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Why isn't TcpListener listening?

We have a C++ v100 application that is processing every event in our system, listening on port 1705, running off the Hostname. (it works perfectly for the C++ app, and we don't want to change anything in the c++ code) We are trying to intercept some of those events into a C# 4.5.2 solution, simply to display specific events in our new web system.
I have coded the following, in an attempt to listen to port 1705 traffic... but I never receive any data. (I can create events that get sent to 1705)
The following code runs, and it makes it to 'Waiting for a connection', but never makes it to 'Connected!'. If you see any reason in the following code as to why I wouldn't be receiving data, please let me know:
private void PortListener()
{
TcpListener server = null;
try
{
// Set the TcpListener on port 13000.
var port = 1705;
var localAddr = IPAddress.Parse(Dns.GetHostAddresses(Environment.MachineName)[0].ToString());
server = new TcpListener(localAddr, port);
// Start listening for client requests.
server.Start();
// Buffer for reading data
var bytes = new byte[256];
// Enter the listening loop.
while (true)
{
Console.Write("Waiting for a connection... ");
// Perform a blocking call to accept requests.
// You could also user server.AcceptSocket() here.
var client = server.AcceptTcpClient();
Console.WriteLine("Connected!");
// Get a stream object for reading and writing
var stream = client.GetStream();
int i;
// Loop to receive all the data sent by the client.
while ((i = stream.Read(bytes, 0, bytes.Length)) != 0)
{
// Translate data bytes to a ASCII string.
var data = System.Text.Encoding.ASCII.GetString(bytes, 0, i);
Console.WriteLine("Received: {0}", data);
// Process the data sent by the client.
data = data.ToUpper();
//TODO: Process the data
}
// Shutdown and end connection
client.Close();
}
}
catch (SocketException e)
{
Console.WriteLine("SocketException: {0}", e);
}
finally
{
// Stop listening for new clients.
server?.Stop();
}
}

Make sure that you are binding/listening to the right ip-address. If you bind/listen on localhost (127.0.0.1) you can only connect from the same host.
Check what
Dns.GetHostAddresses(Environment.MachineName)[0].ToString());
really produces.

I'm doing this all wrong. In order to listen to an already opened Port, I need to use a TcpClient to connect and listen. Only a single TcpListener is allowed per port. Several TcpClients can connect at once. Sigh.

Why does my C# TcpClient fail to receive content from the server if the server sends before reading everything from the client?

