blocks|key|359598|text|您可以使用HttpListener类和HttpWebRequest类构建一个监听传入请求和中继请求的类。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|359599|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/system.net.httplistener.aspx|1|http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.aspx^0|5|C|J|E|5|C|0|J|E|1|0^^$0|@$1|2|3|4|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@$9|U|A|V|1|W]|$9|X|A|Y|1|Z]]|E|$]]|$1|F|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]|N|$5|I|J|K|E|$L|O]]]]

You can build one with the <a href="http://msdn.microsoft.com/en-us/library/system.net.httplistener.aspx" rel="noreferrer"><code>HttpListener</code></a> class to listen for incoming requests and the <a href="http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.aspx" rel="noreferrer"><code>HttpWebRequest</code></a> class to relay the requests.

blocks|key|357415|text|浏览器连接到代理，因此代理从web服务器获取的数据仅通过浏览器向代理发起的相同连接发送。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|357416|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

The browser is connected to the proxy so the data that the proxy gets from the web server is just sent via the same connection that the browser initiated to the proxy.

blocks|key|3359545|text|Proxy可以通过以下方式工作。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3359546|Step1，将客户端配置为使用代理主机:代理端口。|3359547|Proxy是监听proxyHost:proxyPort的TCP服务器。浏览器打开与代理的连接并发送Http请求。代理解析此请求并尝试检测"Host“标头。此标头将告诉代理在何处打开连接。|3359548|步骤2:代理打开到"Host“报头中指定的地址的连接。然后，它向该远程服务器发送HTTP请求。读取响应。|3359549|步骤3:从远程HTTP服务器读取响应后，Proxy通过之前打开的与浏览器的TCP连接发送响应。|3359550|从原理上讲，它看起来像这样：|3359551|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
Render+content|code-block|syntax|javascript|3359552|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|U|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|V|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|W|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|X|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|Y|8|@]|9|@]|A|$O|P]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

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:

<pre><code>Browser Proxy HTTP server
 Open TCP connection 
 Send HTTP request -----------&gt; 
 Read HTTP header
 detect Host header
 Send request to HTTP -----------&gt;
 Server
 &lt;-----------
 Read response and send
 &lt;----------- it back to the browser
Render content
</code></pre>

blocks|key|3146788|text|我不会使用HttpListener或类似的东西，那样你会遇到很多问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3146789|最重要的是，支持它将是一个巨大的痛苦：|3146790|3146791|Proxy+RFC将无法工作(正确地说，您将获得弹出窗口)|unordered-list-item|3146792|.NET库严格遵循RFC，这会导致一些请求失败(即使IE、FF和世界上任何其他浏览器都可以工作。)|3146793|3146794|您需要做的是：|3146795|3146796|侦听TCP端口|3146797|解析浏览器请求|3146798|提取主机在TCP中与该主机连接所有内容，除非您想要添加自定义标头等。|3146799|3146800|我用.NET编写了两个不同的HTTP代理，有不同的需求，我可以告诉你，这是最好的方法。|3146801|Mentalis这样做，但他们的代码是“委托意大利面”，比GoTo更糟糕:)|3146802|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|12|8|@]|9|@]|A|$]]|$1|E|3|F|5|G|7|13|8|@]|9|@]|A|$]]|$1|H|3|I|5|G|7|14|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|15|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|16|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|17|8|@]|9|@]|A|$]]|$1|N|3|O|5|G|7|18|8|@]|9|@]|A|$]]|$1|P|3|Q|5|G|7|19|8|@]|9|@]|A|$]]|$1|R|3|S|5|G|7|1A|8|@]|9|@]|A|$]]|$1|T|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1C|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1D|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]]|Z|$]]

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:

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

What you need to do is:

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

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 :)

blocks|key|359627|text|同意邪恶博士如果你使用HTTPListener，你会有很多问题，你必须解析请求，并将涉及到头部和...|type|unstyled|depth|inlineStyleRanges|entityRanges|data|359628|359629|使用tcp侦听器侦听浏览器请求|ordered-list-item|359630|359631|359632|仅解析请求的第一行，并获取要连接的主机域和端口，然后在浏览器的第一行上向找到的主机发送确切的原始请求，以便从目标站点获取数据(我在此section)|359633|send中遇到了问题，即从主机接收到的确切数据到browser|359634|359635|您甚至不需要知道浏览器请求中有什么并解析它，只需从第一行获取目标站点地址通常类似于get+SSLHTTP1.1或CONNECT+facebook.com:443+(这是针对http://google.com请求的)|offset|length|359636|entityMap|0|LINK|mutability|MUTABLE|url|http://google.com/^0|0|0|0|0|0|0|0|0|2E|H|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|10|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|11|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|12|8|@]|9|@]|A|$]]|$1|H|3|I|5|E|7|13|8|@]|9|@]|A|$]]|$1|J|3|K|5|E|7|14|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|15|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|16|8|@]|9|@$O|17|P|18|1|19]]|A|$]]|$1|Q|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

Agree to dr evil
if you use HTTPListener you will have many problems, you have to parse requests and will be engaged to headers and ...

