Communication between two servers for a ddos challenge page?

I need to develop a challenge page much similar to the Cloudflare firewall challenge.

I know how to make the front end and the back end of the challenge app and I know how to set it up on the server.

The problem comes on how to implement it to the website which is on one server, while the challenge app is on another server. How should I make the communication between the servers? Both servers will be Ubuntu 16.4. Thank you all in advance.

via Chebli Mohamed