137" -Direction Inbound -Protocol UDP -LocalPort 137 -Action Allow
Port assignment per the IANA Service Name and Transport Protocol Port Number Registry. Test several ports at once with the port scanner, or any port with the open port checker.
Questions about port 137
What is port 137 used for?
NetBIOS Name Service. Legacy Windows name resolution on local networks, alongside 138 (datagram) and 139 (session). Modern Windows uses DNS and SMB on 445 directly.
Is port 137 TCP or UDP?
UDP. NetBIOS name uses UDP port 137.
Is it safe to open port 137?
High: do not expose to the internet. Never on the internet; it leaks host names and workgroup details.
How do I check if port 137 is open?
Port 137 is UDP-only, which has no handshake, so an online TCP checker can't confirm it. Use the service's own client to test, and check what is listening locally with
ss -ulpn | grep :137 (Linux) or netstat -ano | findstr :137 (Windows).