<ol>
<li>Use tcp listener to listen to browser requests </li>
<li>parse only the first line of the request and get the host domain and port to connect</li>
<li>send the exact raw request to the found host on the first line of browser request</li>
<li>receive the data from the target site(I have problem in this section)</li>
<li>send the exact data received from the host to the browser</li>
</ol>

you see you dont need to even know what is in the browser request and parse it, only get the target site address from the first line
first line usually likes this
GET <a href="http://google.com" rel="nofollow noreferrer">http://google.com</a> HTTP1.1
or
CONNECT facebook.com:443 (this is for ssl requests)

blocks|key|362178|text|Socks4是一个实现起来非常简单的协议。您侦听初始连接，连接到客户端请求的主机/端口，将成功代码发送到客户端，然后跨套接字转发传出和传入的流。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|362179|如果你使用HTTP，你将不得不读取并可能设置/删除一些HTTP头，所以这需要更多的工作。|362180|如果我没记错的话，SSL可以跨HTTP和Socks代理工作。对于HTTP代理，您实现CONNECT谓词，它的工作方式与上面描述的socks4非常相似，然后客户端通过代理的tcp流打开SSL连接。|362181|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

Socks4 is a very simple protocol to implement. You listen for the initial connection, connect to the host/port that was requested by the client, send the success code to the client then forward the outgoing and incoming streams across sockets.

If you go with HTTP you'll have to read and possibly set/remove some HTTP headers so that's a little more work.

If I remember correctly, SSL will work across HTTP and Socks proxies. For a HTTP proxy you implement the CONNECT verb, which works much like the socks4 as described above, then the client opens the SSL connection across the proxied tcp stream.

blocks|key|359663|text|如果你只是想截获流量，你可以使用fiddler核心来创建一个代理...|type|unstyled|depth|inlineStyleRanges|entityRanges|data|359664|http://fiddler.wikidot.com/fiddlercore|offset|length|359665|首先使用UI运行fiddler，看看它做了什么，它是一个代理，允许您调试http/https流量。它是用c#编写的，并且有一个核心，您可以将其构建到您自己的应用程序中。|359666|请记住，FiddlerCore对于商业应用程序来说不是免费的。|359667|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|12|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@$D|S|E|T|1|U]]|A|$]]|$1|F|3|G|5|6|7|V|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|W|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|X|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|C]]]]

If you are just looking to intercept the traffic, you could use the fiddler core to create a proxy...

<a href="http://fiddler.wikidot.com/fiddlercore" rel="noreferrer">http://fiddler.wikidot.com/fiddlercore</a>

run fiddler first with the UI to see what it does, it is a proxy that allows you to debug the http/https traffic. It is written in c# and has a core which you can build into your own applications.

Keep in mind FiddlerCore is not free for commercial applications.

blocks|key|359679|text|我最近使用TcpListener和TcpClient在c#+.net中编写了一个轻量级代理。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|359680|https://github.com/titanium007/Titanium-Web-Proxy|359681|它以正确的方式支持安全HTTP，客户端需要信任代理所使用的根证书。还支持WebSockets中继。除管道外，HTTP+1.1的所有功能都受支持。无论如何，大多数现代浏览器都不使用流水线。还支持windows身份验证(普通、摘要)。|359682|您可以通过引用项目来挂接您的应用程序，然后查看和修改所有流量。(请求和响应)。|359683|至于性能，我已经在我的机器上进行了测试，没有任何明显的延迟。|359684|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/system.net.sockets.tcplistener%2528v=vs.110%2529.aspx|1|https://msdn.microsoft.com/en-us/library/system.net.sockets.tcpclient(v=vs.110).aspx)|2^0|5|B|0|H|9|1|0|0|1D|2|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]|$A|10|B|11|1|12]]|C|$]]|$1|D|3|E|5|6|7|13|8|@]|9|@$A|14|B|15|1|16]]|C|$]]|$1|F|3|G|5|6|7|17|8|@]|9|@]|C|$]]|$1|H|3|I|5|6|7|18|8|@]|9|@]|C|$]]|$1|J|3|K|5|6|7|19|8|@]|9|@]|C|$]]|$1|L|3|-4|5|6|7|1A|8|@]|9|@]|C|$]]]|M|$N|$5|O|P|Q|C|$R|S]]|T|$5|O|P|Q|C|$R|U]]|V|$5|O|P|Q|C|$R|E]]]]

