Port / TCP
43WHOIS
WHOIS answers plain-text queries about domain and IP registration data from registry and registrar servers.
If it is internet-facing: Port 43 is routine where it is expected. Its risk lives in the service configuration behind it, not the port itself.
Exposure
Internet-facing
Transport
Cleartext
Risk if public
Low
Exposure verdict
Port 43 normally answers from the internet, but the transport is not confidential by default. Enforce the encrypted variant before the traffic leaves your network.
Traffic crosses the network readable and modifiable. Anything sensitive on this port needs a tunnel, a VPN, or a different protocol.
Assigned by IANA through a review process. On Unix systems a process needs root or the CAP_NET_BIND_SERVICE capability to bind here, which is why containers so often remap these to a higher port.
Grouped with the other network services ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
1 note- Queries and answers are unauthenticated plaintext; RDAP over HTTPS is the modern replacement.
Recommended firewall rule
Allow it, and harden the service
This port is expected to answer from anywhere, so the firewall is not where its security comes from.
ufw
sudo ufw allow 43/tcpiptables
sudo iptables -A INPUT -p tcp --dport 43 -j ACCEPTCheck what is really there
Listening locally
sudo ss -tlpn 'sport = :43'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 -iTCP:43 -sTCP:LISTENmacOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sV -Pn -p 43 <host>A version probe confirms whether WHOIS is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vz <host> 43The quick yes/no when you only need to know whether a path exists through the firewall.
The safer path
port 443Move to port 443 — HTTPS
RDAP serves the same registration data over HTTPS on port 443.