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TELNET

TELNET. TELNET SSH. TELNET. TELNET is an abbreviation for TErminaL NETwork. It is the standard TCP/IP protocol for virtual terminal service as proposed by ISO.

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TELNET

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  1. TELNET • TELNET • SSH

  2. TELNET • TELNET is an abbreviation for TErminaL NETwork. • It is the standard TCP/IP protocol for virtual terminal service as proposed by ISO. • TELNET enables the establishment of a connection to a remote system in such a way that the local terminal appears to be a terminal at the remote system.

  3. Topics to be Discussed • Concepts • Time-Sharing Environment • Network Virtual Terminal (NVT) • Embedding • Options and Suboption Negotiation • Controlling the Server • Out-of-Band Signaling • Escape Character • Modes of Operation • User Interface • Security Issue

  4. TELNET is a general-purpose client-server application program.

  5. Figure 1Local login

  6. Figure 2Remote login

  7. Figure 3Concept of NVT

  8. Figure 4Format of data and control characters

  9. Figure 5An example of embedding

  10. Figure 6Offer to enable an option

  11. Figure 7Request to enable an option

  12. Figure 8Offer to disable an option

  13. Figure 9Request to disable an option

  14. Example1 • Figure 10 shows an example of option negotiation. • In this example, the client wants the server to echo each character sent to the server. • In other words, when a character is typed at the user keyboard terminal, it goes to the server and is sent back to the screen of the user before being processed. • The echo option is enabled by the server because it is the server that sends the characters back to the user terminal. • Therefore, the client should request from the server the enabling of the option using DO. The request consists of three characters: IAC, DO, and ECHO. The server accepts the request and enables the option. It informs the client by sending the three-character approval: IAC, WILL, and ECHO.

  15. Figure 10Example 1: Echo option

  16. Figure 11Example of sub-option negotiation

  17. Figure 12Example of interrupting an application program

  18. Figure 13Out-of-band signaling

  19. Figure 14Two different interruptions

  20. Example2 • In this example, we use the default mode to show the concept and its deficiencies even though it is almost obsolete today. • The client and the server negotiate the terminal type and terminal speed and then the server checks the login and password of the user (see Figuren15).

  21. Figure 15Example 2

  22. Example 20.3 • In this example, we show how the client switches to the character mode. • This requires that the client request the server to enable the SUPPRESS GO AHEAD and ECHO options (see Figure 16).

  23. Figure 16Example 3

  24. 2 SECURE SHELL (SSH) • Another popular remote login application program is Secure Shell (SSH). SSH, like TELNET, uses TCP as the underlying transport protocol. • SSH is more secure and provides more services than TELNET.

  25. Topics to be Discussed • Versions • Components • Port Forwarding • Format of the SSH Packet

  26. Figure 17Components of SSH

  27. Figure 18Port forwarding

  28. Figure 19SSH packet format

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