SSH host key checking cannot disable when using proxy jump

bastionhostkeyssh

I am trying to SSH through a jumpbox, but SSH seems to be intent on checking host keys for the jumpbox, even though I'm telling it not to, using the normal -o StrictHostKeyChecking=no -o UserKnownHostsFile=no command line options.

If I SSH directly to the jumpbox, I can have SSH ignore the error as expected:

ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -i ~/.ssh/id_jumpuser_rsa jumpuser@jumpbox

However, if I add the proxy jump option, I suddenly get the error. The error is NOT coming from the jumpbox there are no known_hosts files in any .ssh directory on the jumpbox, nor am I logging in as the jumpuser:

ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -i ~/.ssh/id_jumpuser_rsa -J jumpuser@jumpbox jumpuser@10.10.0.5

The error message:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is
<redacted>.
Please contact your system administrator.
Add correct host key in /home/user/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /home/user/.ssh/known_hosts:10
  remove with:
  ssh-keygen -f "/home/user/.ssh/known_hosts" -R jumpbox
ECDSA host key for jumpbox has changed and you have requested strict checking.
Host key verification failed.
ssh_exchange_identification: Connection closed by remote host

Where user is my regular user, not the user I am attempting to SSH as.

I have no clue what's going on here. Does SSH have a special override forcing hostkey checking for proxy jump situations? If so, it's supremely irritating, as it's going to make local VM provisioning a real pain.

Best Answer

The ProxyJump issues another ssh process, that does not inherit the command-line arguments that you specify on the command-line of the first ssh command. There are two possible ways out:

  • Use these options in configuration file in ~/.ssh/config -- it can save you a lot of typing too!

    Host jumpbox
  User jumpuser
  StrictHostKeyChecking=no
  UserKnownHostsFile=/dev/null
  IdentityFile ~/.ssh/id_jumpuser_rsa

    and then you can connect just as ssh -J jumpbox jumpuser@10.10.0.5.

  • Use ProxyCommand option instead -- it does the same job, but more transparently so you can see what is actually going on there:

    ssh -o ProxyCommand="ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -i ~/.ssh/id_jumpuser_rsa -W %h:%p jumpuser@jumpbox" -i ~/.ssh/id_jumpuser_rsa jumpuser@10.10.0.5

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