Ytdyklly

I am trying to understand the difference between "local port forwarding" and "dynamic port forwarding".

In the ssh command for "local port forwarding", is it always required to specify the destination host?

Does "dynamic" in "dynamic port forwarding" mean that, in the ssh command for "dynamic port forwarding", there is no need to specify the destination host? if yes, when is the destination specified?

Yes, you have to specify a destination IP and port when using local forwarding. From man ssh:

 -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.

Clearly, only the bind address is optional.

No, you can't specify a destination host or port when using dynamic forwarding. In dynamic forwarding, SSH acts as a SOCKS proxy. Again from the manpage (emphasis mine):

 -D [bind_address:]port

         Specifies a local “dynamic” application-level port forwarding.

         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 the application protocol is then used to

         determine where to connect to from the remote machine.  Currently

         the SOCKS4 and SOCKS5 protocols are supported, and ssh will act

         as a SOCKS server.

With -L, SSH makes no attempt to understand the traffic. It just sends everything it receives on the local port to the target port - you determine the target port at the time the connection is made. With -D, SSH acts as a proxy server, and therefore can handle connections from multiple ports (for example, a browser configured to use it as a SOCKS proxy can then access HTTP, HTTPS, FTP, etc. over the same connection). And like with other proxy servers, it will use the traffic to determine the destination.

Another way to learn this concept would be to take a look at how a client connects to amazon's elastic map reduce (EMR) cluster. EMR has a bunch of local apps that it exposes and these are normally accessed via SSH tunnels.

There are 2 options for a client:
A) Local port forward
SSH command:
ssh -i key.pem -L 8157:a.b.c.d:8088 hadoop@a.b.c.d

Here the client says for example, that 8157 on localhost gets forwarded to a.b.c.d:8088
The client has to make a request to the localhost:
http://localhost:8157
Other applications could be listening on ports such as 8089,8090, and the client has to make ssh connections for each one of these.

B) Dynamic port forward
Here a single SSH command is used:
ssh -i key.pem -D 8157 hadoop@a.b.c.d

Any traffic going in on port 8157 will be routed via the ssh tunnel. The destination of the traffic will be the URL's destination. For example, you can use a proxy-client on your webserver and redirect some http URI's to use the proxy. You can access all your applications on 8089,8090 via a single command.

For reference, see this doc:
https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-web-interfaces.html