I have recently written a light weight proxy in c# .net using <a href="http://msdn.microsoft.com/en-us/library/system.net.sockets.tcplistener%28v=vs.110%29.aspx" rel="noreferrer">TcpListener</a> and <a href="https://msdn.microsoft.com/en-us/library/system.net.sockets.tcpclient(v=vs.110).aspx" rel="noreferrer">TcpClient</a>.

<a href="https://github.com/titanium007/Titanium-Web-Proxy" rel="noreferrer">https://github.com/titanium007/Titanium-Web-Proxy</a>

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.

blocks|key|358917|text|使用OWIN和WebAPI，事情变得非常简单。在我寻找C#代理服务器的过程中，我也偶然发现了这篇文章http://blog.kloud.com.au/2013/11/24/do-it-yourself-web-api-proxy/。这就是我要走的路。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|358918|entityMap|0|LINK|mutability|MUTABLE|url|http://blog.kloud.com.au/2013/11/24/do-it-yourself-web-api-proxy/^0|1E|1T|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Things have become really easy with OWIN and WebAPI. In my search for a C# Proxy server, I also came across this post <a href="http://blog.kloud.com.au/2013/11/24/do-it-yourself-web-api-proxy/">http://blog.kloud.com.au/2013/11/24/do-it-yourself-web-api-proxy/</a> . This will be the road I'm taking.

blocks|key|3359788|text|这里是一个基于HttpListener和HttpClient的C#异步实现示例(我使用它来连接Android设备中的Chrome到IIS+Express，这是我找到的唯一方法...)。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3359789|如果你需要HTTPS支持，应该不需要更多的代码，只需要证书配置：Httplistener+with+HTTPS+support|3359790|//+define+http://localhost:5000+and+http://127.0.0.1:5000/+to+be+proxies+for+http://localhost:53068
using+(var+server+=+new+ProxyServer("http://localhost:53068",+"http://localhost:5000/",+"http://127.0.0.1:5000/"))
{
++++server.Start();
++++Console.WriteLine("Press+ESC+to+stop+server.");
++++while+(true)
++++{
++++++++var+key+=+Console.ReadKey(true);
++++++++if+(key.Key+==+ConsoleKey.Escape)
++++++++++++break;
++++}
++++server.Stop();
}

....

public+class+ProxyServer+:+IDisposable
{
++++private+readonly+HttpListener+_listener;
++++private+readonly+int+_targetPort;
++++private+readonly+string+_targetHost;
++++private+static+readonly+HttpClient+_client+=+new+HttpClient();

++++public+ProxyServer(string+targetUrl,+params+string[]+prefixes)
++++++++:+this(new+Uri(targetUrl),+prefixes)
++++{
++++}

++++public+ProxyServer(Uri+targetUrl,+params+string[]+prefixes)
++++{
++++++++if+(targetUrl+==+null)
++++++++++++throw+new+ArgumentNullException(nameof(targetUrl));

++++++++if+(prefixes+==+null)
++++++++++++throw+new+ArgumentNullException(nameof(prefixes));

++++++++if+(prefixes.Length+==+0)
++++++++++++throw+new+ArgumentException(null,+nameof(prefixes));

++++++++RewriteTargetInText+=+true;
++++++++RewriteHost+=+true;
++++++++RewriteReferer+=+true;
++++++++TargetUrl+=+targetUrl;
++++++++_targetHost+=+targetUrl.Host;
++++++++_targetPort+=+targetUrl.Port;
++++++++Prefixes+=+prefixes;

++++++++_listener+=+new+HttpListener();
++++++++foreach+(var+prefix+in+prefixes)
++++++++{
++++++++++++_listener.Prefixes.Add(prefix);
++++++++}
++++}

++++public+Uri+TargetUrl+{+get;+}
++++public+string[]+Prefixes+{+get;+}
++++public+bool+RewriteTargetInText+{+get;+set;+}
++++public+bool+RewriteHost+{+get;+set;+}
++++public+bool+RewriteReferer+{+get;+set;+}+//+this+can+have+performance+impact...

