Red Hat

can you please suggest for both?

The solution I described in my first post works from the same network with user D having a different port configuration.

Then again, if you want to connect to him from outside of the network, you'll have to configure port redirection in your router.

Let's say user D has lan IP : 192.168.0.10, you'd have to configure a port redirection so incoming requests get redirected to the relevent port at this address.

Additionnally, you can configure your internal ssh server to listen on multiple port numbers, so you can still use port 22 internally.

In /etc/ssh/sshd_config you'd have something like :

Port 22
Port 49952

Meaning you can still use port 22 internally and use the second one to connect to this server from the outside after setting up the right port redirection in your router.

But I think you can't stop any other, internal user from connecting to this new port...

You can, but you have to define user authentication rules i.e. in /etc/ssh/ssh_known_hosts

To this extent, you really should take a look at the man page and go to the authentication section.

Code:

man ssh

Anyway, still not sure what you want to achieve.