Telnet is a network protocol used to access remote computers over a network
Works on a client–server model
Allows users to execute commands on a remote machine
How Telnet Works
Uses TCP for communication
Default port: 23
User connects using:
IP address or hostname
Username and password
Provides a command-line interface to the remote system
Advantages of Telnet
Simple and easy to use
Low resource usage
Useful for testing network services
Supported on many operating systems
Disadvantages of Telnet
No encryption (data sent in plain text)
Vulnerable to:
Packet sniffing
Man-in-the-middle attacks
Not safe for sensitive data
Security Concerns
Passwords can be easily intercepted
Not recommended for modern secure communication
Mostly replaced by SSH (Secure Shell)
Telnet vs SSH
Telnet:
No encryption
Insecure
SSH:
Encrypted communication
Secure authentication
Widely used today
Telnet Configuration Tutorial - Step by Step
1. Network Topology Overview:
2. Configure the Router (R1) for Telnet Access
Double-click the Router to open its CLI tab.
Enter the following commands step by step (press Enter after each):
3. Verify Basic Connectivity
From the Client PC's CLI (Desktop > Command Prompt):text
You should see successful replies, confirming Layer 3 connectivity.
4. Initiate the Telnet Session from the Client
On the Client PC, go to Desktop > Command Prompt (or Terminal).
Enter:
When prompted:
Username:nerd
Password:cafe (it won't echo on screen).
You should see R1> prompt, indicating successful login.
Type exit to close the session.
5. Capture the Packets in GNS3 with Wireshark
In GNS3, right-click the link between the switch and router (or client and switch) → Start capture.
GNS3 launches Wireshark automatically, capturing live traffic on the selected virtual link.
Apply a filter in Wireshark: tcp.port == 23 or telnet.
Immediately perform (or repeat) the Telnet login from the client (step 4) to capture the session.
Stop capture when done: Right-click the link → Stop capture.
6. Analyze the Capture
In Wireshark, filter for the Telnet stream (e.g., tcp.stream eq 1).
Right-click a packet → Follow → TCP Stream → set view to ASCII.
You will see the full clear-text conversation:
"User Access Verification"
"Username: nerd"
"Password: cafe"
Router prompt and any typed commands
Packet details show Ethernet/IP/TCP headers + unencrypted Telnet payload (credentials visible in plain text).
This matches the insecure Telnet behavior in your screenshot.
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