I am getting Access Denied exception while trying to open a socket.
My connect function is as shown.
internal void Connect()
{
try
{
//AccessPolicy = new SocketClientAccessPolicyProtocol();
args = new SocketAsyncEventArgs();
args.UserToken = socket;
args.RemoteEndPoint = endPoint;
args.Completed += new EventHandler<SocketAsyncEventArgs>(OnConnect);
**isConnected = socket.ConnectAsync(args);**
//autoEvent.WaitOne();
if (args.SocketError != SocketError.Success)
throw new SocketException((int)args.SocketError);
if(isConnected)
global::System.Windows.MessageBox.Show("Connected");
}
catch (Exception ex)
{
global::System.Windows.MessageBox.Show(ex.Message);
}
}
The function ConnectAsync however is executing fine as isConnected is coming true but socket is not getting connected.
http://drdobbs.com/windows/208403238
Looking through the code you'll see that it uses the TcpListener class to listen for incoming client connections. Once a client connects the code checks the request for the following value: <policy-file-request/>
Silverlight automatically sends this text to the policy file socket once it connects. If the request contains the proper value the code writes the contents of the client access policy back to the client stream (see the OnReceiveComplete() method). Once the policy file is received, Silverlight parses it, checks that it allows access to the desired port, and then accepts or rejects the socket call that the application is trying to make.
If this is Silverlight in the browser, you need a socket policy server in the mix. See http://msdn.microsoft.com/en-us/library/cc645032%28v=vs.95%29.aspx for details.
Related
What is the correct way to handle a disconnection and re-connection event with the windows StreamSocket class (TCP)?
I have an issue where calling "Invalid Operation, method was called at an unexpected time" when calling async_connect after a disconnection event
do I need to create a new streamsocket, or wait for some amount of time before attempting to re-connect?
https://learn.microsoft.com/en-us/uwp/api/windows.networking.sockets.streamsocket
The solution was to add the following code:
// on catching an exception
socket.dispose();
connect();
// connect function
connect():
socket = new StreamSocket ...
It was necessary to 1) call socket.dispose() on the socket which the client disconnected from and 2) create a new socket (socket = new StreamSocket(...)). Reusing the same socket to connect did not work.
I have just recently started development for a server - multiple clients application using Socket.
The server doesn't need to keep track of the connected clients; If there is a client that requests for connection, server accepts it. If there is a request from any client (to get some data), server will response to that client.
/// <summary>
/// Callback when server accepts a new incoming connection.
/// </summary>
/// <param name="result">Incoming connection result object.</param>
private void AcceptedCallback(IAsyncResult result)
{
try
{
Socket clientSocket = _socket.EndAccept(result); // Asynchronously accepts an incoming connection attempt
if (clientSocket.Connected) // Check if the client is in 'Connected' state
{
StateObject state = new StateObject();
state.clientSocket = clientSocket;
clientSocket.BeginReceive(state.buffer, 0, StateObject.BufferSize, SocketFlags.None, // Start listening to client request
ReceiveCallback, state);
}
else
{
clientSocket.Close(); // Terminate that client's connection
Log.writeLog("TCPServer(AcceptedCallback)"
, "Client's status is not connected.");
}
}
catch (Exception ex)
{
Log.writeLog("TCPServer(AcceptedCallback)"
, ex.Message);
clientSocket.Close();
}
finally
{
Accept(); // Start to accept new connection request
}
}
I have 3 questions about this:
For each BeginReceive that I create for the newly connected client, does my server application creates a new thread/object to hold that client?
If after the client is connected, and the network cable is pulled off at the client side and plug back in, the client will connect to the server again and this is considered a new connection on the server, if this scenario occurs again and again, will my server program crashes?
Hence, do I need to keep track of each client that is connected to the server, and find a way to track their state so I can call Close/Dispose on them?
So far in my testing for scenario 2, there are no abnormalities detected in my server program, but I hope someone would help clarify this for me. Thank you.
No, it will use a IO completion thread from a pool of threads.
No, you can code and should code to cater for this. If something happens on the client side that the OS can detect, it will send a TCP fin/ack to the server. This should result in any BeginXXX method still waiting to go the Async Callback Method. From there your call to EndXXX method should either throw an exception or return zero bytes being read from the socket.
