Other SSH Tricks
OpenSSH respects TCP wrapper configurations, described in Chapter 23.
sshd, like the Linux login program, denies logins when the file /etc/nologin exists. When remotely maintaining hosts in a way that may disrupt user activities, you should create this file with a helpful explanation of what is happening. This will stop all nonroot logins by any method, so you can do your maintenance undisturbed. The file is usually created by the shutdown command as well, to keep users from logging in while the machine is shutting down. The file is removed after a complete boot:
If there is any reason to suspect that your maintenance work can disconnect you or break the login mechanism, you should keep multiple login sessions open while doing the work. Test logging in again before closing them. Otherwise, doing a tiny PAM change that breaks all authentication could force you to reboot the machine into single-user mode to recover.
Consider scheduling an at or cron job to remove /etc/nologin at a particular time, in the event you log yourself out. Such a job can be handy when restarting sshd from a remote location as well.