++++public+void+Start()
++++{
++++++++_listener.Start();
++++++++_listener.BeginGetContext(ProcessRequest,+null);
++++}

++++private+async+void+ProcessRequest(IAsyncResult+result)
++++{
++++++++if+(!_listener.IsListening)
++++++++++++return;

++++++++var+ctx+=+_listener.EndGetContext(result);
++++++++_listener.BeginGetContext(ProcessRequest,+null);
++++++++await+ProcessRequest(ctx).ConfigureAwait(false);
++++}

++++protected+virtual+async+Task+ProcessRequest(HttpListenerContext+context)
++++{
++++++++if+(context+==+null)
++++++++++++throw+new+ArgumentNullException(nameof(context));

++++++++var+url+=+TargetUrl.GetComponents(UriComponents.SchemeAndServer,+UriFormat.Unescaped);
++++++++using+(var+msg+=+new+HttpRequestMessage(new+HttpMethod(context.Request.HttpMethod),+url+%2B+context.Request.RawUrl))
++++++++{
++++++++++++msg.Version+=+context.Request.ProtocolVersion;

++++++++++++if+(context.Request.HasEntityBody)
++++++++++++{
++++++++++++++++msg.Content+=+new+StreamContent(context.Request.InputStream);+//+disposed+with+msg
++++++++++++}

++++++++++++string+host+=+null;
++++++++++++foreach+(string+headerName+in+context.Request.Headers)
++++++++++++{
++++++++++++++++var+headerValue+=+context.Request.Headers[headerName];
++++++++++++++++if+(headerName+==+"Content-Length"+&&+headerValue+==+"0")+//+useless+plus+don't+send+if+we+have+no+entity+body
++++++++++++++++++++continue;

++++++++++++++++bool+contentHeader+=+false;
++++++++++++++++switch+(headerName)
++++++++++++++++{
++++++++++++++++++++//+some+headers+go+to+content...
++++++++++++++++++++case+"Allow":
++++++++++++++++++++case+"Content-Disposition":
++++++++++++++++++++case+"Content-Encoding":
++++++++++++++++++++case+"Content-Language":
++++++++++++++++++++case+"Content-Length":
++++++++++++++++++++case+"Content-Location":
++++++++++++++++++++case+"Content-MD5":
++++++++++++++++++++case+"Content-Range":
++++++++++++++++++++case+"Content-Type":
++++++++++++++++++++case+"Expires":
++++++++++++++++++++case+"Last-Modified":
++++++++++++++++++++++++contentHeader+=+true;
++++++++++++++++++++++++break;

++++++++++++++++++++case+"Referer":
++++++++++++++++++++++++if+(RewriteReferer+&&+Uri.TryCreate(headerValue,+UriKind.Absolute,+out+var+referer))+//+if+relative,+don't+handle
++++++++++++++++++++++++{
++++++++++++++++++++++++++++var+builder+=+new+UriBuilder(referer);
++++++++++++++++++++++++++++builder.Host+=+TargetUrl.Host;
++++++++++++++++++++++++++++builder.Port+=+TargetUrl.Port;
++++++++++++++++++++++++++++headerValue+=+builder.ToString();
++++++++++++++++++++++++}
++++++++++++++++++++++++break;

++++++++++++++++++++case+"Host":
++++++++++++++++++++++++host+=+headerValue;
++++++++++++++++++++++++if+(RewriteHost)
++++++++++++++++++++++++{
++++++++++++++++++++++++++++headerValue+=+TargetUrl.Host+%2B+":"+%2B+TargetUrl.Port;
++++++++++++++++++++++++}
++++++++++++++++++++++++break;
++++++++++++++++}

++++++++++++++++if+(contentHeader)
++++++++++++++++{
++++++++++++++++++++msg.Content.Headers.Add(headerName,+headerValue);
++++++++++++++++}
++++++++++++++++else
++++++++++++++++{
++++++++++++++++++++msg.Headers.Add(headerName,+headerValue);
++++++++++++++++}
++++++++++++}

++++++++++++using+(var+response+=+await+_client.SendAsync(msg).ConfigureAwait(false))
++++++++++++{
++++++++++++++++using+(var+os+=+context.Response.OutputStream)
++++++++++++++++{
++++++++++++++++++++context.Response.ProtocolVersion+=+response.Version;
++++++++++++++++++++context.Response.StatusCode+=+(int)response.StatusCode;
++++++++++++++++++++context.Response.StatusDescription+=+response.ReasonPhrase;