This depends on what you mean by keep track of to dispose of them properly. If you mean dispose of them if you detect an error, no, you can put clean up code in your EndXXX methods. If you mean so that you can signal clients gracefully if you shut the server down, then yes.
I've got a little problem with the .Net Sockets in C#.
I programmed a client and a server working with TCP.
As the client is opened it sends a handshake to the server. The server answers with it's state (clientexists, clientaccepted,...). After that the application sends a getdata-request, abandons the connection and listens for the server's 'response'. Now, the server builds a connection to the client and sends all the data the client needs.
The code and everything else works, but the problem:
On our company testserver it works fine, on the live server only the handshake works. After it the client doesn't receive any more data. Serverapplication is the same on both servers.
I thought the problem was caused by some firewall (server wants to build a tcp connection to the client -> not good), but the system administrator said there is no firewall that could block that.
Now I'm searching for a ('cheap') solution that doesn't take too much time and changes in code. If anyone knows how to theoretically solve that, that would be great.
BTW: I am not allowed to do anything on the live server other than run the serverapplication. I don't have the possibility to debug on this server.
I can't publish all of my code, but if you need to see specific parts of it, ask for it please.
---EDIT---
Client-Server communication
1) Client startup
Client send handshake (new tcp connection)
2) Server validates handshake and saves IP
Server responds with it's client state (same tcp connection)
3) Client acknowledges this response and abandons this connection
Client sends getdata-request (new tcp connection)
Client abandons this tcp connection, too
4) Server receives getdata-request and collects the needed data in the main database
Server sends all the collected data to the client (multiple tcp connections)
5) Client receives all data and displays it in it's GUI (multiple tcp connections and the order of the data is kept by working with AutoResetEvents and Counts of sockets to send)
This is the main part my code does. It's by far not the best but it was for me as I wrote it I guess. Step one, two and three work as intended. The processing of the data works fine, too.
Another thing i forgot to mention is that the solution uses two Ports '16777' and '16778'. One to receive/listen and one to send.
My code is based on the MSDN example of the asynchronous server and client.
Sending a handshake (and getdata-request)
public void BeginSend(String data)
{
try
{
StateObject state = new StateObject();
state.workSocket = sender;
byte[] byteData = Encoding.UTF8.GetBytes(data);
sender.BeginSend(byteData, 0, byteData.Length, 0,
new AsyncCallback((IAsyncResult e) =>
{
Socket socket = (Socket)e.AsyncState;
SocketBase.StateObject stateObject = new SocketBase.StateObject();
stateObject.workSocket = socket;
socket.BeginReceive(stateObject.buffer, 0, 256, SocketFlags.None, new AsyncCallback(this.ReadCallback), (object)stateObject);
}), sender);
sender = RetrieveSocket(); //Socketreset
Thread.Sleep(100);
}
catch /*(Exception e)*/
{
//--
}
}
Server listener
public void StartListening()
{
listener = new Socket(AddressFamily.InterNetwork,
SocketType.Stream, ProtocolType.Tcp);
// Bind the socket to the local endpoint and listen for incoming connections.
try
{
listener.Bind(localEndPoint);
listener.Listen(System.Int32.MaxValue);
while (true)
{
// Set the event to nonsignaled state.
allDone.Reset();
// Start an asynchronous socket to listen for connections.
listener.BeginAccept(
new AsyncCallback(AcceptCallback),
listener);
// Wait until a connection is made before continuing.
allDone.WaitOne();
}
}
catch (Exception e)
{
//--
}
}
public void AcceptCallback(...);
public void ReadCallback(...);
Socket send
private void Send(Socket handler, String data)
{
Socket t = RetrieveSocket(((IPEndPoint)handler.RemoteEndPoint).Address);
// Convert the string data to byte data using ASCII encoding.
byte[] byteData = Encoding.UTF8.GetBytes(data);
// Begin sending the data to the remote device.
t.BeginSend(byteData, 0, byteData.Length, 0,
new AsyncCallback(SendCallback), t);
}
Socket send all data part (answer to getdata-request | socToHandle should be the socket of the previous connection of the getdata-request)
private void SendAllData(Socket socToHandle, string PakContent)
{
#region IsThereADatetime? //Resolve a given datetime
#region GiveClientNumberOfPackets //Send the client info about how much he has to receive (See line below)
Send(socToHandle, "ALERT#TASKCOUNT;OPT-" + GetBestDate(dateStart) + EndSocket);
#region #SendResouces
#region #SendGroups
#region #SendTasks
}
Looking through my old code I have one idea =>
Could I send everything over the same connection by changing:
Socket t = RetrieveSocket(((IPEndPoint)handler.RemoteEndPoint).Address);
(which creates a new connection) to something that uses the same connection?
