SSH keypairs are normally generated using a key generation tool that comes with the SSH client software. (If you're using the free PuTTY SSH client, the key generation tool is called PuTTYgen.)
The keypair includes two keys: the private key and the public key. The private key stays with the client host, and the public key is inserted into the appropriate file in the remote server.
Usually the key generation tool will suggest that you set a passphrase on the private key. But it is possible to create a key without a passphrase, if you need to execute commands with no typing of passwords or -phrases at all. In that case, it is recommended that you configure restrictions to the server to make it harder to damage your systems if an unauthorized or malicious person gets the private key.
Most Linux hosts use a SSH server implementation based on OpenSSH. With it, you have the option of configuring a specific command to be executed when a particular SSH key is used. When that command completes, the session ends. This may be useful if you need to allow only a few specific actions to be executed remotely.
If you want more information about this, run "man sshd" on your Linux system and read the chapter titled "AUTHORIZED_KEYS FILE FORMAT": it lists all the server-side restrictions you can configure on a SSH key.
MK
MK