In an application I'm working on I want to disconnect clients that are trying to send me packets that are too large.
Just before disconnecting them I want to send them a message informing them about the reason for disconnecting them.
The issue I am running into is that the client cannot receive this server message, if the server does not read everything the client has send him first. I do not understand why this is happening.
I've managed to narrow it down to a very small test setup where the problem is demonstrated.
The StreamUtil class is a simple wrapper class that helps to get around the TCP message boundary problem, basically on the sender side it sends the size of each message first and then the message itself, and on the receiver side it receives the size of the message first and then the message.
The client uses a ReadKey command to simulate some time between sending and receiving, seeing in my real application these two actions are not immediately back to back either.
Here is a test case that works:
Run server as shown below
Run client as shown below, it will show a "Press key message", WAIT do not press key yet
Turn off server since everything is already in the clients receive buffer anyway (I validated this using packet sniffer)
Press key on the client -> client correctly shows the messages from the server.
This is what I was expecting, so great so far no problem yet.
Now in the server code, comment out the 2nd receive call and repeat the steps above.
Step 1 and 2 complete successfully, no errors sending from client to server.
On step 3 however the client crashes on the read from the server, EVEN though the server reply HAS arrived on the client (again validated with packet sniffer).
If I do a partial shutdown (eg socket.Shutdown (...send...)) without closing the socket on the server, everything works.
1: I just cannot get my head around WHY not processing the line of text from the client on the server causes the client to fail on receiving the text send back from the server.
2: If I send content from server to client but STOP the server before actually closing the socket, this content never arrives, but the bytes have already been transmitted to the server side... (see ReadKey in server to simulate, basically I block there and then just quit the server)
If anyone could shed light on these two issues, I'd deeply appreciate it.
Client:
class TcpClientDemo
{
public static void Main (string[] args)
{
Console.WriteLine ("Starting....");
TcpClient client = new TcpClient();
try
{
client.Connect("localhost", 56789);
NetworkStream stream = client.GetStream();
StreamUtil.SendString(stream, "Client teststring...");
Console.WriteLine("Press key to initiate receive...");
Console.ReadKey();
Console.WriteLine("server reply:" + StreamUtil.ReceiveString(stream));
}
catch (Exception e)
{
Console.WriteLine(e.Message);
}
finally
{
client.Close();
}
Console.WriteLine("Client ended");
Console.ReadKey(true);
}
}
Server:
class TcpServerDemo
{
public static void Main (string[] args)
{
TcpListener listener = new TcpListener (IPAddress.Any, 56789);
listener.Start ();
Console.WriteLine ("Waiting for clients to serve...");
while (true)
{
TcpClient client = null;
NetworkStream stream = null;
try
{
client = listener.AcceptTcpClient();
stream = client.GetStream();
//question 1: Why does commenting this line prevent the client from receiving the server reply??
Console.WriteLine("client string:" + StreamUtil.ReceiveString(stream));
StreamUtil.SendString(stream, "...Server reply goes here...");
//question 2: If I close the server program without actually calling client.Close (while on this line), the client program crashes as well, why?
//Console.ReadKey();
}
catch (Exception e)
{
Console.WriteLine(e.Message);
break;
}
finally
{
if (stream != null) stream.Close();
if (client != null) client.Close();
Console.WriteLine("Done serving this client, everything closed.");
}
}
listener.Stop();
Console.WriteLine("Server ended.");
Console.ReadKey(true);
}
}
StreamUtil:
public class StreamUtil
{
public static byte[] ReadBytes (NetworkStream pStream, int byteCount) {
byte[] bytes = new byte[byteCount];
int bytesRead = 0;
int totalBytesRead = 0;
try {
while (
totalBytesRead != byteCount &&
(bytesRead = pStream.Read (bytes, totalBytesRead, byteCount - totalBytesRead)) > 0
) {
totalBytesRead += bytesRead;
Console.WriteLine("Read/Total:" + bytesRead + "/" + totalBytesRead);
}
} catch (Exception e) {
Console.WriteLine(e.Message);
}
return (totalBytesRead == byteCount) ? bytes : null;
}
public static void SendString (NetworkStream pStream, string pMessage) {
byte[] sendPacket = Encoding.ASCII.GetBytes (pMessage);
pStream.Write (BitConverter.GetBytes (sendPacket.Length), 0, 4);
pStream.Write (sendPacket, 0, sendPacket.Length);
}
public static string ReceiveString (NetworkStream pStream) {
int byteCountToRead = BitConverter.ToInt32(ReadBytes (pStream, 4), 0);
Console.WriteLine("Byte count to read:"+byteCountToRead);
byte[] receivePacket = ReadBytes (pStream, byteCountToRead);
return Encoding.ASCII.GetString (receivePacket);
}
}

The client fails because it detects the socket was already closed.
If C# socket operations detect a closed connection during earlier operations, an exception is thrown on the next operation which can mask data which would otherwise have been received
The StreamUtil class does a couple of things when the connection is closed before/during a read:
Exceptions from the reads are swallowed
A read of zero bytes isn't treated
These obfuscate what's happening when an unexpected close hits the client.
Changing ReadBytes not to swallow exceptions and to throw a mock socket-closed exception (e.g. if (bytesRead == 0) throw new SocketException(10053);) when it reads zero bytes I think makes the outcome more clear.
Edit
I missed something subtle in your examples - your first example causes a TCP RST flag to be sent as soon as the server closes connection, due to the socket being closed with data waiting to be read.
The RST flag results in a closedown that doesn't preserve pending data.
This blog has some discussion based on a very similar scenario (web server sending a HTTP error).
So I don't think there's an easy fix, options are:
As you already tried, shutdown the socket on the server before closing to force a FIN to be sent before the RST
Read the data in question but never process it (taking up bandwidth for no reason)

