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TELNET & SSH Workshop for Network Administration

This workshop introduces the TELNET protocol, its options and sub-options, modes of operation, and out-of-band signaling. It also covers SSH as a secure alternative to TELNET and explores port-forwarding in SSH for enhanced security.

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TELNET & SSH Workshop for Network Administration

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  1. Workshop Administrasi Jaringan TELNET ＆SSH Ferry Astika Saputra

  2. OBJECTIVES: • To introduce the TELNET protocol and show how it implements local and remote login. • To discuss options and sub-options used in TELNET and how they are negotiated. • To define out-of-band signaling in TELNET. • To define different modes of operations in TELNET. • To introduce SSH as an alternative to TELNET. • To show how different components of SSH are combined to provide a secure connection over an insecure TCP connection. • To discuss port-forwarding in SSH and how it can be used to provide security for other applications. TCP/IP Protocol Suite

  3. 20-1 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.

  4. Topics Discussed in the Section • 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 TCP/IP Protocol Suite

  5. Note TELNET is a general-purpose client-server application program. TCP/IP Protocol Suite

  6. Figure 20.1Local login TCP/IP Protocol Suite

  7. Figure 20.2Remote login TCP/IP Protocol Suite

  8. Figure 20.3Concept of NVT TCP/IP Protocol Suite

  9. Figure 20.4Format of data and control characters TCP/IP Protocol Suite

  10. TCP/IP Protocol Suite

  11. Figure 20.5An example of embedding TCP/IP Protocol Suite

  12. TCP/IP Protocol Suite

  13. TCP/IP Protocol Suite

  14. Figure 20.6Offer to enable an option TCP/IP Protocol Suite

  15. Figure 20.7Request to enable an option TCP/IP Protocol Suite

  16. Figure 20.8Offer to disable an option TCP/IP Protocol Suite

  17. Figure 20.9Request to disable an option TCP/IP Protocol Suite

  18. Example 20.1 Figure 20.10 shows an example of option negotiation. In this example, the client wants the serverto 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. TCP/IP Protocol Suite

  19. Figure 20.10Example 20.1: Echo option TCP/IP Protocol Suite

  20. TCP/IP Protocol Suite

  21. Figure 20.11Example of sub-option negotiation TCP/IP Protocol Suite

  22. TCP/IP Protocol Suite

  23. Figure 20.12Example of interrupting an application program TCP/IP Protocol Suite

  24. Figure 20.13Out-of-band signaling TCP/IP Protocol Suite

  25. Figure 20.14Two different interruptions TCP/IP Protocol Suite

  26. Example 20.2 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 Figure 20.15). TCP/IP Protocol Suite

  27. Figure 20.15Example 20.2 TCP/IP Protocol Suite

  28. 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 20.16). TCP/IP Protocol Suite

  29. Figure 20.16Example 20.3 TCP/IP Protocol Suite

  30. TCP/IP Protocol Suite

  31. 20-2 SECURE SHELL (SSH) Another popular remote login application program is Secure Shell (SSH). SSH, like TELNET, uses TCP as the underlying transport protocol, but SSH is more secure and provides more services than TELNET. TCP/IP Protocol Suite

  32. Topics Discussed in the Section • Versions • Components • Port Forwarding • Format of the SSH Packet TCP/IP Protocol Suite

  33. Figure 20.17Components of SSH TCP/IP Protocol Suite

  34. Figure 20.18Port forwarding TCP/IP Protocol Suite

  35. Figure 20.19SSH packet format TCP/IP Protocol Suite

  36. How SSH Works • (1) Client contacts server • (2) If SSH protocol versions do not agree, no connection • (3) Server identifies itself. Server sends host key, server key, check bytes, list of methods. Client looks in its DB for hosts. • (4) Client sends a secret key, encrypted using server’s public key • Both begins encryption. Server authentication is completed • Client authentication on the server side. Example, password and public-key authentication

  37. SSH-2 Protocol

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