++++++++++++++++++++foreach+(var+header+in+response.Headers)
++++++++++++++++++++{
++++++++++++++++++++++++context.Response.Headers.Add(header.Key,+string.Join(",+",+header.Value));
++++++++++++++++++++}

++++++++++++++++++++foreach+(var+header+in+response.Content.Headers)
++++++++++++++++++++{
++++++++++++++++++++++++if+(header.Key+==+"Content-Length")+//+this+will+be+set+automatically+at+dispose+time
++++++++++++++++++++++++++++continue;

++++++++++++++++++++++++context.Response.Headers.Add(header.Key,+string.Join(",+",+header.Value));
++++++++++++++++++++}

++++++++++++++++++++var+ct+=+context.Response.ContentType;
++++++++++++++++++++if+(RewriteTargetInText+&&+host+!=+null+&&+ct+!=+null+&&
++++++++++++++++++++++++(ct.IndexOf("text/html",+StringComparison.OrdinalIgnoreCase)+>=+0+%7C%7C
++++++++++++++++++++++++ct.IndexOf("application/json",+StringComparison.OrdinalIgnoreCase)+>=+0))
++++++++++++++++++++{
++++++++++++++++++++++++using+(var+ms+=+new+MemoryStream())
++++++++++++++++++++++++{
++++++++++++++++++++++++++++using+(var+stream+=+await+response.Content.ReadAsStreamAsync().ConfigureAwait(false))
++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++await+stream.CopyToAsync(ms).ConfigureAwait(false);
++++++++++++++++++++++++++++++++var+enc+=+context.Response.ContentEncoding+??+Encoding.UTF8;
++++++++++++++++++++++++++++++++var+html+=+enc.GetString(ms.ToArray());
++++++++++++++++++++++++++++++++if+(TryReplace(html,+"//"+%2B+_targetHost+%2B+":"+%2B+_targetPort+%2B+"/",+"//"+%2B+host+%2B+"/",+out+var+replaced))
++++++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++++++var+bytes+=+enc.GetBytes(replaced);
++++++++++++++++++++++++++++++++++++using+(var+ms2+=+new+MemoryStream(bytes))
++++++++++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++++++++++ms2.Position+=+0;
++++++++++++++++++++++++++++++++++++++++await+ms2.CopyToAsync(context.Response.OutputStream).ConfigureAwait(false);
++++++++++++++++++++++++++++++++++++}
++++++++++++++++++++++++++++++++}
++++++++++++++++++++++++++++++++else
++++++++++++++++++++++++++++++++{
++++++++++++++++++++++++++++++++++++ms.Position+=+0;
++++++++++++++++++++++++++++++++++++await+ms.CopyToAsync(context.Response.OutputStream).ConfigureAwait(false);
++++++++++++++++++++++++++++++++}
++++++++++++++++++++++++++++}
++++++++++++++++++++++++}
++++++++++++++++++++}
++++++++++++++++++++else
++++++++++++++++++++{
++++++++++++++++++++++++using+(var+stream+=+await+response.Content.ReadAsStreamAsync().ConfigureAwait(false))
++++++++++++++++++++++++{
++++++++++++++++++++++++++++await+stream.CopyToAsync(context.Response.OutputStream).ConfigureAwait(false);
++++++++++++++++++++++++}
++++++++++++++++++++}
++++++++++++++++}
++++++++++++}
++++++++}
++++}

++++public+void+Stop()+=>+_listener.Stop();
++++public+override+string+ToString()+=>+string.Join(",+",+Prefixes)+%2B+"+=>+"+%2B+TargetUrl;
++++public+void+Dispose()+=>+((IDisposable)_listener)?.Dispose();

++++//+out-of-the-box+replace+doesn't+tell+if+something+*was*+replaced+or+not
++++private+static+bool+TryReplace(string+input,+string+oldValue,+string+newValue,+out+string+result)
++++{
++++++++if+(string.IsNullOrEmpty(input)+%7C%7C+string.IsNullOrEmpty(oldValue))
++++++++{
++++++++++++result+=+input;
++++++++++++return+false;
++++++++}

++++++++var+oldLen+=+oldValue.Length;
++++++++var+sb+=+new+StringBuilder(input.Length);
++++++++bool+changed+=+false;
++++++++var+offset+=+0;
++++++++for+(int+i+=+0;+i+<+input.Length;+i%2B%2B)
++++++++{
++++++++++++var+c+=+input[i];