If that would work, how can I do that?
And would the listener part of the client still receive single packets?
Servers and their environment are configured to handle incoming requests properly. Clients are usually behind a router, which by default make them unable to receive incoming connections from outside their network (a good thing).
To enable incoming connections, you could configure your router to forward all requests for a certain port number to your machine. No one else on your network would be able to run the client then, though.
This is why in a typical multiple clients-single server environment, the client makes all the connections, and only the server requires any changes to the network landscape.
I don't know why you chose to connect to the clients from the server side, but I would strongly advise against this - any cheap solution that uses this mechanism may turn out to be very expensive in the end.
I'm writing a small C# Sockets application. Actually I have two, a server and a client.
The user runs the client, enters the IP and port for the server, presses 'connect', and then once connected they can enter text into a textbox and send it to the server.
The server simply displays either "No connection" or "Connection from [ip]:[port]", and the most recent received message underneath.
The server successfully receives messages, and even handles the client disconnect fine.
Now I'm trying to make it listen again after the client has disconnected but for some reason nothing I try will allow it to start listening again.
Here is part of my code:
Socket socket;
private void listen()
{
socket = new Socket(AddressFamily.InterNetwork,
SocketType.Stream, ProtocolType.Tcp);
socket.Bind(new IPEndPoint(IPAddress.Any, 12345));
socket.Listen(10);
socket.BeginAccept(new AsyncCallback(acceptAsync), socket);
}
and
private void receiveAsync(IAsyncResult res)
{
Socket socket = (Socket)res.AsyncState;
try
{
int nBytes = socket.EndReceive(res);
if (nBytes > 0)
{
Invoke(new MethodInvoker(delegate()
{
lMessage.Text = encoder.GetString(buffer);
}));
setupReceiveAsync(socket);
}
else
{
Invoke(new MethodInvoker(delegate()
{
lConnections.Text = "No Connections.";
lMessage.Text = "No Messages.";
socket.Shutdown(SocketShutdown.Both);
socket.Close();
listen();
}));
}
}
catch { }
}
The last line: listen(); is what throws the error.
I have tried simply calling socket.BeginAccept() again, but that also throws an exception.
The message I'm getting is:
Only one usage of each socket address (protocol/network address/port) is normally permitted
If I don't call my listen() function and instead just call socket.BeginAccept(), then I get "You must first call socket.listen()"
If I call the socket.listen() function, then it tells me it's already connected and cart start listening.
Once I have made an asynchronous connection, and received several asynchronous messages, how then do I begin receiving again?
Your socket variable already has an listening socket assigned to it the second time you call listen(), which is why it tells you only one usage is permitted. All you need to repeat is the socket.BeginAccept(new AsyncCallback(acceptAsync), socket) call. So try replacing the call to listen() inside your receiveAsync(...) method with socket.BeginAccept(new AsyncCallback(acceptAsync), socket).
In async Begin* is always followed by End*. See Using an Asynchronous Server Socket. Your accept method should be something like:
try {
listener.Bind(localEP);
listener.Listen(10);
while (true) {
allDone.Reset();
Console.WriteLine("Waiting for a connection...");
listener.BeginAccept(
new AsyncCallback(SocketListener.acceptCallback),
listener );
allDone.WaitOne();
}
} catch (Exception e) {
Server, means that an app that listens on specified port/ip. Is usually, always in a listening mode - that's why it is called a server. It can connect and disconnect a client, but is always in listening mode.
This means, when a server disconnects a client - even then - it is in listening mode; meaning it can accept the incoming connections as well.
Though, disconnection request can come from client or can be forcefully applied by server.
The process for a server is:
Bind to socket
Listen
Accept connections
The process for client is:
Connect to the server
Send/receive messages
There are several ways for server to handle the incoming clients, couple, as follows:
Incoming connections are maintained in a list, for instance within a List<TcpClient>.
