[Bug 1872145] Re: explicit key offered after all agent keys, auth can fail before explicit key used

[Christian Ehrhardt ]

Without checking deeper I'm unsure if keys listed in e.g.
/etc/ssh/ssh_config are considered as "userprovided" but lets give this
a shot.

Gladly the debug already contains "explicit" if id->userprovided is
true.

I usually get:
debug1: Will attempt key: /home/paelzer/.ssh/id_rsa RSA ... agent
debug1: Will attempt key: ubuntu at cpaelzer-bastion RSA ... agent
debug1: Will attempt key: paelzer at lap RSA ... agent
debug1: Will attempt key: paelzer at swarm.naarz.dyndns.org RSA ... agent
debug1: Will attempt key: /home/paelzer/.ssh/id_dsa 
debug1: Will attempt key: /home/paelzer/.ssh/id_ecdsa 
debug1: Will attempt key: /home/paelzer/.ssh/id_ecdsa_sk 
debug1: Will attempt key: /home/paelzer/.ssh/id_ed25519 
debug1: Will attempt key: /home/paelzer/.ssh/id_ed25519_sk 
debug1: Will attempt key: /home/paelzer/.ssh/id_xmss 

with an -i set to a key that is in the agent, the defaults vanish.
And since the key is listed & in the agent it is preferred already

debug1: Will attempt key: /home/paelzer/.ssh/id_rsa.n RSA ... explicit agent
debug1: Will attempt key: /home/paelzer/.ssh/id_rsa RSA ... agent
debug1: Will attempt key: ubuntu at cpaelzer-bastion RSA ... agent
debug1: Will attempt key: paelzer at lap RSA ... agent

But if I create a new key and add it it is NOT preferrd (this is the bug in discussion here)
debug1: Will attempt key: /home/paelzer/.ssh/id_rsa RSA ... agent
debug1: Will attempt key: ubuntu at cpaelzer-bastion RSA ... agent
debug1: Will attempt key: paelzer at lap RSA ... agent
debug1: Will attempt key: paelzer at swarm.naarz.dyndns.org RSA ... agent
debug1: Will attempt key: /tmp/testkey RSA ... explicit

The explicit one is down below in the order since it isn't in the agent.

A final solution might want to insert them somewhere else than the very
top and/or might want to do similar with certificates. But for the test
the attached patch could be enough.

** Patch added: "experimental-1872145.debdiff"
   https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1872145/+attachment/5357863/+files/experimental-1872145.debdiff

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

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

Status in openssh package in Ubuntu:
  Triaged

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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