Port 138 NetBIOS datagram · UDP

What runs on port 138

NetBIOS Datagram Service: connectionless messages between Windows hosts, used for browser elections (the "Network Neighbourhood" master browser) and legacy messaging. Noise on a modern network and a fingerprinting aid for anyone scanning it.

Software that uses it: Windows networking · Samba (nmbd)

Should port 138 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

• Block 137–139 at the perimeter; SMB today uses 445 directly.

Check port 138

# on the host: is anything listening, and which process?
ss -lunp 'sport = :138'
sudo lsof -i :138

# from another machine: is it reachable?
nc -zuv host 138
sudo nmap -sU -p 138 host

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 138

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 138 proto udp
sudo ufw deny 138/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 udp dport 138 accept
nft add rule inet filter input udp dport 138 drop

# iptables (legacy)
iptables -A INPUT -p udp --dport 138 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 138 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=138 protocol=udp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 138" with no source is just "open".

Related ports

• 137 NetBIOS name — Windows name service
• 139 NetBIOS session — Old SMB over NetBIOS
• 445 SMB — Windows file sharing, Samba

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions