How to create a simple proxy in C#?

I wouldn't use HttpListener or something like that, in that way you'll come across so many issues.

Most importantly it'll be a huge pain to support:

• Proxy Keep-Alives
• SSL won't work (in a correct way, you'll get popups)
• .NET libraries strictly follows RFCs which causes some requests to fail (even though IE, FF and any other browser in the world will work.)

What you need to do is:

• Listen a TCP port
• Parse the browser request
• Extract Host connect to that host in TCP level
• Forward everything back and forth unless you want to add custom headers etc.

I wrote 2 different HTTP proxies in .NET with different requirements and I can tell you that this is the best way to do it.

Mentalis doing this, but their code is "delegate spaghetti", worse than GoTo :)

You can build one with the HttpListener class to listen for incoming requests and the HttpWebRequest class to relay the requests.

I have recently written a light weight proxy in c# .net using TcpListener and TcpClient.

https://github.com/titanium007/Titanium-Web-Proxy

It supports secure HTTP the correct way, client machine needs to trust root certificate used by the proxy. Also supports WebSockets relay. All features of HTTP 1.1 are supported except pipelining. Pipelining is not used by most modern browsers anyway. Also supports windows authentication (plain, digest).

You can hook up your application by referencing the project and then see and modify all traffic. (Request and response).

As far as performance, I have tested it on my machine and works without any noticeable delay.

Proxy can work in the following way.

Step1, configure client to use proxyHost:proxyPort.

Proxy is a TCP server that is listening on proxyHost:proxyPort. Browser opens connection with Proxy and sends Http request. Proxy parses this request and tries to detect "Host" header. This header will tell Proxy where to open connection.

Step 2: Proxy opens connection to the address specified in the "Host" header. Then it sends HTTP request to that remote server. Reads response.

Step 3: After response is read from remote HTTP server, Proxy sends the response through an earlier opened TCP connection with browser.

Schematically it will look like this:

Browser                            Proxy                     HTTP server
  Open TCP connection  
  Send HTTP request  ----------->                       
                                 Read HTTP header
                                 detect Host header
                                 Send request to HTTP ----------->
                                 Server
                                                      <-----------
                                 Read response and send
                   <-----------  it back to the browser
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