Port 43: WHOIS
Port 43 is WHOIS, the protocol registrars and registries use to answer queries about domain name and IP address ownership. Command-line whois clients and lookup websites both connect on this port.
| Port number | 43 |
|---|---|
| Service | WHOIS protocol |
| Protocol | TCP |
| Category | Legacy protocols |
| Default encryption | No |
How do I check whether port 43 is open?
Port 43 carries TCP, so a connection either completes its handshake or it does not, which makes the check definitive.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | nc -vz example.com 43 | A TCP handshake either completes or it does not, so netcat gives a definitive answer in one line. |
| Same check on Windows | Test-NetConnection example.com -Port 43 | Built into PowerShell. TcpTestSucceeded in the output is the answer; telnet is not installed by default on modern Windows. |
| What is listening locally? | ss -tlnp | grep ':43 ' | Lists the process bound to port 43 on Linux. On macOS use lsof -nP -iTCP:43 -sTCP:LISTEN, on Windows netstat -ano | findstr :43 |
Replace example.com with the host you are testing. A blocked port and a port with nothing listening on it look identical from the outside, so if a service should be running, check locally before blaming the firewall.
Frequently asked questions
What is port 43 used for?
Port 43 is WHOIS, the protocol registrars and registries use to answer queries about domain name and IP address ownership. Command-line whois clients and lookup websites both connect on this port.
Is port 43 TCP or UDP?
Port 43 (WHOIS) uses TCP.
Is port 43 secure?
Port 43 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 43 on my firewall?
Only if you specifically need WHOIS. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.