How to work with local proxy outside the box

I have a local proxy software running at the host OS, and want to tunnel Tor inside the box through the local proxy outside the box, would be difficult to do so?

Not sure I understand your configuration. You need to provide more details. Depending on the proxy, it might be difficult or even impossible because Virtualbox networking resides on OSI Layer 2 (ie doesn’t speak TCP/IP),

If it’s an SSH tunnel you are talking about, then it’s possible using instructions like this: Tunnelling VirtualBox guest's network traffic - Marcin Kulik. This is neither supported nor tested so proceed at your own risk. Generally, it is not a good idea to alter networking unless you understand what you’re doing.

One can ssh into a Whonix-Gateway.

File Transfer - Whonix

As well as into a Whonix-Workstation.

File Transfer - Whonix

Perhaps reverse ssh something?

Or install the local proxy in the gateway instead? Use case?

Perhaps bridged networking or maybe better a host-only network adapter can help? Both have warnings. Which could use some more research, rewrite and documentation. Don’t hold your breath for it to happen anytime soon.

thank you to both of you, the proxy software running in the whonix-gateway is not open source, is it better to create a new user account for the software?

Linux user account isolation is not very strong anyhow. See:

We don’t have it fully documented how to run a custom proxy tunnel software on a Non-Qubes-Whonix-Gateway. Very much likely doable.

You would have to run it under the linux user account tunnel. (Otherwise the tunnel software would have no clearnet (non-torified) internet access for itself.)

See also generally:

More specifically on how to configure Tor to use the proxy:
(host would be port: you name it)