Why Every Network Engineer Still Needs To Master Telnet (And Why They Shouldn't) + Video

Introduction:

In an era of zero‑trust architectures and encrypted‑by‑default communications, the Telnet protocol remains a surprisingly persistent – and dangerous – presence in many corporate networks. With a single packet capture, an attacker can harvest administrative credentials and session data sent in plain text across the wire. For network professionals, truly mastering remote access protocols means understanding precisely why Telnet is a gaping security hole and how to systematically replace it with SSH, while still being able to recognise, troubleshoot and harden both protocols in real‑world environments.

Learning Objectives:

Explain the core technical differences between Telnet and SSH, including port assignments, encryption mechanisms, and authentication capabilities.
Configure Cisco IOS routers and switches for secure SSH‑only remote management, then verify and test the configuration.
Apply a complete security hardening checklist to a production network, eliminating Telnet and implementing industry best practices.

You Should Know:

Telnet vs. SSH – A Technical Deep Dive

Telnet (port 23) transmits every byte of data – including usernames, passwords and full session content – in unencrypted plain text. This makes it trivial for an attacker on the same network to perform a man‑in‑the‑middle attack and capture all credentials. SSH (port 22) was designed from the ground up to provide confidentiality, strong authentication and data integrity through robust cryptography. It uses a client‑server model where the server generates encryption keys automatically, and all communications, including passwords, are encrypted before they leave the client. Furthermore, SSH supports public‑key authentication, completely eliminating the need for password transmission.

Step‑by‑step – What this means in practice and how to use it

When you connect to a remote device, a Telnet session is like shouting your password across a crowded room – anyone who listens will hear it exactly as you said it. An SSH session, by contrast, scrambles every word before it leaves your mouth.

To start a Telnet session (for testing only):
On Windows, first enable the Telnet client via Control Panel → Programs → Turn Windows features on or off → Telnet Client. Then open a command prompt and run:
```
telnet 192.168.1.100 23
```
On Linux/macOS, simply open a terminal and use:
```
telnet 192.168.1.100 23
```
Never use this on a production network or over the internet.
To start an SSH session (the correct way):
Linux/macOS (built‑in):
```
ssh [email protected]
```

or with a specific port:

ssh -p 2222 [email protected]

– Windows (PowerShell or CMD with OpenSSH installed):

ssh [email protected]

– If you are managing Cisco devices, you can also use the SSH client from the Cisco CLI to establish an encrypted outbound connection to another device that runs an SSH server.

Lab tip: Use a packet sniffer like Wireshark while performing a Telnet login to see your password appear in clear text. Then repeat the same test using SSH – you will see only encrypted gibberish.

Cisco Configuration – From Insecure Plain‑Text Access to Fully Hardened SSH

On Cisco IOS and NX‑OS devices, you can enable both Telnet and SSH servers simultaneously, but industry best practice is to completely disable Telnet and configure SSH with strong parameters. Cisco devices can accept SSH connections from any commercially available SSH client, and the SSH server can authenticate users locally or via RADIUS, TACACS+ or LDAP.

Step‑by‑step – Secure SSH configuration on a Cisco router/switch

All commands are entered from privileged EXEC mode (enable).

Set the device hostname and an IP domain name

configure terminal
hostname R1
ip domain-name company.local

Generate an RSA key pair for SSH (the device automatically generates the required server keys)
```
crypto key generate rsa modulus 2048
```
Create a local user account with a strong encrypted password
```
username admin privilege 15 secret StrongP@ssw0rd!
```

Enable SSH version 2 and set session timeouts

ip ssh version 2
ip ssh time-out 60
ip ssh authentication-retries 2

Configure the VTY lines to require local authentication and accept only SSH
```
line vty 0 4
login local
transport input ssh
exec-timeout 5 0
exit
```

6. Disable the Telnet server completely

no ip telnet server

7. Save the configuration

copy running-config startup-config

After these steps, a remote SSH client can connect securely, while any attempt to use Telnet will be rejected. The device can optionally be configured as an SSH server that automatically generates its own encryption keys, ensuring immediate secure communication.

Client Hardening on Linux and Windows – Locking Down the Endpoints

Even if you secure the network devices, the client side must also be hardened. Telnet clients should be removed or disabled by default, and SSH clients should be configured to reject insecure legacy protocols and weak ciphers.

> Step‑by‑step – Linux and Windows hardening

On Linux (e.g., Ubuntu/Debian):
Remove the Telnet client and firewall block port 23:
```
sudo apt remove telnet
sudo iptables -A INPUT -p tcp --dport 23 -j DROP
```
Harden the SSH client configuration in `/etc/ssh/ssh_config` to only use modern key exchange algorithms:
```
Host 
KexAlgorithms [email protected]
Ciphers [email protected]
Protocol 2
```
For the SSH server (if you run one), disable root login and enforce public‑key authentication by editing /etc/ssh/sshd_config:
```
PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
```

Then restart the SSH service:

sudo systemctl restart sshd

On Windows:
Uninstall the Telnet client via Control Panel → Programs → Turn Windows features on or off by unchecking the Telnet Client box. Use the built‑in OpenSSH client instead. To enable it:
```
Add-WindowsCapability -Online -Name OpenSSH.Client~~~~0.0.1.0
```
You can also manage SSH server settings through the `sshd_config` file located in %ProgramData%\ssh\.