One way of handling the incoming clients is through threads. For instance, for each incoming client, spawn a thread that would handle the communication between server and client. For instance, checkout this example.
_
private void ListenForClients()
{
this.tcpListener.Start();
while (true)
{
//blocks until a client has connected to the server
TcpClient client = this.tcpListener.AcceptTcpClient();
//create a thread to handle communication
//with connected client
Thread clientThread = new Thread(new ParameterizedThreadStart(HandleClientComm));
clientThread.Start(client);
}
}
Use single thread and use context switching to manage client communications (TX/RX).
How do I go about making a socket policy file server in C#. All it has to do is listen on port 843 for the string "<policy-file-request/>" followed by a NULL byte and then return an XML string (which is the socket policy file).
I haven't coded this sort of thing before and am unsure of where to start. Do I create it in a windows service? Any tips or links are welcome.
Background:
To contact a web service from flash I am using the 'as3httpclient' library instead of the URLRequest/URLLoader. This is because it gives me the ability to send custom headers with GET requests. This library uses low-level sockets to do its stuff.
When flash uses low-level sockets to connect to a server it looks for a socket policy file - and this needs to be served up by a socket policy file server.
Socket Policy File Article from Adobe
A few things to be aware of using your suggested architecture:
Trying to send an HTTP request over sockets
Principally, you need to be aware that even though you can chat http at a lower level using sockets, there are a large number of cases where communication in this fashion will fail. Mainly these failures will occur if the user has a proxy server enabled in their browser, as there is no effective means of discovering and subsequently using the proxy when connecting via a socket.
In order to make a policy server, you can use the TcpListener class. You would start listening as follows:
var tcpListener = new TcpListener(IPAddress.Any, 843 );
tcpListener.start();
tcpListener.BeginAcceptTcpClient(new AsyncCallback(NewClientHandler), null);
The method NewClientHandler would have the form:
private void NewClientHandler(IAsyncResult ar)
{
TcpClient tcpClient = tcpListener.EndAcceptTcpClient(ar);
...
At which point you might want to supply the tcpClient object to a class of your own creation to handle the validation of the data coming from the socket. I'm going to call it RemoteClient.
In RemoteClient, you'd have something like this:
var buffer=new byte[BUFFER_SIZE];
tcpClient.GetStream().BeginRead(buffer, 0, buffer.Length, Receive, null);
and a Receive method:
private void Receive(IAsyncResult ar)
{
int bytesRead;
try
{
bytesRead = tcpClient.GetStream().EndRead(ar);
}
catch (Exception e)
{
//something bad happened. Cleanup required
return;
}
if (bytesRead != 0)
{
char[] charBuffer = utf8Encoding.GetChars(buffer, 0, bytesRead);
try
{
tcpClient.GetStream().BeginRead(buffer, 0, buffer.Length, Receive, null);
}
catch (Exception e)
{
//something bad happened. Cleanup required
}
}
else
{
//socket closed, I think?
return;
}
}
and some send methods:
public void Send(XmlDocument doc)
{
Send(doc.OuterXml);
}
private void Send(String str)
{
Byte[] sendBuf = utf8Encoding.GetBytes(str);
Send(sendBuf);
}
private void Send(Byte[] sendBuf)
{
try
{
tcpClient.GetStream().Write(sendBuf, 0, sendBuf.Length);
tcpClient.GetStream().WriteByte(0);
tcpClient.GetStream().WriteByte(13); //very important to terminate XmlSocket data in this way, otherwise Flash can't read it.
}
catch (Exception e)
{
//something bad happened. cleanup?
return;
}
}
That's all the important details I think. I wrote this some time ago... the Receive method looks like it could do with a rework, but it should be enough to get you started.
Create a listening socket.
When a connection is opened, perform a receive and wait for the expected string. When received send the file content and then close the socket.
Wrap this in a service (running as a low privilege account).
Most of the work is done with the System.Net.Sockets.Socket class, the documentation contains a sample, The API is very similar to the BSD socket API overall (largely there is a 1:1 mapping from BSD API to Socket (or help type) method) so any background should be easily translatable.
I had to to this task with both Java and C#, they are quite similar.
You can have a look at java policy file.
Some issue can see at this answer: https://stackoverflow.com/a/12854204/1343667