Reset TCP connection if server closes/crashes mid connection

I have a tcp connection like follows:
public void ConnectToServer()
{
string mac = GetUID();
while(true)
{
try
{
tcpClient = new TcpClient("xx.x.xx.xxx", xxxx);
networkstream = new SslStream(tcpClient.GetStream());
networkstream.AuthenticateAsClient("xx.x.xx.xxx");
networkstream.Write(Encoding.UTF8.GetBytes("0002:" + mac + "\r\n"));
networkstream.Flush();
string serverMessage = ReadMessage(networkstream);
Console.WriteLine("MESSAGE FROM SERVER: " + serverMessage);
}
catch (Exception e)
{
tcpClient.GetStream().Close();
tcpClient.Close();
}
}
}
This works fine and can send a receive data to/from the server.
What I need help with, if the server isn't running when the client starts, it'll wait and then connect once the server is up. But, if both the client and server are running and everything is working, if I close the server, the client will not reconnect(because I don't have anything to handle the event yet).
I have seen some answers on here that suggest polling and such. Is that the only way? The ReadMessage method that I call get into an infinite loop as well. I can post that code if need be.
I would really like to detect when the server closes/crashes and close the stream and the tcpclient and reconnect ASAP.
Here is my readmessage:
static string ReadMessage(SslStream sslStream)
{
if (sslStream.CanRead)
{
byte[] buffer = new byte[2048];
StringBuilder messageData = new StringBuilder();
int bytes = -1;
string message_type = null;
string actual_message = null;
do
{
try
{
Console.WriteLine("LENGTH: " + buffer.Length);
bytes = sslStream.Read(buffer, 0, buffer.Length);
Decoder decoder = Encoding.UTF8.GetDecoder();
char[] chars = new char[decoder.GetCharCount(buffer, 0, bytes)];
decoder.GetChars(buffer, 0, bytes, chars, 0);
messageData.Append(chars);
message_type = messageData.ToString().Substring(0, 5);
actual_message = messageData.ToString().Substring(5);
if (message_type.Equals("0001:"))
{
m_Window pop = new m_Window();
pop.callHttpPost(null, new EventArgs());
}
if (messageData.ToString().IndexOf("\r\n") != -1)
{
break;
}
}
catch (Exception e)
{
Console.WriteLine("ERROR: " + e.Message);
}
} while (bytes != 0);
return messageData.ToString();
}
return("CONNECTION HAS BEEN LOST");
}

With TCP you have 2 kinds of a server disconnect:
the server is closed
the server crashes
When the server is closed, you are going to receive 0 bytes on your client socket, this is the way you know that the peer has closed its end of the socket, which is called a half close.
But thing get more ugly if the server crashes.
When that happens again you have several possibilities.
If you don't send anything from the client to the server, the you have not way to find out that the server has indeed crashed.
The only way to find out that the server crashed is by letting the client send something or by activating keep alive. If you send something to a server socket that does not exist, you will have to wait a rather long period, because TCP is going to try several times, with retransmits untill there is a server response. When TCP has retried several times, then it will finally bail out and if you have a blocking socket you will see that the send failed, which means you should close your socket.
Actually there is a third possible server disconnect, that is a reset, but this is exceptionally used. I assume here that if there is a gracefull server shutdown, a normal close on the socket on the server end is executed. Which will end up in a FIN being sent instead of a RST, which is the exceptional case.
Now back to your situation, if the server crashes, it is inherently in the design of TCP, because of all those retransmission timeouts and increasing delays, that you will have to wait some time to actually detect that there is a problem. If the server is gracefully closed and startup again, this is not the case, this you detect immediately by receiving 0 bytes.

Read unknown length by DataReader

I've been working with windows app store programming in c# recently, and I've come across a problem with sockets.
I need to be able to read data with an unknown length from a DataReader().
It sounds simple enough, but I've not been able to find a solution after a few days of searching.
Here's my current receiving code (A little sloppy, need to clean it up after I find a solution to this problem. And yes, a bit of this is from the Microsoft example)
DataReader reader = new DataReader(args.Socket.InputStream);
try
{
while (true)
{
// Read first 4 bytes (length of the subsequent string).
uint sizeFieldCount = await reader.LoadAsync(sizeof(uint));
if (sizeFieldCount != sizeof(uint))
{
// The underlying socket was closed before we were able to read the whole data.
return;
}
reader.InputStreamOptions
// Read the string.
uint stringLength = reader.ReadUInt32();
uint actualStringLength = await reader.LoadAsync(stringLength);
if (stringLength != actualStringLength)
{
// The underlying socket was closed before we were able to read the whole data.
return;
}
// Display the string on the screen. The event is invoked on a non-UI thread, so we need to marshal
// the text back to the UI thread.
//MessageBox.Show("Received data: " + reader.ReadString(actualStringLength));
MessageBox.updateList(reader.ReadString(actualStringLength));
}
}
catch (Exception exception)
{
// If this is an unknown status it means that the error is fatal and retry will likely fail.
if (SocketError.GetStatus(exception.HResult) == SocketErrorStatus.Unknown)
{
throw;
}
MessageBox.Show("Read stream failed with error: " + exception.Message);
}

