Resolving ssh: connect to host xxx.xxx.xxx.xxx port 22: Connection refused

There are a number of reasons why an SSH server may fail to allow a client to connect. Many aren't readily apparent, even from tailing system log files or using ssh -v on the client. Here are some of the ones I've encountered:

1. Incorrect permissions / ownership on the key files in /etc/ssh/
2. Incorrect permissions / ownership on the ~/.ssh/id_rsa private key file of the user as which we're trying to connect. I'll add more to this list as I encounter them.
3. systemd just plain being a piece of shit. Running 'systemctl restart sshd.socket' has fixed the problem in the past.

Resolving slow SSH login times

At work, like most other organizations, we have a Linux server that we access via SSH. However, lately, I've found my use of it skyrocketing in order to test software, and log-ins and file copying have been very slow. In my efforts to find out why, I found out that sshd has in its configuration file a setting called 'UseDNS'. The default is 'yes', so even if this setting is commented out, it will try to perform reverse DNS lookups on the IP addresses of users logging in. This would be a bad thing for us, especially since we have this box locked down and unable to contact any DNS servers. After disabling DNS lookups, my login and file copy time dropped to near 0. I hope this helps somebody.

*EDIT:* I've also encountered this problem on the BeagleBone Black boards which I've recently acquired for use in a project.