++++++++++++if+(offset+>+0)
++++++++++++{
++++++++++++++++if+(c+==+oldValue[offset])
++++++++++++++++{
++++++++++++++++++++offset%2B%2B;
++++++++++++++++++++if+(oldLen+==+offset)
++++++++++++++++++++{
++++++++++++++++++++++++changed+=+true;
++++++++++++++++++++++++sb.Append(newValue);
++++++++++++++++++++++++offset+=+0;
++++++++++++++++++++}
++++++++++++++++++++continue;
++++++++++++++++}

++++++++++++++++for+(int+j+=+0;+j+<+offset;+j%2B%2B)
++++++++++++++++{
++++++++++++++++++++sb.Append(input[i+-+offset+%2B+j]);
++++++++++++++++}

++++++++++++++++sb.Append(c);
++++++++++++++++offset+=+0;
++++++++++++}
++++++++++++else
++++++++++++{
++++++++++++++++if+(c+==+oldValue[0])
++++++++++++++++{
++++++++++++++++++++if+(oldLen+==+1)
++++++++++++++++++++{
++++++++++++++++++++++++changed+=+true;
++++++++++++++++++++++++sb.Append(newValue);
++++++++++++++++++++}
++++++++++++++++++++else
++++++++++++++++++++{
++++++++++++++++++++++++offset+=+1;
++++++++++++++++++++}
++++++++++++++++++++continue;
++++++++++++++++}

++++++++++++++++sb.Append(c);
++++++++++++}
++++++++}

++++++++if+(changed)
++++++++{
++++++++++++result+=+sb.ToString();
++++++++++++return+true;
++++++++}

++++++++result+=+input;
++++++++return+false;
++++}
}|code-block|syntax|javascript|3359791|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.microsoft.com/en-us/dotnet/api/system.net.httplistener?view=netframework-4.8|1|https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclient?view=netframework-4.8|2|https://stackoverflow.com/questions/11403333/httplistener-with-https-support^0|7|C|0|K|A|1|0|W|V|2|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]|$A|10|B|11|1|12]]|C|$]]|$1|D|3|E|5|6|7|13|8|@]|9|@$A|14|B|15|1|16]]|C|$]]|$1|F|3|G|5|H|7|17|8|@]|9|@]|C|$I|J]]|$1|K|3|-4|5|6|7|18|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]|S|$5|N|O|P|C|$Q|T]]|U|$5|N|O|P|C|$Q|V]]]]

For what it's worth, here is a C# sample async implementation based on <a href="https://docs.microsoft.com/en-us/dotnet/api/system.net.httplistener?view=netframework-4.8" rel="nofollow noreferrer">HttpListener</a> and <a href="https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclient?view=netframework-4.8" rel="nofollow noreferrer">HttpClient</a> (I use it to be able to connect Chrome in Android devices to IIS Express, that's the only way I found...).

And If you need HTTPS support, it shouldn't require more code, just certificate configuration: <a href="https://stackoverflow.com/questions/11403333/httplistener-with-https-support">Httplistener with HTTPS support</a>

<pre><code>// define http://localhost:5000 and http://127.0.0.1:5000/ to be proxies for http://localhost:53068
using (var server = new ProxyServer("http://localhost:53068", "http://localhost:5000/", "http://127.0.0.1:5000/"))
{
 server.Start();
 Console.WriteLine("Press ESC to stop server.");
 while (true)
 {
 var key = Console.ReadKey(true);
 if (key.Key == ConsoleKey.Escape)
 break;
 }
 server.Stop();
}

....

public class ProxyServer : IDisposable
{
 private readonly HttpListener _listener;
 private readonly int _targetPort;
 private readonly string _targetHost;
 private static readonly HttpClient _client = new HttpClient();

 public ProxyServer(string targetUrl, params string[] prefixes)
 : this(new Uri(targetUrl), prefixes)
 {
 }

 public ProxyServer(Uri targetUrl, params string[] prefixes)
 {
 if (targetUrl == null)
 throw new ArgumentNullException(nameof(targetUrl));

 if (prefixes == null)
 throw new ArgumentNullException(nameof(prefixes));

 if (prefixes.Length == 0)
 throw new ArgumentException(null, nameof(prefixes));

