Port / UDP
1434SQL Server browser
The SQL Server Browser service tells clients which dynamic port a named instance is listening on.
If it is internet-facing: Finding 1434 open to the internet is a finding in its own right — it warrants an explanation, not just a rule.
Exposure
Trusted network only
Transport
Cleartext
Risk if public
High
Exposure verdict
Port 1434 belongs on a trusted internal network. If a perimeter scan finds it answering, treat that as a firewall defect rather than a configuration preference.
Traffic crosses the network readable and modifiable. Anything sensitive on this port needs a tunnel, a VPN, or a different protocol.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other databases & caches ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- It enumerates every instance on the host, and has served as a UDP amplification reflector.
- Disable it and configure static ports for named instances.
Recommended firewall rule
Allow only from 10.0.0.0/8, drop the rest
Swap in the CIDR your administrators or application tier actually come from — the point is that the default answer is no.
ufw
sudo ufw allow from 10.0.0.0/8 to any port 1434 proto udpiptables
sudo iptables -A INPUT -p udp -s 10.0.0.0/8 --dport 1434 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 1434 -j DROPIf you do need it open to everyone
sudo ufw allow 1434/udpWrite down why. An unexplained allow rule outlives the person who added it, and the next reviewer has no way to tell a deliberate exception from an accident.
Check what is really there
Listening locally
sudo ss -ulpn 'sport = :1434'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iUDP:1434macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 1434 <host>A version probe confirms whether SQL Server browser is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 1434The quick yes/no when you only need to know whether a path exists through the firewall.