Develop Reference

Removing TCP Options Using TcpClient in C#

I am using a TcpClient in C# to communicate with a server.
When I view the first SYN request in Wireshark, I see 12 bytes specifying 6 TCP Options in the following order:
-Maximum Segment size
-NOP
-Window Scale
-NOP
-NOP
-SACK Permitted
TCP_Options <<< A screenshot of the Wireshark capture
I am using the default constructor of the TcpClient object being passed into the function.
I believe there is a way to specify an option using the Socket.SetSocketOption Method (described here), however, I wish to remove these options to verify that the server doesn't require them to establish connections.
Is it possible to remove (or perhaps modify) the options that I am seeing in Wireshark?
Here is a small sample of how I am using the TcpClient (instantiated with defaults):
public static IPStatus PingThenConnect(TcpClient tcpClient, IPEndPoint serverEndpoint)
{
IPStatus ipStatus = IPStatus.Unknown;
// Ping the given IP Address
ipStatus = Network.PingIP(serverEndpoint.Address);
if (ipStatus == IPStatus.Success)
{
// Connect TcpClient to given endpoint.
tcpClient.Connect(serverEndpoint.Address, serverEndpoint.Port);
tcpClient.LingerState = new LingerOption(true, 10);
}
return ipStatus;
}

UdpClient Receive not receiving datagram

I trying to write a TFTP client for a class project.
Using the UdpClient class I can successfully request data from the server but the returned packet never reaches my code.
My firewall is turned off.
I can observe the returned packet in wireshark but UdpClient.Receive blocks indefinitely.
mUdpClient is initialized like this in the constructor: mUdpClient = new UdpClient();
mUdpClient is connected like this
public void connect(String host, int port) {
mServerAddress = System.Net.Dns.GetHostAddresses(host)[0];
var endPoint = new IPEndPoint(mServerAddress, port);
mUdpClient.Connect(endPoint);
}
After the connect I send my request which is successful (as observed in wireshark)
This is what my receive code looks like
private void receiveResponse() {
var newEndpoint = new IPEndPoint(IPAddress.Any, 0);
byte[] response = mUdpClient.Receive(ref newEndpoint);
Console.Out.WriteLine(response);
}
This has been tested on my Surface Pro and a Windows 8.1 VirtualBox VM running under Debian.

Note that since you are using the Connect() method on your UDP socket, you will only see datagrams actually sent from that IPEndPoint. If your remote host for some reason uses a different IPEndPoint to send data back to you, you won't see it. So maybe try not using the default host feature (i.e. don't call Connect...just provide the remote IPEndPoint on each Send() call).

Why can't I receive text sent to localhost using UdpClient in C#?

I am trying to write a simple UDP program in C# which sends and receives data on localhost. I am a beginner in C# but much better at MATLAB so instead of writing the server and the client in C#, I decided to send data using C# and receive it in MATLAB.
I tried two approaches to send data. Using Socket class worked, but using UdpClient class failed.
Before running this code, I run the MATLAB code to set the callback function to print the received datagram.
Only one region is active in each run. I comment out the other one.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net.Sockets;
using System.Net;
namespace udp1
{
class Program
{
const int port = 62745; //Chosen at random
static void Main(string[] args)
{
string str = "Hello World!";
byte[] sendBytes = Encoding.ASCII.GetBytes(str);
#region 1 Send data using socket class
Socket sock = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
IPEndPoint ipEndPoint = new IPEndPoint(IPAddress.Parse("127.0.0.1"), port);
sock.SendTo(sendBuff, ipEndPoint);
Console.ReadLine();
#endregion
#region 2 Send data using UdpClient class
UdpClient sendingClient = new UdpClient(port);
sendingClient.Send(sendBytes, sendBytes.Length);
#endregion
}
}
}
I am getting
Only one usage of each socket address (protocol/network address/port)
is normally permitted
error when I run the code in region 2.
However, when I run the code in region 1 everything works as I expect and I receive the data in MATLAB without any problems.
Here is my MATLAB code. I have used this code in other applications, so I highly doubt there is anything wrong with it.
fclose(instrfindall); %Close all udp objects
%UDP Configuration
udpConfig.ipAddress = '127.0.0.1';
udpConfig.portAddress = 62745;
udpObj = udp(udpConfig.ipAddress, udpConfig.portAddress, ...
'LocalPort', udpConfig.portAddress, ...
'ByteOrder', 'bigEndian');
set(udpObj, 'datagramTerminateMode', 'on');
set(udpObj, 'datagramReceivedFcn', {#cbDataReceived, udpObj});
fopen(udpObj);
And the callback function:
function cbDataReceived(hObj, eventdata, udpObj)
bytesAvailable = get(udpObj, 'BytesAvailable');
receivedDatagram = fread(udpObj, bytesAvailable);
disp(char(receivedDatagram));
end
So, why am I getting the error in UdpClient case and not getting it in Socket case? Is there a way to avoid that error?