 RewriteTargetInText = true;
 RewriteHost = true;
 RewriteReferer = true;
 TargetUrl = targetUrl;
 _targetHost = targetUrl.Host;
 _targetPort = targetUrl.Port;
 Prefixes = prefixes;

 _listener = new HttpListener();
 foreach (var prefix in prefixes)
 {
 _listener.Prefixes.Add(prefix);
 }
 }

 public Uri TargetUrl { get; }
 public string[] Prefixes { get; }
 public bool RewriteTargetInText { get; set; }
 public bool RewriteHost { get; set; }
 public bool RewriteReferer { get; set; } // this can have performance impact...

 public void Start()
 {
 _listener.Start();
 _listener.BeginGetContext(ProcessRequest, null);
 }

 private async void ProcessRequest(IAsyncResult result)
 {
 if (!_listener.IsListening)
 return;

 var ctx = _listener.EndGetContext(result);
 _listener.BeginGetContext(ProcessRequest, null);
 await ProcessRequest(ctx).ConfigureAwait(false);
 }

 protected virtual async Task ProcessRequest(HttpListenerContext context)
 {
 if (context == null)
 throw new ArgumentNullException(nameof(context));

 var url = TargetUrl.GetComponents(UriComponents.SchemeAndServer, UriFormat.Unescaped);
 using (var msg = new HttpRequestMessage(new HttpMethod(context.Request.HttpMethod), url + context.Request.RawUrl))
 {
 msg.Version = context.Request.ProtocolVersion;

 if (context.Request.HasEntityBody)
 {
 msg.Content = new StreamContent(context.Request.InputStream); // disposed with msg
 }

 string host = null;
 foreach (string headerName in context.Request.Headers)
 {
 var headerValue = context.Request.Headers[headerName];
 if (headerName == "Content-Length" &amp;&amp; headerValue == "0") // useless plus don't send if we have no entity body
 continue;

 bool contentHeader = false;
 switch (headerName)
 {
 // some headers go to content...
 case "Allow":
 case "Content-Disposition":
 case "Content-Encoding":
 case "Content-Language":
 case "Content-Length":
 case "Content-Location":
 case "Content-MD5":
 case "Content-Range":
 case "Content-Type":
 case "Expires":
 case "Last-Modified":
 contentHeader = true;
 break;

 case "Referer":
 if (RewriteReferer &amp;&amp; Uri.TryCreate(headerValue, UriKind.Absolute, out var referer)) // if relative, don't handle
 {
 var builder = new UriBuilder(referer);
 builder.Host = TargetUrl.Host;
 builder.Port = TargetUrl.Port;
 headerValue = builder.ToString();
 }
 break;

 case "Host":
 host = headerValue;
 if (RewriteHost)
 {
 headerValue = TargetUrl.Host + ":" + TargetUrl.Port;
 }
 break;
 }

 if (contentHeader)
 {
 msg.Content.Headers.Add(headerName, headerValue);
 }
 else
 {
 msg.Headers.Add(headerName, headerValue);
 }
 }

 using (var response = await _client.SendAsync(msg).ConfigureAwait(false))
 {
 using (var os = context.Response.OutputStream)
 {
 context.Response.ProtocolVersion = response.Version;
 context.Response.StatusCode = (int)response.StatusCode;
 context.Response.StatusDescription = response.ReasonPhrase;

 foreach (var header in response.Headers)
 {
 context.Response.Headers.Add(header.Key, string.Join(", ", header.Value));
 }

 foreach (var header in response.Content.Headers)
 {
 if (header.Key == "Content-Length") // this will be set automatically at dispose time
 continue;

 context.Response.Headers.Add(header.Key, string.Join(", ", header.Value));
 }

 var ct = context.Response.ContentType;
 if (RewriteTargetInText &amp;&amp; host != null &amp;&amp; ct != null &amp;&amp;
 (ct.IndexOf("text/html", StringComparison.OrdinalIgnoreCase) &gt;= 0 ||
 ct.IndexOf("application/json", StringComparison.OrdinalIgnoreCase) &gt;= 0))
 {
 using (var ms = new MemoryStream())
 {
 using (var stream = await response.Content.ReadAsStreamAsync().ConfigureAwait(false))
 {
 await stream.CopyToAsync(ms).ConfigureAwait(false);
 var enc = context.Response.ContentEncoding ?? Encoding.UTF8;
 var html = enc.GetString(ms.ToArray());
 if (TryReplace(html, "//" + _targetHost + ":" + _targetPort + "/", "//" + host + "/", out var replaced))
 {
 var bytes = enc.GetBytes(replaced);
 using (var ms2 = new MemoryStream(bytes))
 {
 ms2.Position = 0;
 await ms2.CopyToAsync(context.Response.OutputStream).ConfigureAwait(false);
 }
 }
 else
 {
 ms.Position = 0;
 await ms.CopyToAsync(context.Response.OutputStream).ConfigureAwait(false);
 }
 }
 }
 }
 else
 {
 using (var stream = await response.Content.ReadAsStreamAsync().ConfigureAwait(false))
 {
 await stream.CopyToAsync(context.Response.OutputStream).ConfigureAwait(false);
 }
 }
 }
 }
 }
 }

