[Bug 1872145] [NEW] explicit key offered after all agent keys, auth fails before explicit key used

[Robert C Jennings]

Public bug reported:

A user creates an ssh key and specifies it on the cmdline with 'ssh -i
new_key user at host'.  The connection fails with the message "Too many
authentication failures" displayed to the user.

This would lead the user to believe that they failed to put the public
portion of the new key on the destination and it will probably be hard
for the average user to debug this.

The root of this issue is that the user has a number of keys in ~/.ssh/
registered with their ssh agent.  The ssh command is offering each of
these keys from the agent to the remote system before trying the
explicit key from the command line.  There are enough agent keys to
reach the failure limit (usually 5 keys) with the remote before they get
to the explicit key.

The solution today for the user is to head down into the ssh_config man
page to find '-o IdentitiesOnly=yes' to skip the agent keys and only use
the specified key.  But they're unlikely to do this because '-i' in the
ssh man page doesn't suggest this and they'd only look for this if they
actually understood the root cause of the problem, which is a bit cruel.

We should consider changing the order of the keys offered to the remote
to use explicit keys first followed by agent keys.  It would seem to me
that this would honor the users intent of explicitly specifying a key to
use.

The current order makes this difficult for anyone fielding a user's
authentication failure report as they must double check that ssh managed
to actually try the key the user specified before it raised an error
message about authentication failures.

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

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https://bugs.launchpad.net/bugs/1872145

Title:
  explicit key offered after all agent keys, auth fails before explicit
  key used

Status in openssh package in Ubuntu:
  New

Bug description:
  A user creates an ssh key and specifies it on the cmdline with 'ssh -i
  new_key user at host'.  The connection fails with the message "Too many
  authentication failures" displayed to the user.

  This would lead the user to believe that they failed to put the public
  portion of the new key on the destination and it will probably be hard
  for the average user to debug this.

  The root of this issue is that the user has a number of keys in
  ~/.ssh/ registered with their ssh agent.  The ssh command is offering
  each of these keys from the agent to the remote system before trying
  the explicit key from the command line.  There are enough agent keys
  to reach the failure limit (usually 5 keys) with the remote before
  they get to the explicit key.

  The solution today for the user is to head down into the ssh_config
  man page to find '-o IdentitiesOnly=yes' to skip the agent keys and
  only use the specified key.  But they're unlikely to do this because
  '-i' in the ssh man page doesn't suggest this and they'd only look for
  this if they actually understood the root cause of the problem, which
  is a bit cruel.

  We should consider changing the order of the keys offered to the
  remote to use explicit keys first followed by agent keys.  It would
  seem to me that this would honor the users intent of explicitly
  specifying a key to use.

  The current order makes this difficult for anyone fielding a user's
  authentication failure report as they must double check that ssh
  managed to actually try the key the user specified before it raised an
  error message about authentication failures.

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