[Bug 1863930] [NEW] SSH 1.99 clients fail to connect to openssh-server 1:7.6p1-4ubuntu0.3

[Kyle Birkeland]

Public bug reported:

SSHD closes the connection and logs the error message below when a
client presents a protoversion of "1.99":

    Protocol major versions differ for X.X.X.X port X:
SSH-2.0-OpenSSH_7.6p1 Ubuntu-4ubuntu0.3 vs. SSH-1.99-XXX

RFC 4253 only states that clients should treat a server's protoversion
of "1.99" as equivalent to "2.0"; however, some backward-compatible
clients send a protoversion of "1.99" and expect the server to treat it
as "2.0".

This regression was introduced in openssh-portable 7.6p1 from commit
97f4d3083; fixes were implemented in commits 9e9c4a7e5 and c9c1bba06.
I've attached a patch with both of those fixes.

** Affects: openssh (Ubuntu)
     Importance: Undecided
         Status: New

** Patch added: "protocol_major_version_mismatch_regression.patch"
   https://bugs.launchpad.net/bugs/1863930/+attachment/5329542/+files/protocol_major_version_mismatch_regression.patch

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https://bugs.launchpad.net/bugs/1863930

Title:
  SSH 1.99 clients fail to connect to openssh-server 1:7.6p1-4ubuntu0.3

Status in openssh package in Ubuntu:
  New

Bug description:
  SSHD closes the connection and logs the error message below when a
  client presents a protoversion of "1.99":

      Protocol major versions differ for X.X.X.X port X:
  SSH-2.0-OpenSSH_7.6p1 Ubuntu-4ubuntu0.3 vs. SSH-1.99-XXX

  RFC 4253 only states that clients should treat a server's protoversion
  of "1.99" as equivalent to "2.0"; however, some backward-compatible
  clients send a protoversion of "1.99" and expect the server to treat
  it as "2.0".

  This regression was introduced in openssh-portable 7.6p1 from commit
  97f4d3083; fixes were implemented in commits 9e9c4a7e5 and c9c1bba06.
  I've attached a patch with both of those fixes.

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