-t flag tests a configuration file without restarting the
sshd daemon. This is helpful if you are
ssh-ed into a server and need to change the
sshd_config file before updating your configuration management scripts - an error may prevent your server from accepting your next
ssh session, or kick you off the box.
-f option allows you to specify a file. Excluding it tests the current configuration file (i.e .
$ sudo sshd -t -f /tmp/sshd_config
This command can also be used to test for installed ciphers when changing cryptographic options, as well as syntax errors.