 public void Stop() =&gt; _listener.Stop();
 public override string ToString() =&gt; string.Join(", ", Prefixes) + " =&gt; " + TargetUrl;
 public void Dispose() =&gt; ((IDisposable)_listener)?.Dispose();

 // out-of-the-box replace doesn't tell if something *was* replaced or not
 private static bool TryReplace(string input, string oldValue, string newValue, out string result)
 {
 if (string.IsNullOrEmpty(input) || string.IsNullOrEmpty(oldValue))
 {
 result = input;
 return false;
 }

 var oldLen = oldValue.Length;
 var sb = new StringBuilder(input.Length);
 bool changed = false;
 var offset = 0;
 for (int i = 0; i &lt; input.Length; i++)
 {
 var c = input[i];

 if (offset &gt; 0)
 {
 if (c == oldValue[offset])
 {
 offset++;
 if (oldLen == offset)
 {
 changed = true;
 sb.Append(newValue);
 offset = 0;
 }
 continue;
 }

 for (int j = 0; j &lt; offset; j++)
 {
 sb.Append(input[i - offset + j]);
 }

 sb.Append(c);
 offset = 0;
 }
 else
 {
 if (c == oldValue[0])
 {
 if (oldLen == 1)
 {
 changed = true;
 sb.Append(newValue);
 }
 else
 {
 offset = 1;
 }
 continue;
 }

 sb.Append(c);
 }
 }

 if (changed)
 {
 result = sb.ToString();
 return true;
 }

 result = input;
 return false;
 }
}
</code></pre>

I have downloaded Privoxy few weeks ago and for the fun I was curious to know how a simple version of it can be done.
I understand that I need to configure the browser (client) to send request to the proxy. The proxy send the request to the web (let say it's a http proxy). The proxy will receive the answer... but how can the proxy send back the request to the browser (client)?
I have search on the web for C# and http proxy but haven't found something that let me understand how it works behind the scene correctly. (I believe I do not want a reverse proxy but I am not sure).
Does any of you have some explication or some information that will let me continue this small project?
<h2>Update</h2>
This is what I understand (see graphic below).
Step 1 I configure the client (browser) for all request to be send to 127.0.0.1 at the port the Proxy listen. This way, request will be not sent to the Internet directly but will be processed by the proxy.
Step2 The proxy see a new connection, read the HTTP header and see the request he must executes. He executes the request.
Step3 The proxy receive an answer from the request. Now he must send the answer from the web to the client but how???
<img src="https://i.stack.imgur.com/zNWDc.png" alt="alt text" />
<h3>Useful link</h3>
<a href="http://www.mentalis.org/soft/projects/proxy/" rel="noreferrer">Mentalis Proxy</a> : I have found this project that is a proxy (but more that I would like). I might check the source but I really wanted something basic to understand more the concept.
<a href="http://www.codeproject.com/KB/aspnet/asproxy.aspx" rel="noreferrer">ASP Proxy</a> : I might be able to get some information over here too.
<a href="http://bartdesmet.net/blogs/bart/archive/2007/02/22/httplistener-for-dummies-a-simple-http-request-reflector.aspx" rel="noreferrer">Request reflector</a> : This is a simple example.
Here is a <a href="https://github.com/MrDesjardins/SimpleHttpProxy" rel="noreferrer">Git Hub Repository with a Simple Http Proxy</a>.

How to create a simple proxy in C#?

我几周前下载了Privoxy，为了好玩，我好奇地想知道它的一个简单版本是如何做到的。我知道我需要配置浏览器(客户端)来向代理发送请求。代理将请求发送到web (假设它是一个http代理)。代理服务器将收到答案...但是代理如何将请求发送回浏览器(客户端)呢？我已经在网络上搜索了C#和http proxy，但还没有找到能...

问如何在C#中创建一个简单的代理？
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