ssh multiplexing via port forwarding

Question

I'm trying to create a setup where one could "ssh" to a specific port on localhost and then get forwarded to a remote server and into a shell, without the need to enter credentials (nope, secret key is not an option, unfortunately).

So far I have set up ssh multiplexing, with ControlMaster, so I can start a remote ssh shell without any prompts, from the local shell.

Is there a way to use that socket when new client connects to a specific port on localhost?

I've tried various combinations with -D, -R and -S, with no luck. Is such a setup possible, on either Linux or Mac?

edit:

I don't mind entering localhost credentials at any point, but I'm trying to avoid entering remote credentials for all connections following the initial control connection.

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localhost ----> localhost:2222 ----> remote:22 $ ssh -p 2222 forward to remote $ _ :) using an existing control socket 

Answer 1

If you want a basic shell, you could use netcat to create a shell listening on a certain port on remote server, and use Port Forwarding on local ssh to create the tunnels:

On remote:

while true ; do netcat -l -p 15000 -e /bin/bash ; done 

On local:

ssh -L 5000:remoteip:15000 user@server netcat localhost 5000 

You will not be able to use some commands (vim behaviour is funny), but it works for most commands. Every time the connection is closed, the loop will spawn a new one.

Answer 2

You could always reroute (forward) localhost:2222 localhost:22 using iptables, eg.

iptables -t nat -A PREROUTING -p tcp --dport 2222 -j REDIRECT --to-port 22

See alias port / port forwarding to another local port or this Google search. This, in addition to ControlMaster auto, should not require any additional passwords (either local or remote).

However to me this seems unnecessary and makes me wonder if you don't have a case of the XY problem.