I understand that you are using the same port for both MATLAB and C# on the same computer. Thus, operating system does not allow to open the same port from different applications.
UDP allows sending and receiving datagrams from different ports, so use different ports for different applications if both applications are running on the same computer.
UdpClient sendingClient = new UdpClient(62746); // Some different port to listen
sendingClient.Send(sendBytes, sendBytes.Length, ipEndPoint);

UDP hole punching. Have server talk to client

I been reading a lot on how to implement UDP hole punching but fore some reason I cannot make it work.
For those that are not familiar of what udp hole punching is here is my own definition:
The goal is to be able to transfer data between two clients (Client A
and client B) with the help of a server. So client A connects to the server and sends its info. Client B does the same. The server has the nessesary info so that Client A is able to send data to Client B and vise versa . Therefore the server gives that info to both clients. Once both clients have that info about each other it is possible to start sending and receiving data between those clients without the help of the server.
My goal is to be able to do what I just described (udp hole punching). Before doing so I think it will be helpful to be able to connect from the server to the client. In order to do so I plan to send the server the info about the client. Once the server receives that info attempt to connect to the client from scratch. Once I am able to perform that I should have everything I need to start implementing the real udp hole punching.
Here is how I have things set up:
The top router has the server and bottom router connected to LAN ports. The bottom router (NAT) is connected to the top router via it's WAN port. And the client computer is connected to the bottom router to one of its LAN ports.
So in that connection the client is able to see the server but the server is not able to see the client.
So the algorithm I have done in pseudo code is:
Client connects to server.
Client send some UDP packages to the server in order to open some ports on the NAT
Send information to the server on what ports the client is listening to.
Once the server receives that info attempt to connect to the client from scratch.
Here is the implementation in code:
Server:
static void Main()
{
/* Part 1 receive data from client */
UdpClient listener = new UdpClient(11000);
IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, 11000);
string received_data;
byte[] receive_byte_array = listener.Receive(ref groupEP);
received_data = Encoding.ASCII.GetString(receive_byte_array, 0, receive_byte_array.Length);
// get info
var ip = groupEP.Address.ToString();
var port = groupEP.Port;
/* Part 2 atempt to connect to client from scratch */
// now atempt to send data to client from scratch once we have the info
Socket sendSocket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
IPEndPoint endPointClient = new IPEndPoint(IPAddress.Parse(ip), port);
sendSocket.SendTo(Encoding.ASCII.GetBytes("Hello"), endPointClient);
}
Client:
static void Main(string[] args)
{
/* Part 1 send info to server */
Socket sending_socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
IPAddress send_to_address = IPAddress.Parse("192.168.0.132");
IPEndPoint sending_end_point = new IPEndPoint(send_to_address, 11000);
sending_socket.SendTo(Encoding.ASCII.GetBytes("Test"), sending_end_point);
// get info
var port = sending_socket.LocalEndPoint.ToString().Split(':')[1];
/* Part 2 receive data from server */
IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, int.Parse(port));
byte[] buffer = new byte[1024];
sending_socket.Receive(buffer);
}
For some reason it worked a few times! It works when the client receives data successfully on the line: sending_socket.Receive(buffer);
Things to note:
If on the server on the second part I used the instance variable listner instead of creating the new variable sendSocket and send the bytes through that variable the client is able to receive the data being sent. Remember that the second part of the server is going to be implemented by a second client B that's why I am initializing variables again from scratch...
Edit:
Here is a different way of looking at the same problem. When I initialize a new object instead of using the same object the client does not receives the response.
I have a object of type UdpClient. I am able to send data with that object to the other peer. If I create another object of the same type with the same properties and attempt to send data it does not work! I might be missing to initialize some variables. I am able to set private variables with reflection so I should not have a problem. anyways here is the server code:
public static void Main()
{
// wait for client to send data
UdpClient listener = new UdpClient(11000);
IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, 11000);
byte[] receive_byte_array = listener.Receive(ref groupEP);
// connect so that we are able to send data back
listener.Connect(groupEP);
byte[] dataToSend = new byte[] { 1, 2, 3, 4, 5 };
// now let's atempt to reply back
// this part does not work!
UdpClient newClient = CopyUdpClient(listener, groupEP);
newClient.Send(dataToSend, dataToSend.Length);
// this part works!
listener.Send(dataToSend, dataToSend.Length);
}
static UdpClient CopyUdpClient(UdpClient client, IPEndPoint groupEP)
{
var ip = groupEP.Address.ToString();
var port = groupEP.Port;
var newUdpClient = new UdpClient(ip, port);
return newUdpClient;
}
the client code basically sends data to the server and then waits for a response:
string ipOfServer = "192.168.0.132";
int portServerIsListeningOn = 11000;
// send data to server
Socket sending_socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
IPAddress send_to_address = IPAddress.Parse(ipOfServer);
IPEndPoint sending_end_point = new IPEndPoint(send_to_address, portServerIsListeningOn);
sending_socket.SendTo(Encoding.ASCII.GetBytes("Test"), sending_end_point);
// get info
var port = sending_socket.LocalEndPoint.ToString().Split(':')[1];
// now wait for server to send data back
IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, int.Parse(port));
byte[] buffer = new byte[1024];
sending_socket.Receive(buffer); // <----- keeps waiting in here :(
note that the client is behind a router (NAT) otherwise I will not have this problem. The reason why I will like to copy udpClient is so that I can send that variable to another computer enabling the other computer to send data to the client.
So my question is why is the original object listener able to send data but newClient is not able to? The client keeps waiting at line sending_socket.Receive(buffer); even after the server executes the line: newClient.Send(dataToSend, dataToSend.Length);. the client successfully receives data when listener sends the data but not newClient. Why is this if both variables have the same destination IP and port? how do the variables differ?
Note:
If the server and client are on the same network then the copy works and variable newClient is able to send data to the client. To simulate this problem the client must be behind a NAT (router). An example of such network may consist of two routers. let's call them router X and router Y. You also need a Server call that S. and a client C. so S can be connected to one of the LAN ports of X. C can be connected to one of the LAN ports of Y. Finally connect the WAN port of Y to one of the LAN ports of X.

