Port 23: Telnet

What is port 23 used for?

Unencrypted remote terminal access, a predecessor of SSH.

Should port 23 be open?

Don’t expose to the internet. Keep this port on private networks and reach it through a VPN or SSH tunnel. Everything, including passwords, crosses the network in plain text. Telnet left open on routers and IoT devices is a classic botnet entry point. Use SSH.

See what is listening on port 23

# Linux
sudo ss -tlnp | grep ':23 '

# macOS
sudo lsof -nP -i TCP:23 -sTCP:LISTEN

# Windows (PowerShell)
Get-NetTCPConnection -LocalPort 23 -State Listen

Open or block port 23 in a firewall

# Ubuntu / Debian (ufw)
sudo ufw allow 23/tcp
sudo ufw deny 23/tcp

# Windows (PowerShell, as administrator)
New-NetFirewallRule -DisplayName "Port 23" -Direction Inbound -Protocol TCP -LocalPort 23 -Action Allow

On cloud servers, also open the port in the provider’s security group or network firewall. At home, a service behind a router needs a port-forwarding rule to the device’s local IP.

Test it from outside

A port can be open on the server but blocked upstream. The Port Checker connects from the internet, so it shows what the world sees. From your own machine, use nc -vz example.com 23 (macOS/Linux) or Test-NetConnection example.com -Port 23 (Windows).