Port 1434 SQL Server Browser · UDP

What runs on port 1434

SQL Server Browser (UDP): answers "which port is instance X on?" and was the vector for the SQL Slammer worm in 2003. Only needed for named instances; disable it or firewall it otherwise.

Software that uses it: Microsoft SQL Server Browser service

Should port 1434 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.

• Connect with host,port explicitly and you do not need it at all.

Check port 1434

# on the host: is anything listening, and which process?
ss -lunp 'sport = :1434'
sudo lsof -i :1434

# from another machine: is it reachable?
nc -zuv host 1434
sudo nmap -sU -p 1434 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 1434

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 1434 proto udp
sudo ufw deny 1434/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 udp dport 1434 accept
nft add rule inet filter input udp dport 1434 drop

# iptables (legacy)
iptables -A INPUT -p udp --dport 1434 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 1434 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=1434 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 1434" with no source is just "open".

Related ports

• 1433 MS SQL Server

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