Port 23 — Telnet

TCP well-known

What runs on port 23

Unencrypted remote terminal access, superseded by SSH decades ago.

Security considerations

Everything including the password is plain text on the wire. There is no safe way to expose this. If something still needs it, tunnel it.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :23
sudo ss -lntp | grep :23

# Windows
netstat -ano | findstr :23
Get-NetTCPConnection -LocalPort 23

# is it reachable from outside?
nc -zv example.com 23
curl -v telnet://example.com:23

Freeing the port

# find the process, then stop it
sudo lsof -ti :23 | xargs kill        # Linux / macOS
netstat -ano | findstr :23            # note the PID, then:
taskkill /PID <pid> /F                    # Windows

Other common ports

• Port 20 — FTP data
• Port 21 — FTP control
• Port 22 — SSH / SFTP
• Port 25 — SMTP
• Port 53 — DNS
• Port 67 — DHCP server
• Port 68 — DHCP client
• Port 69 — TFTP
• Port 80 — HTTP
• Port 110 — POP3
• Port 123 — NTP
• Port 143 — IMAP
• Port 161 — SNMP
• Port 389 — LDAP
• Port 443 — HTTPS
• Port 445 — SMB
• Port 465 — SMTPS
• Port 514 — Syslog

All port numbers