Troubleshooting Remote Access – Essential Commands and Methodology

When remote access fails, a structured troubleshooting approach saves hours of frustration. Knowing the right Cisco CLI commands is a must for any network engineer. Use the following commands in sequence to diagnose issues.

> Step‑by‑step – Remote access troubleshooting checklist

1. Verify interface status and IP configuration

show ip interface brief

– Check that the management interface is up and has the correct IP address.

2. Test basic connectivity

ping <destination-ip>

– If ping fails, the problem is likely at Layer 2 or Layer 3.

Check that the SSH/Telnet service is running and properly configured
```
show ip ssh
```
– Displays SSH version, timeout settings and authentication retries.

4. Examine the VTY line configuration

show running-config | section line vty

– Confirm that `transport input ssh` is present and that `login local` is set.

Check access lists applied to the VTY lines
```
show access-lists
```
– An ACL that accidentally blocks your source IP will prevent any login.

6. View logs for authentication failures

show log | include SSH

– Look for messages such as “SSH authentication failed” or “%SSH‑3‑NO_MATCH”.

If you are troubleshooting from a Linux or Windows client, use the following commands to isolate the issue:

On Linux:

Test if the SSH port is open
nc -zv 192.168.1.100 22
Run SSH in verbose mode to see detailed handshake messages
ssh -vvv [email protected]
Capture traffic to see if packets are dropped
sudo tcpdump -i eth0 host 192.168.1.100 and port 22

On Windows (PowerShell):

Test TCP connectivity to port 22
Test-NetConnection 192.168.1.100 -Port 22
Run SSH with debug output (if OpenSSH is installed)
ssh -vvv [email protected]

Security Hardening Checklist – Moving from Telnet to a Zero‑Trust Model

Many organisations still have legacy devices or automation scripts that rely on Telnet. In such cases, a layered security approach is essential. The ultimate goal, however, is to completely eliminate Telnet from the production network.

Step‑by‑step – Hardening remote access for production networks

Phase 1 – Immediate replacement:
Identify every device that has Telnet enabled.
Configure SSH using the steps in section 2.
Disable the Telnet server (no ip telnet server) and remove it from startup configurations.
Phase 2 – Network‑level protection (if Telnet must be kept temporarily):

Use an IP access list to restrict Telnet access to a single, trusted management host:

access-list 99 permit host 192.168.1.10
line vty 0 4
access-class 99 in
transport input telnet

Encapsulate Telnet inside an SSH tunnel. For example, using local port forwarding:
```
ssh -L 2323:old-device:23 admin@gateway
```
Then connect to `localhost:2323` using Telnet – the traffic is encrypted through the SSH tunnel.
Phase 3 – Defence in depth:
Enforce multi‑factor authentication (MFA) for SSH access using RADIUS with DUO or OKTA.
Run SSH services on a non‑standard port to reduce automated scanning noise.
Combine VPN or IP‑based allow‑listing with strong authentication for a layered defence.
Regularly audit SSH keys and remove unused or orphaned keys from the `.ssh/authorized_keys` files.

What Undercode Say:

The single most critical action any network engineer can take today is to disable Telnet everywhere and enforce SSH‑only access with public‑key authentication.
Beyond the obvious security benefits, mastering SSH configuration and troubleshooting directly elevates one’s ability to manage large‑scale, multi‑vendor networks efficiently and securely.

When Mohamed Abdelgadr shared that he had revised and practiced Telnet and SSH, he highlighted a fundamental truth: remote access protocols are the front doors to every network device. Practicing hands‑on labs with a simulator like Cisco Packet Tracer is the most effective way to internalise these concepts before touching live equipment. The provided GitHub repository demonstrates a complete working configuration of both Telnet (for learning comparison) and SSH (for production) on a Cisco router, including SSH version 2, access lists, timeout values and disabling of unnecessary services. This practical, step‑by‑step approach is exactly what prepares engineers for real‑world interviews and operational roles.

Expected Output:

After applying the SSH configuration outlined in section 2, a network administrator should see the following when connecting from a remote client:

$ ssh [email protected]
Password: 
R1> enable
Password: 
R1 show ssh
Connection Version State Username Encryption Authentication
0 2.0 SSH-2 admin aes256-ctr password

From the device itself, verifying the SSH server status:

R1 show ip ssh
SSH Enabled - version 2.0
Authentication timeouts: 60 secs; Authentication retries: 2
Minimum expected Diffie Hellman key size : 1024
IOS Keys in SECSH format(ssh-rsa, base64 encoded): TRUNCATED

Trying to Telnet to the same device will fail, as expected:

C:> telnet 192.168.1.100 23
Connecting To 192.168.1.100...Could not open connection to the host, on port 23: Connect failed

Prediction:

As cyber‑insurance policies become more stringent and compliance frameworks such as NIS2 and PCI DSS 4.0 explicitly require encrypted management channels, any remaining Telnet usage will be forced out of the enterprise within the next two to three years. Network automation tools will increasingly enforce “SSH‑only” policies at the orchestration layer, and any device that cannot support SSH will be treated as a legacy risk that must be isolated or replaced. The role of the network engineer will evolve from simply knowing how to type configuration commands to architecting secure, verifiable remote access solutions that integrate with centralised identity management, logging and continuous compliance monitoring.

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