Networks: Telnet and SSH

Introduction

When managing networks and servers, administrators often need to access and control devices remotely. Two common protocols for remote terminal access are Telnet (Telecommunication Network) and SSH (Secure Shell). Both allow users to connect to remote systems and execute commands as if they were sitting in front of them, but they differ significantly in terms of security and features.

In this article, we'll explore both protocols, understand their differences, and learn how to use them effectively for network management and remote access.

Telnet: The Original Remote Access Protocol

What is Telnet?

Telnet is one of the oldest network protocols, developed in 1969 and standardized as part of the TCP/IP suite. It operates on port 23 and provides a bidirectional interactive text-oriented communication facility using a virtual terminal connection.

How Telnet Works

Telnet uses a client-server model:

1. A Telnet client establishes a connection with a Telnet server
2. The client sends keystrokes to the server
3. The server processes these commands as if they were entered locally
4. The server sends the output back to the client

Using Telnet

Here's how to use Telnet from a command line:

bash

telnet hostname_or_ip port

Example connecting to a server:

bash

telnet 192.168.1.1 23

Output:

Trying 192.168.1.1...
Connected to 192.168.1.1.
Escape character is '^]'.

login: admin
Password: 
Welcome to RouterOS!
[admin@MikroTik] >

Telnet Security Issues

The major drawback of Telnet is its lack of security:

• No encryption: All data, including usernames and passwords, is transmitted in plain text
• No authentication of the server (vulnerable to man-in-the-middle attacks)
• No data integrity checking

Due to these security concerns, Telnet has largely been replaced by SSH for most remote administration tasks. However, it's still used for:

• Simple network debugging
• Accessing legacy systems
• Internal networks where security is less of a concern
• Connecting to network devices that don't support SSH

SSH: The Secure Alternative

What is SSH?

SSH (Secure Shell) was developed in 1995 specifically to address the security flaws in protocols like Telnet. It operates on port 22 by default and provides:

• Encrypted communications
• Strong authentication
• Data integrity verification
• Port forwarding capabilities

How SSH Works

SSH also uses a client-server model but adds encryption and security:

1. Client and server establish a connection and negotiate encryption parameters
2. They exchange encryption keys using secure key exchange algorithms
3. All subsequent communication is encrypted
4. The client authenticates to the server (password, key-based, or other methods)
5. Upon successful authentication, a secure shell session is established

SSH Authentication Methods

SSH supports multiple authentication methods:

1. Password Authentication: Simple username/password authentication (encrypted)
2. Public Key Authentication: Uses a pair of cryptographic keys for authentication
3. Host-Based Authentication: Authentication based on the host rather than the user
4. Keyboard-Interactive: Can include multi-factor authentication methods

Using SSH

Basic SSH command syntax:

bash

ssh username@hostname_or_ip -p port_number

Example of connecting to a server:

bash

ssh [email protected]

Output:

The authenticity of host '192.168.1.1' can't be established.
RSA key fingerprint is SHA256:uNiVztksCsDhcc0u9e8BujQXVUpKZIDTMczCvj3tD2s.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.1' (RSA) to the list of known hosts.
[email protected]'s password: 
Welcome to Ubuntu 20.04.2 LTS
admin@server:~$ 

Generating SSH Keys

To use public key authentication, you need to generate a key pair:

bash

ssh-keygen -t rsa -b 4096

Output:

Generating public/private rsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/user/.ssh/id_rsa
Your public key has been saved in /home/user/.ssh/id_rsa.pub
The key fingerprint is:
SHA256:l4Ue3f+6fBEIdhvKcLYjDR90ZQGw9q0LY2XP2GgIbZ0 user@localhost

Then, copy your public key to the server:

bash

ssh-copy-id username@remote_host

SSH Advanced Features

SSH offers several advanced features beyond basic remote terminal access:

