Debugging SSH without-password

Sometimes, we want to set up passwordless login across nodes. In this post, I will use the root user as an example.

Common Practice

Easiest way:
$ ssh-copy-id remotehostname

Sometimes, we may encounter
/usr/bin/ssh-copy-id: ERROR: No identities found

In that case, do:
$ ssh-copy-id -i ~/.ssh/id_rsa.pub remotehostname

If known_hosts has offending key
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ 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.
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Offending key in /root/.ssh/known_hosts: 6
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