On what to set up and how, an intermediate proxy server so that each client accessing it accesses the network under its own proxy server?

Question

linux

Good day to all.
There is a task to raise a cloud server on VDS or locally.
On the PC of several employees, software for promotion on the network is installed, Proxycap is installed, it has its own proxies for each program, free proxies do not last long, paid ones do not give a lot of money and you need to change them often.
You need a server (router if it is pfsense or freebsd) whose address and its deferred ports will be specified in the program (about 100 ports). The server sends each port of the incoming proxy to the outgoing proxy, which it took from a text file, after first checking these proxies, maybe even spars it from free proxy sites. As a bonus, it controlled the passage of traffic, if it went from the entrance and did not exit, then change the proxy at the exit.
On what OS to build such a server, what add-ons are needed for it.

Answer 1

In fact, any linux / freebsd / windows allows you to configure port forwarding programmatically on the firewall. With the simplest script, you periodically scan proxies and generate commands to iptables to change redirect tables.
Of course, this is also possible on routers (almost everyone gives a terminal one way or another, or you can see in the browser what requests their admin panel sends when changing nat settings), but in this case, proxy scanning itself should be done on a separate machine, you do not need to load weak hardware.

Answer 2

What has been found so far.
There is such an easy simple proxy server 3proxy, judging by the description, it can do the same "cascading" (yes, what I need is called like that).
There is software for Windows CCproxy, it can do cascading to one port for sure, and you need to test for several. I will search further.