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Quite often i put servers into a rescue mode and that obviously changes the host key. So there are situations when i know that SSH host key will be changed temporarily or permanently. And each time i need to do:

  • ssh-keygeyn -R x.x.x.x
  • ssh x.x.x.x and confirm addition of a new key
  • Do something in rescue mode and reboot the server
  • ssh-keygen -R x.x.x.x
  • ssh x.x.x.x if needed and accept new host key

I wounder if somebody came up with a smart alias or there is an ssh client's config option which in case of different host key asks to replace curent hostkey or just ignore the problem temporarily and proceed.

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Solution 1

You can scan remote host new public key before login with ssh-keyscan command.

ssh-keygen -R x.x.x.x
ssh-keyscan x.x.x.x >> ~/.ssh/known_hosts
ssh x.x.x.x

Then you can make a script from that, using the host as an argument and put it in your PATH.

To check if public keys differ you can do this :

diff -q <(ssh-keygen -F x.x.x.x | sed '1d') <(ssh-keyscan x.x.x.x 2>/dev/null)

Solution 2

Now, if you have a DNS server in your infrastructure, you should set up SSHFP DNS records to handle your machine's public key changes a centralized way and avoid the hassle of homemade scripts everywhere.

Retrieve DNS entries to configure :

ssh-keygen -r /etc/ssh/ssh_host_key.pub

The result will look like :

IN SSHFP 1 1 d3fa9bcf2d51979c53bcac2961f38b60e4e60886
IN SSHFP 2 1 f1f09814dd79eea523f490808cf3c096f1d1a432

Little explanation :

  • First field : IN = Internet class
  • Second field : SSHFP record type
  • Third field : Algorithm (1=RSA, 2=DSA, 3=ECDSA)
  • Fourth field : Fingerprint type (1=SHA-1, 2=SHA256)

Prefix these records with the server name and put them in your DNS configuration.

Then make sure all your machines will contact your DNS server in /etc/resolv.conf.

Finally, put VerifyHostKeyDNS=yes option in .ssh/config file on each server.

  • Did not know about ssh-keyscan, thanks. The only missing part is to compare received key via ssh-keyscan and a local copy. Only manually using grep/awk/... ? – Radium Sep 19 '14 at 17:40
  • @user2660496 Depends what you have at your disposal, check my updated answer. – Xavier Lucas Sep 19 '14 at 18:47
  • Your solution is almost there. The "-F" key is a solution. Just need to add so prompt questions and i am there. Thank you. – Radium Sep 20 '14 at 9:41
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ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null  

will trick ssh into thinking its keylist is /dev/null and won't ask you to confirm to "add" the key to the (nonexistant) file. This has the advantage that you don't add the temporay key to the real file.

You could add an bash alias to use it.

alias sshnk="ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null"
  • I did something like that using alias "sshno" but that does not really satisfy my needs. I want that to be transparent and i want to automate host key replacement. Your solution just ignores verification for one time. – Radium Sep 19 '14 at 17:18

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