Troubleshooting "ssh: connection closed by remote host"

So the error "ssh: connection closed by remote host" is one of the most common & pesky errors I've experienced as a system administrator.

Some of the troubleshooting & diagnostics I've learned over the years are as follows:

Note: You'll need console access to the server you're trying to ssh into.

• Do an ssh -vvv <hostname> to get more detailed information as to why the connection is not getting established.

• Check the status of ssh service on the destination server.

• Check for space utilization of /,/var & /tmp file systems with df. Space constraints in these mount points can prevent logins.

• Check permissions of /var & /etc & make sure they are not 777. The 'secure' in 'secure shell' kicks in & won't allow logins if permissions of /var & /etc are world readable. Also if the permissions of home directory of login user on destination server is set to 777 that would again cause a 'no login'.

• Check the status of login user on destination server to make sure its not expired or locked. I know it sounds basic but still a valid check.

• If you are using ssh keys for authentication make sure the authorized_keys file on destination server is syntactically correct.

• Finally, check for nproc values on the destination server. Sometimes if the server is under heavy load & has a lot of processes running, the nproc value can inhibit the creation of new shell processes to conserve system resources thereby preventing login.

I hope these tips help save time the next time you see this error.