Spring mvc by pass stream to another server

I have 2 web app using spring mvc in different location, here's some info:

1. server A:
    - have ip public

2. server B:
    - no ip public

3. Client:
    - want to access server B via server A using browser
    

Diagram:

Client  ---(request)--->  server A  ---(by pass the request)--->  server B

I want to client access server B through server A and i dont want to use VPN, what should i do?

and what we call this mechanism?

can i use netty for it?