You are going down the right lines - read the first INT to find out how many bytes are to be sent.
Franky Boyle is correct - without a signalling mechanism it is impossible to ever know the length of a stream. Thats why it is called a stream!
NO socket implementation (including the WinSock) will ever be clever enough to know when a client has finished sending data. The client could be having a cup of tea half way through sending the data!
Your server and its sockets will never know! What are they going to do? Wait forever? I suppose they could wait until the client had 'closed' the connection? But your client could have had a blue screen and the server will never get that TCP close packet, it will just be sitting there thinking it is getting more data one day?
I have never used a DataReader - i have never even heard of that class! Use NetworkStream instead.
From my memory I have written code like this in the past. I am just typing, no checking of syntax.
using(MemoryStream recievedData = new MemoryStream())
{
using(NetworkStream networkStream = new NetworkStream(connectedSocket))
{
int totalBytesToRead = networkStream.ReadByte();
// This is your mechanism to find out how many bytes
// the client wants to send.
byte[] readBuffer = new byte[1024]; // Up to you the length!
int totalBytesRead = 0;
int bytesReadInThisTcpWindow = 0;
// The length of the TCP window of the client is usually
// the number of bytes that will be pushed through
// to your server in one SOCKET.READ method call.
// For example, if the clients TCP window was 777 bytes, a:
// int bytesRead =
// networkStream.Read(readBuffer, 0, int.Max);
// bytesRead would be 777.
// If they were sending a large file, you would have to make
// it up from the many 777s.
// If it were a small file under 777 bytes, your bytesRead
// would be the total small length of say 500.
while
(
(
bytesReadInThisTcpWindow =
networkStream.Read(readBuffer, 0, readBuffer.Length)
) > 0
)
// If the bytesReadInThisTcpWindow = 0 then the client
// has disconnected or failed to send the promised number
// of bytes in your Windows server internals dictated timeout
// (important to kill it here to stop lots of waiting
// threads killing your server)
{
recievedData.Write(readBuffer, 0, bytesReadInThisTcpWindow);
totalBytesToRead = totalBytesToRead + bytesReadInThisTcpWindow;
}
if(totalBytesToRead == totalBytesToRead)
{
// We have our data!
}
}
}

How to reconnect a TcpClient as soon as network unplugged and then plugged

I have a TcpClient that i want automatically re-connect as soon as network disconnects and then reconnect,but i am not getting how to achieve it..
Here is my function ..
private void Conn()
{
try
{
client = new TcpClient();
client.Connect(new IPEndPoint(IPAddress.Parse(ip), intport));
//Say thread to sleep for 1 secs.
Thread.Sleep(1000);
}
catch (Exception ex)
{
// Log the error here.
client.Close();
}
try
{
using (NetworkStream stream = client.GetStream())
{
byte[] notify = Encoding.ASCII.GetBytes("Hello");
stream.Write(notify, 0, notify.Length);
}
byte[] data = new byte[1024];
while (true)
{
{
int numBytesRead = stream.Read(data, 0, data.Length);
if (numBytesRead > 0)
{
data= Encoding.ASCII.GetString(data, 0, numBytesRead);
}
}
}
}
catch{Exception ex}
Also how reliable is while (true) to get the continuous data from the Tcpip machine.Till my testing this codes automatically exits from responding or getting data after a while.
Please help me to get the uninterrupted data .
Thanks..

You are immediately disposing of the NetworkStream after you have written something. This closes the socket. Don't do that. Rather, put the TcpClient in a using statement.
The way you read data is exactly right. The loop will exit when Read returns 0 which indicated a graceful shutdown of the connection by the remote side. If this is unexpected, the problem lies with the remote side.
Catch SocketException only and examine the status code property to find out the exact error.
It is not possible to reliably detect network errors. You have to wait for an exception to notice connection failure. After that, you need to periodically try establishing a connection again to find out when the network becomes available again.
I believe Windows provides some network interface level events to detect unplugged cabled but those are unreliable.