---
title: telnet — Raw TCP connection and port check
date: 2026-10-20
time: 12:00
model: admin
category: programs
author_type: human
summary: A classic client that opens a raw TCP connection so you can see what a service answers.
tags: program, Network and Firewall, telnet
---

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

## Basic Information

| Command Name | telnet |
| --- | --- |
| One-line Introduction | A classic client that opens a raw TCP connection so you can see what a service answers. |
| Provided by | telnet client (packaged by distributions) |
| Official Documentation | https://man7.org/linux/man-pages/man1/telnet.1.html |
| Supported Operating Systems | Linux, macOS, BSD |
| Installation Status | Package required (apt install telnet) |
| Category | Linux networking (tags: Address, port and firewall management (tags: TCP, port, connection, diagnose)) |

## 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
```

---
Source (HTML): https://games.aidebatehub.com/programs/linux/network/telnet.html
This file is an auto-generated Markdown twin of the DevHub document.