telnet — Raw TCP connection and port check

Linux / Network and Firewall / telnet.html

A classic client that opens a raw TCP connection so you can see what a service answers.

Basic Information

What is this command?

telnet opens a TCP connection and sends whatever you type without encryption.

It is used for port checks and legacy text protocols, not for secure remote login.

What problems does it solve?

• Check whether a service listens on a port.
• Verify that a firewall rule allows the path.
• Exercise plain-text protocols such as SMTP or HTTP by hand.
• Read the banner and response exactly as sent.

Key features

Port connection

Give a host and port to attempt a connection.

• When to use: When you need a quick open/closed check.
• What you get: A banner or prompt appears when it connects.
• Limitations: Nothing is encrypted, so never send secrets.

Protocol test

After connecting, type protocol commands and read the replies.

• When to use: When inspecting plain-text service replies.
• What you get: You see the raw response from the server.
• Limitations: Being interactive, it is poor for automation.

Exit

Close the session with quit, or Ctrl+] followed by quit.

• When to use: When cleaning up after a test.
• What you get: The connection closes cleanly.
• Limitations: Some servers keep the connection open.

Installation and usage

Check whether it is installed

telnet -h

sudo apt install telnet

Real-world examples

Port check

Connect to a web port and read the banner.

telnet example.com 80

SMTP test

Read the greeting of a mail server.

telnet mail.example.com 25

See a refusal

A closed port is refused immediately.

telnet 127.0.0.1 9