remote_host 에서 9820 포트로 서비스 하는 웹서비스가 있는 경우,
localhost 4500 포트로 SSH 터널링을 걸어주는 명령어.

ssh -L 4500:localhost:9820 user_name@remote_host

-L [bind_address:]port:host:hostport
    Specifies that the given port on the local (client) host is to be forwarded to the given host and port 
    on the remote side.  This works by allocating a socket to listen to port on the local side, optionally 
    bound to the specified bind_address.

    Whenever a connection is made to this port, the connection is forwarded over the secure channel, 
    and a connection is made to host port hostport from the remote machine.

    Port forwardings can also be specified in the configuration file. IPv6 addresses can be specified with an
    alternative syntax: [bind_address/]port/host/hostport or by enclosing the address in square brackets.
    Only the superuser can forward privileged ports.

    By default, the local port is bound in accordance with the GatewayPorts setting.  However, an explicit
    bind_address may be used to bind the connection to a specific address. The bind_address of “localhost” 
    indicates that the listening port be bound for local use only, while an empty address or ‘*’ indicates that
    the port should be available from all interfaces.

로컬 PC 에서 (windows 인 경우 cygwin 에서...) 위 명령어를 내려주면,
http://localhost:4500 이 http://remote_host:9820 과 같은 효과.