Question

I have set up ssh key pairs between my desktop and two servers, and from the servers to my desktop, but after reinstalling the OS on my desktop, I can’t re-establish the key-pair going into my desktop by this:

mkdir ~/.ssh
chmod 700 ~/.ssh
ssh-keygen -t 
ssh-copy-id username@server

I get the following error:

(names in italics changed to protect the innocent My desktop is Ubuntu, and I can’t find the answer here)

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ 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 the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
ab:cd:ef:gh
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:1
RSA host key for user.server has changed and you have requested strict
checking.
Host key verification failed.

Answer 1

The problem occurs if the host key has expired or been altered (for example, a new install on the server side), so it no longer matches the key in your known_hosts

You can delete the key(s) currently associated with the host and attempt to SSH once more. This ensures both the client and server recognize the new key.

ssh-keygen -R hostname

This deletes the offending key from the known_hosts

The man page entry reads:

-R hostname Removes all keys belonging to hostname from a known_hosts file. This option is useful to delete hashed hosts (see the -H option
above).