Assuming you’re using a Debian or Ubuntu based Linux.
While the SSH daemon is secure enough for most people, some may wish to further enhance their security by changing certain sshd settings. As there are tools out there (such as the ones found in Backtrack and ArchPwn Linux) that can attempt to guess your password, either doing damage by succeeding or starting a DoS attack (bombard your server so much that it stops doing its job as it becomes overwhelmed) All changes, unless otherwise stated, are made in the /etc/ssh/sshd_config file. Lines with a pound sign (#) are commented and not read.
To edit this file from a terminal:
sudo nano /etc/ssh/sshd_config
Please remember, after making any changes, sshd must be restarted, which can be done from the terminal with this command:
sudo /etc/init.d/ssh restart