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Each user creates and destroys 15+ VM's per day. The VM's are created in the company's internal OpenStack cloud.

Every time a new vm is assigned an ip address which has previously been handed out, the user gets the dreaded host key verification failed error. This is because the ssh key does not match the IP address in the users known_hosts file.

Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
Please contact your system administrator.
Add correct host key in /home/user/.ssh/known_hosts to get rid of this message.
Offending key in /home/user/.ssh/known_hosts:4
RSA host key for has changed and you have requested strict checking.
Host key verification failed.

The two solutions I can see are:

  • Turn off strict checking - (Security risk)
  • Have the users run ssh-keygen -RipAddress - (users are getting tired of this solution, since then run into it multiple times per day)

Is there any way to prevent this error message, yet stay secure? perhaps turn off security checking for just a specific subnet?

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@MichaelHampton This is no duplicate; this question is much broader. The other one was about solving the problem for a single key. This is about handling frequent key changes. – Hauke Laging May 10 '13 at 23:12
...any reason you can't use Puppet or something equivalent to put a standard per-user key on each VM? – voretaq7 May 11 '13 at 1:24
up vote 10 down vote accepted

The great feature HostKeyAlias solves your problem:

ssh -o HostKeyAlias=hostkeyalias__vm_2013-05-11_07 user@host

creates an entry hostkeyalias__vm_2013-05-11_07 (without IP) in known_hosts. Of course, you could write a script or shell function which sets this value before each ssh call. Or you use a shell variable:

ssh -o HostKeyAlias=$HOSTKEYALIAS user@host

and change $HOSTKEYALIAS whenever the VM is changed. From time to time the old entries should be deleted from known_hosts.

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create ~/.ssh/config

with the contents: Host * StrictHostKeyChecking no

Alternately you can create an alias for ssh to: ssh -o StrictHostKeyChecking=no

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You could retrieve the new host key from the VM console and update the known hosts file after an instance boots up.

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