Port 137 (NetBIOS Name Service)

NetBIOS name registration and resolution on Windows LANs.

Well-known (system) port, 0-1023 UDP Windows networking Localhost or LAN only

Port 137 is normally used over UDP; the checker tests TCP only, so a "filtered" result does not mean the UDP service is down.

What port 137 is used for

NetBIOS name registration and resolution on Windows LANs.

Security: should port 137 be open?

Localhost or LAN only. Should never reach the internet. Exposed NBNS leaks host and domain names and can be used for small reflection attacks; on a LAN, NBNS spoofing (Responder) captures NTLM hashes.

IANA assignment

Service nameProtoDescriptionAssigneeReference

netbios-ns TCP NETBIOS Name Service - -

netbios-ns UDP NETBIOS Name Service - -

Check what is listening on port 137

Linux: UDP 137

sudo ss -ulnp 'sport = :137'

Linux / macOS: which process owns the port

sudo lsof -nP -i :137

Windows (Command Prompt)

netstat -ano | findstr :137

Windows (PowerShell)

Get-NetUDPEndpoint -LocalPort 137

Test it from outside

UDP probe (no reply does not prove it is closed)

nc -vzu -w 5 HOST 137

Block port 137 in the firewall

ufw

sudo ufw deny 137/udp

iptables

sudo iptables -A INPUT -p udp --dport 137 -j DROP

nftables

sudo nft add rule inet filter input udp dport 137 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block UDP 137' -Direction Inbound -Protocol UDP -LocalPort 137 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.