Ask HN: I want to create my own web gateway, where should I start?

I'm having a hard time finding resources on creating my own web gateway. It feels like I'm googling the wrong terms? I'm talking about the website that limits connectivity until a username/password is provided, usually found in Free WiFi Access Points.

I'm a C# Developer, specializing on ASP.Net Web Api, if it makes any difference. Any help is appreciated, just point me in the right direction :) Thanks in advance.