-f: Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background.
-N: Do not execute a remote command. This is useful for just forwarding ports.
-n: Redirects stdin from /dev/null
-L: Specifies that connections to the given TCP port or Unix socket on the local (client) host are to be forwarded to the given host and port, or Unix socket, on the remote side.
-D [bind_address:]port: Specifies a local ‘dynamic’ application-level port forwarding.
-R: Specifies that connections to the given TCP port or Unix socket on the remote (server) host are to be forwarded to the local side.