How to set up a proxy server from one interface to another?

Question

linux

Hello
On the server, VPN connection and ppp connection via USB modem are up. There are also a few more connections to the network and the Internet.
What proxy server can be set so that it listens to the connection on the interface with the VPN and sends it to the interface with the modem?
And so that there are no problems with the fact that this and that connection can sometimes "fall off" so that the proxy server does not "crumble"

Answer 1

iptables masquerade