NetBIOS Name Service
What is Port 137?
Port 137 is used by the NetBIOS Name Service (NBNS), which provides name registration and resolution for NetBIOS names on local networks. NetBIOS, originally developed by IBM in 1983, allows applications on different computers to find and communicate with each other by name. When a Windows computer joins a network, it registers its NetBIOS name on port 137. Other machines query this port to resolve names to IP addresses. While DNS has largely replaced NetBIOS name resolution in modern networks, many Windows systems still fall back to NetBIOS for local name resolution, especially in workgroup (non-domain) environments. The service is part of the broader NetBIOS over TCP/IP (NBT) suite.
Common Uses
- Windows computer name resolution on local networks
- Network neighborhood browsing in Windows
- Legacy application name resolution
- Workgroup-based file and printer sharing discovery
Technical Details
Copy-paste commands to check port 137 (NetBIOS Name Service) from your terminal
Test connectivity
nmap -p 137 -sU -sV example.com
(echo > /dev/tcp/example.com/137) 2>/dev/null && echo "open" || echo "closed"
Open port in firewall
sudo ufw allow 137/udp
sudo iptables -A INPUT -p udp --dport 137 -j ACCEPT
sudo firewall-cmd --permanent --add-port=137/udp && sudo firewall-cmd --reload
Security Considerations
NetBIOS name services expose computer names and can be used for network reconnaissance. NBNS poisoning attacks (like those performed by Responder) can capture credentials by impersonating legitimate network resources. Block ports 137-139 at the network perimeter. Disable NetBIOS over TCP/IP on systems that don't require it. Use DNS for name resolution instead.
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