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SSH, The Secure Shell: The Definitive Guide, 2nd Edition by Robert G. Byrnes, Richard E. Silverman, Daniel J. Barrett

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Inside SSH-2

The SSH protocol has two major, incompatible versions, called Version 1[14] and Version 2. [1.5] We refer to these as SSH-1 and SSH-2. The SSH-1 protocol is now a relic; it is less flexible than SSH-2, has unfixable security weaknesses, and has been deprecated for years. Its implementations see no real development aside from bug fixes, and the default protocol for most SSH software has been SSH-2 for some time now. In this chapter, as we describe “the SSH protocol,” we are talking about SSH-2. We limit our treatment of SSH-1 to a summary of its design, its differences with SSH-2, and its weaknesses.

The SSH protocol is actually divided into four major pieces, formally described as four separate protocols in different IETF documents, and in principle independent of one another. In practice, they are layered together to provide the set of services most users associate with SSH as a whole. These are:

  • SSH Transport Layer Protocol (SSH-TRANS)

  • SSH Authentication Protocol (SSH-AUTH)

  • SSH Connection Protocol (SSH-CONN)

  • SSH File Transfer Protocol (SSH-SFTP)

There are other documents that describe other aspects of, or extensions to, the protocols, but the preceding ones represent the core of SSH. As of this writing, these documents are still “Internet-Drafts,” but after much effort by the IETF SECSH working group, they have been submitted to the IESG for consideration as proposed standards and may soon be published as Internet RFCs.

Figure 3-2 outlines the division of labor between ...

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