Telnet
What is Port 23?
Port 23 is the default port for the Telnet protocol, one of the earliest remote access protocols developed for the ARPANET in 1969. Telnet provides a bidirectional, text-based communication channel that allows users to log into and manage remote computers. Defined in RFC 854, Telnet was the primary means of remote system administration for decades before SSH replaced it. Telnet transmits everything in plaintext — including usernames, passwords, and session data — making it fundamentally insecure on modern networks. Despite this, Telnet is still found in legacy industrial control systems, embedded devices, network equipment management interfaces, and as a diagnostic tool for testing TCP connectivity to specific ports.
Common Uses
- Legacy remote administration of older systems
- Network device configuration (switches, routers)
- Testing TCP connectivity to services on specific ports
- Industrial control systems and SCADA interfaces
Technical Details
Copy-paste commands to check port 23 (Telnet) from your terminal
Test connectivity
nc -zv example.com 23
curl -v --max-time 5 http://example.com:23/
telnet example.com 23
nmap -p 23 -sV example.com
(echo > /dev/tcp/example.com/23) 2>/dev/null && echo "open" || echo "closed"
Open port in firewall
sudo ufw allow 23/tcp
sudo iptables -A INPUT -p tcp --dport 23 -j ACCEPT
sudo firewall-cmd --permanent --add-port=23/tcp && sudo firewall-cmd --reload
Security Considerations
Telnet should never be used over untrusted networks. All data, including credentials, is transmitted in plaintext and can be intercepted by anyone on the network path. Telnet-enabled devices are prime targets for botnets like Mirai. Replace Telnet with SSH wherever possible. If Telnet is required, restrict it to isolated management networks and use VPN tunnels.
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