1. Secure File Transfer with SCP

SCP (Secure Copy) uses SSH for file transfers:

bash

scp source_file username@hostname:/path/to/destination

Example:

bash

scp report.pdf [email protected]:/home/admin/documents/

2. Port Forwarding

SSH can create encrypted tunnels for other applications:

bash

# Local port forwarding
ssh -L local_port:destination_host:destination_port username@ssh_server

# Remote port forwarding
ssh -R remote_port:destination_host:destination_port username@ssh_server

Example of accessing a remote web server securely:

bash

ssh -L 8080:internal-server:80 admin@gateway-server

3. SSH Configuration File

Create an ~/.ssh/config file to simplify SSH connections:

Host webserver
    HostName 192.168.1.100
    User admin
    Port 2222
    IdentityFile ~/.ssh/webserver_key

Then simply use:

bash

ssh webserver

Telnet vs SSH: Comparison

Feature	Telnet	SSH
Default Port	23	22
Data Encryption	None (plaintext)	Strong encryption
Authentication	Basic (plaintext)	Multiple secure methods
Data Integrity	No verification	Yes (integrity checking)
Server Authentication	No	Yes
File Transfer	No (requires FTP)	Yes (SCP/SFTP)
Port Forwarding	No	Yes
Developed	1969	1995
Security	Very low	High
Modern Usage	Legacy systems, debugging	Standard for remote administration

Practical Examples

Example 1: Using Telnet for Network Debugging

One common use of Telnet today is to test if a specific port is open on a server:

bash

telnet google.com 80

If the connection succeeds, you can send HTTP commands:

GET / HTTP/1.1
Host: google.com

(Type the commands followed by two enters)

This allows you to directly interact with web servers and other network services for debugging purposes.

Example 2: Securing SSH Further

For production environments, you can enhance SSH security:

1. Disable password authentication in /etc/ssh/sshd_config:

PasswordAuthentication no

2. Restrict SSH access to specific users:

AllowUsers admin maintainer

3. Change the default port:

Port 2222

4. Configure fail2ban to protect against brute force attacks:

bash

sudo apt-get install fail2ban

Example 3: Automating Tasks with SSH

SSH enables secure automation of remote tasks:

bash

# Run a remote command
ssh user@server "ls -la /var/log | grep error"

# Run multiple commands
ssh user@server "cd /var/www && git pull && systemctl restart apache2"

# Execute a local script on a remote server
ssh user@server 'bash -s' < local_script.sh

Security Best Practices

Whether using Telnet (only in secure environments) or SSH, follow these best practices:

1. Always use SSH over Telnet when security matters
2. Use key-based authentication instead of passwords when possible
3. Keep software updated with security patches
4. Use strong passwords and passphrases
5. Limit login attempts and implement account lockouts
6. Restrict access to authorized IPs when possible
7. Use non-standard ports for public-facing services
8. Audit and log all remote access attempts
9. Disable root login over SSH when possible
10. Use SSH version 2 (SSHv1 has security vulnerabilities)

Summary

Telnet and SSH are both protocols for remote terminal access, but they serve different purposes in modern networks:

• Telnet is a legacy protocol with no built-in security, useful primarily for network debugging and accessing legacy systems in secure environments.

• SSH is the secure modern alternative, providing encrypted communications, strong authentication, and additional features like secure file transfers and port forwarding.

For almost all remote administration tasks, SSH is the recommended protocol due to its security features. Understanding both protocols helps network administrators manage different systems effectively and securely.

Additional Resources and Exercises

Resources

• OpenSSH Documentation
• IETF RFC 4251-4254 (SSH Protocol Specifications)
• IETF RFC 854 (Telnet Protocol Specification)

Exercises

1. Install an SSH server on your local machine and connect to it from another device.
2. Set up key-based authentication and disable password login for your SSH server.
3. Use SSH to create a secure tunnel for accessing a web interface on a remote server.
4. Compare the network traffic of Telnet and SSH using Wireshark to observe the differences in encryption.
5. Write a shell script that uses SSH to back up files from multiple remote servers to a central location.