ssh_exchange_identification: Connection closed by remote host

Debug

"ssh -vvv" can be used for verbose debugging info to determine the cause of failing ssh.

Possible Solutions

[1] Concurrent SSH connections

The default number of concurrent copnnections allowed by sshd is usually only 10. If higher concurrent ssh are expected, increase the MaxStartups in /etc/ssh/sshd_config

This case it the most likely if some of the ssh from one machine to another works and rest get connection closed by remote host error.

[2] Corrupted SSH Keys

If even a single ssh from one machine to another doesn't work, then the SSH keys could have been corrupted. The best steps to follow in such a case is to do the following:
- Ensure correctness of /etc/hosts.allow and /etc/hosts.deny
- Remove ~/.ssh from both machines
- Start over ssh setup following my other post on SSH without password.


Excellent reference and additional details is available here.