Firewall or parser with working authorization?

Question

PHP

The customer stumped me with the following task:
Make a white-list of employees who will have access to their personal account. All other visitors must go through the forest. At the same time, the customer does not give me access to the server where the personal account is hosted.
It is assumed that it should work like this:
1) employee ---> 2) pre-authorization server <---> 3) personal account server
In the future, the customer plans to block all incoming IP addresses, except for the IP address of the intermediate server (2).
Question: whether there is a similar ready technology inf. security?
I tried to set up Glype for this task, but the http-authorization of my personal account refuses to work in it (not surprisingly).
Tell me, good people, in which direction to turn the pedals, otherwise I will soon earn a fracture of my fingers trying to make http-authorization work.
Ideally, it should turn out like noblockme.ru but with the opposite meaning.

Answer 1

1) squid
2) VPN