How to create a simple proxy in C

Question

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  Update 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     Useful link Mentalis Proxy   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  ASP Proxy   I might be able to get some information over here too  Request reflector   This is a simple example  Here is a Git Hub Repository with a Simple Http Proxy

User · Answer

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

User · Answer

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  ----------- 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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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       Use tcp listener to listen to browser requests parse only the first line of the request and get the host domain and port to connect send the exact raw request to the found host on the first line of browser request receive the data from the target site I have problem in this section  send the exact data received from the host to the browser   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 http   google com HTTP1 1 or CONNECT facebook com 443  this is for ssl requests

User · Answer

If you are just looking to intercept the traffic  you could use the fiddler core to create a proxy     http   fiddler wikidot com fiddlercore  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

User · Answer

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

User · Answer

For what it s worth  here is a C  sample async implementation based on HttpListener and HttpClient  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  Httplistener with HTTPS support     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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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       Use tcp listener to listen to browser requests parse only the first line of the request and get the host domain and port to connect send the exact raw request to the found host on the first line of browser request receive the data from the target site I have problem in this section  send the exact data received from the host to the browser   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 http   google com HTTP1 1 or CONNECT facebook com 443  this is for ssl requests

User · Answer

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

User · Answer

For what it s worth  here is a C  sample async implementation based on HttpListener and HttpClient  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  Httplistener with HTTPS support     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

User · Answer

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

User · Answer

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  ----------- 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

User · Answer

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

User · Answer

If you are just looking to intercept the traffic  you could use the fiddler core to create a proxy     http   fiddler wikidot com fiddlercore  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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

[c#] How to create a simple proxy in C#?

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