Telnet - TELecommunication NETwork ==================================== .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **What is Telnet?** Telnet (short for TELecommunication NETwork) is a network protocol used to provide a bidirectional interactive text-based communication facility using a virtual terminal connection. It allows users to remotely access and manage devices over a TCP/IP network, typically on port 23. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Why is Telnet useful?** * Remote Access – Enables users to log into remote systems and control them as if they were local. * Device Management – Commonly used for configuring network devices like routers and switches. * Simple Protocol – Lightweight and easy to implement for basic remote control. * Cross-Platform – Available on most operating systems and networked devices. * Testing and Debugging – Useful for testing open ports and services on remote systems. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **How it works?** * Connection Initiation – The client initiates a TCP connection to the server on port 23. * Negotiation – Client and server negotiate options like terminal type and echo settings. * Authentication – The user is prompted to enter a username and password (in plaintext). * Command Execution – The user can execute commands on the remote system via a virtual terminal. * Session Maintenance – The session remains active until the user logs out or the connection is closed. * Session Termination – Either side can terminate the session, closing the TCP connection. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Where is Telnet used?** * Legacy Systems – Still used in older systems that don’t support SSH. * Network Device Configuration – Used to configure routers, switches, and firewalls. * Remote Server Access – For managing servers without graphical interfaces. * Testing TCP Connectivity – Useful for checking if a port is open on a remote host. * Educational Environments – Used in labs to demonstrate basic networking concepts. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow **Which OSI layer does this protocol belong to?** * User Interaction – Provides a user interface for remote communication. * Runs Over TCP – Operates on top of the transport layer (TCP). * Application-Level Protocol – Handles terminal emulation and command execution at the application layer. .. panels:: :container: container pb-4 :column: col-lg-12 p-2 :card: shadow Topics in this section, * :ref:`Learnings in this section ` * :ref:`Terminology ` * :ref:`Version Info ` * :ref:`Telnet Version&RFC Details ` * :ref:`Telnet Basic Setup on Ubuntu using IPv4 ` * :ref:`Telnet Basic Setup on Ubuntu using IPv6 ` * :ref:`Telnet Protocol Packet Details ` * :ref:`Telnet Usecases ` * :ref:`Telnet Basic Features ` * :ref:`Telnet Feature : Text based Communication ` * :ref:`Telnet Feature : Remote Login ` * :ref:`Telnet Feature : Port Based Access ` * :ref:`Telnet Feature : Network Virtual Terminal ` * :ref:`Telnet Feature : Option Negotiation ` * :ref:`Telnet Feature : Subnegotiation support ` * :ref:`Telnet Feature : Cross Platform Compatiblity ` * :ref:`Telnet Feature : Simple Protocol structure ` * :ref:`Reference links ` .. _Telnet_step1: .. tab-set:: .. tab-item:: Learnings in this section * In this section, you are going to learn .. _Telnet_step2: .. tab-set:: .. tab-item:: Terminology * Terminology .. _Telnet_step3: .. tab-set:: .. tab-item:: Version Info * Version Info .. _Telnet_step5: .. tab-set:: .. tab-item:: Telnet Version&RFC Details .. csv-table:: :file: ./Telnet/Telnet_rfcdetails.csv :widths: 10,10,10,30 :header-rows: 1 .. _Telnet_step19: .. tab-set:: .. tab-item:: Telnet Basic Setup on Ubuntu using IPv4 * Setup .. _Telnet_step20: .. tab-set:: .. tab-item:: MQTT Basic Setup on Ubuntu using IPv6 * Setup .. _Telnet_step6: .. tab-set:: .. tab-item:: Telnet Protocol Packet Details **TCP Connection Establishment Packet** .. csv-table:: :file: ./Telnet/Telnet_packetdetails1.csv :widths: 10,20,30,10 :header-rows: 1 **Telnet Data Packet** .. csv-table:: :file: ./Telnet/Telnet_packetdetails2.csv :widths: 10,20,30,10 :header-rows: 1 **Telnet Command Packet (IAC Sequence) Packet** .. csv-table:: :file: ./Telnet/Telnet_packetdetails3.csv :widths: 10,20,30,10 :header-rows: 1 **Subnegotiation Packet** .. csv-table:: :file: ./Telnet/Telnet_packetdetails4.csv :widths: 10,20,30,10 :header-rows: 1 **Telnet Control Commands** .. csv-table:: :file: ./Telnet/Telnet_packetdetails5.csv :widths: 10,20,30,10 :header-rows: 1 **Telnet Option Codes** .. csv-table:: :file: ./Telnet/Telnet_packetdetails6.csv :widths: 10,20,30,10 :header-rows: 1 **Telnet Session Termination** .. csv-table:: :file: ./Telnet/Telnet_packetdetails7.csv :widths: 10,20,30,10 :header-rows: 1 .. _Telnet_step7: .. tab-set:: .. tab-item:: Telnet Usecases .. csv-table:: :file: ./Telnet/Telnet_usecases.csv :widths: 10,20,30 :header-rows: 1 .. _Telnet_step8: .. tab-set:: .. tab-item:: Telnet Basic Features .. csv-table:: :file: ./Telnet/Telnet_basicfeatures.csv :widths: 10,10,30 :header-rows: 1 .. _Telnet_step9: .. tab-set:: .. tab-item:: Telnet Feature : Text based Communication **Text_based_Communication - Testcases** .. csv-table:: :file: ./Telnet/Telnet_feature1_Text_based_Communication.csv :widths: 10,10,30,20 :header-rows: 1 .. _Telnet_step10: .. tab-set:: .. tab-item:: Telnet Feature : Remote Login **Remote Login - Testcases** .. csv-table:: :file: ./Telnet/Telnet_feature2_Remote_Login.csv :widths: 10,10,30,20 :header-rows: 1 .. _Telnet_step11: .. tab-set:: .. tab-item:: Telnet Feature : Port Based Access **Port Based Access - Testcases** .. csv-table:: :file: ./Telnet/Telnet_feature3_Port_Based_Access.csv :widths: 10,10,30,20 :header-rows: 1 .. _Telnet_step12: .. tab-set:: .. tab-item:: Telnet Feature : Network Virtual Terminal **Network Virtual Terminal - Testcases** .. csv-table:: :file: ./Telnet/Telnet_feature4_Network_Virtual_Terminal.csv :widths: 10,10,30,20 :header-rows: 1 .. _Telnet_step13: .. tab-set:: .. tab-item:: Telnet Feature : Option Negotiation **Option Negotiation - Testcases** .. csv-table:: :file: ./Telnet/Telnet_feature5_Option_Negotiation.csv :widths: 10,10,30,20 :header-rows: 1 .. _Telnet_step14: .. tab-set:: .. tab-item:: Telnet Feature : Subnegotiation support **Subnegotiation support - Testcases** .. csv-table:: :file: ./Telnet/Telnet_feature6_Subnegotiation_support.csv :widths: 10,10,30,20 :header-rows: 1 .. _Telnet_step15: .. tab-set:: .. tab-item:: Telnet Feature : Cross Platform Compatiblity **Cross Platform Compatiblity - Testcases** .. csv-table:: :file: ./Telnet/Telnet_feature7_Cross_Platform_Compatiblity.csv :widths: 10,10,30,20 :header-rows: 1 .. _Telnet_step16: .. tab-set:: .. tab-item:: Telnet Feature : Simple Protocol structure **Simple Protocol structure - Testcases** .. csv-table:: :file: ./Telnet/Telnet_feature8_Simple_Protocol_structure.csv :widths: 10,10,30,20 :header-rows: 1 .. _Telnet_step18: .. tab-set:: .. tab-item:: Reference links * Reference links