Port 23: Telnet
Port 23 is Telnet, an old remote-login protocol that sends everything, including passwords, in clear text. It has been replaced by SSH on port 22 and should be disabled where possible.
| Port number | 23 |
|---|---|
| Service | Telnet remote login |
| Protocol | TCP |
| Category | Remote access |
| Default encryption | No |
How do I check whether port 23 is open?
Port 23 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 23 | 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 23 | 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 ':23 ' | Lists the process bound to port 23 on Linux. On macOS use lsof -nP -iTCP:23 -sTCP:LISTEN, on Windows netstat -ano | findstr :23 |
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 23 used for?
Port 23 is Telnet, an old remote-login protocol that sends everything, including passwords, in clear text. It has been replaced by SSH on port 22 and should be disabled where possible.
Is port 23 TCP or UDP?
Port 23 (Telnet) uses TCP.
Is port 23 secure?
Port 23 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 23 on my firewall?
Only if you specifically need Telnet. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.