How to Install Telnet on Linux and Test Ports (Ubuntu, Debian, AlmaLinux, Rocky)

Updated: 30 September 2026 · Applies to: telnet 0.17 on AlmaLinux and Rocky Linux 9, inetutils-telnet 2.5 on Ubuntu 24.04 and 2.6 on Debian 13

Today the telnet command is mainly used as a quick tool to check whether a port on another server answers, for example a mail server on port 25. The telnet server, which gives remote logins, sends everything including passwords unencrypted; use SSH for logins instead. This guide replaces the older CentOS/RHEL 7 version (CentOS 7 reached end of life in June 2024).

Install the telnet client

sudo apt install telnet      # Ubuntu, Debian (installs inetutils-telnet)
sudo dnf install telnet      # AlmaLinux, Rocky Linux, RHEL

Test a port

telnet mail.example.com 25

Connected to mail.example.com means the port is open; a mail server also greets you with a line starting 220. To leave, type quit, or press Ctrl+] and then type quit. Connection refused means nothing listens there, and a long wait ending in a time-out usually means a firewall blocks the port.

Modern alternatives

nc -zv mail.example.com 25                                  # netcat: open or not
openssl s_client -connect mail.example.com:465 -brief       # ports that use TLS
curl -v telnet://mail.example.com:25                        # not available with curl-minimal on RHEL 9

nc is in the netcat-openbsd package on Ubuntu and Debian and nmap-ncat on AlmaLinux and Rocky Linux.

The telnet server (not recommended)

If an old device or program really needs it, install it only on a private network and never expose port 23 to the internet:

sudo dnf install telnet-server && sudo systemctl enable --now telnet.socket   # AlmaLinux, Rocky Linux
sudo apt install inetutils-telnetd                                             # Ubuntu, Debian

Remove it again when you no longer need it. For port numbers used by mail, see SMTP port numbers.