Hmm, I think you are confusing several things here. For one thing, it's really called UDP hole punching. Let me try to explain how this should work.
NAT routers usually do port mapping when forwarding packets from the inside private network to the outside internet.
Say you created a UDP socket on a machine behind NAT, and sent a datagram to some external IP/port. When the IP packet carrying that datagram leaves the sending machine, its IP header has the source address field set to local not-globally-routable private IP address (like 192.168.1.15), and UDP header has the source port field set to whatever port was assigned to the socket (either explicitly via binding, or implicitly picked by the OS from the ephemeral ports). I'll call this source port number P1.
Then when the NAT router sends that packet out on the outside network, it overwrites the source IP address to its own external IP address (otherwise there's no way to route packets back), and often overwrites the source UDP port to some other value (maybe because some other host on the private network uses the same source port, which creates ambiguity). The mapping between the original source port and that new port number (let's label it P2) is preserved in the router to match return packets. This mapping might also be specific to the target IP address and target UDP port.
So now you have "punched a hole" in the router - UDP packets sent back to the router to port P2 are forwarded to internal machine on UDP port P1. Again, depending on NAT implementation, this could be restricted to only packets from the original target IP address and target UDP port.
For client-to-client communication you'll have to tell external IP/port of one to the other through the server, hoping that the NAT router maps same internal source ports to same external source ports. Then the clients will send packets to each other using those.
Hope this helps.

Finally found the answer! Here is the implemenation with just a client and a server. My next attempt will be to use 3 computers. anyways hope this helps:
Server code:
class Program
{
static byte[] dataToSend = new byte[] { 1, 2, 3, 4, 5 };
// get the ip and port number where the client will be listening on
static IPEndPoint GetClientInfo()
{
// wait for client to send data
using (UdpClient listener = new UdpClient(11000))
{
IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, 11000);
byte[] receive_byte_array = listener.Receive(ref groupEP);
return groupEP;
}
}
static void Main(string[] args)
{
var info = GetClientInfo(); // get client info
/* NOW THAT WE HAVE THE INFO FROM THE CLIENT WE ARE GONG TO SEND
DATA TO IT FROM SCRATCH!. NOTE THE CLIENT IS BEHIND A NAT AND
WE WILL STILL BE ABLE TO SEND PACKAGES TO IT
*/
// create a new client. this client will be created on a
// different computer when I do readl udp punch holing
UdpClient newClient = ConstructUdpClient(info);
// send data
newClient.Send(dataToSend, dataToSend.Length);
}
// Construct a socket with the info received from the client
static UdpClient ConstructUdpClient(IPEndPoint clientInfo)
{
var ip = clientInfo.Address.ToString();
var port = clientInfo.Port;
// this is the part I was missing!!!!
// the local end point must match. this should be the ip this computer is listening on
// and also the port
UdpClient client = new UdpClient(new IPEndPoint( IPAddress.Any, 11000));
// lastly we are missing to set the end points. (ip and port client is listening on)
// the connect method sets the remote endpoints
client.Connect(ip, port);
return client;
}
}
client code:
string ipOfServer = "192.168.0.139";
int portServerIsListeningOn = 11000;
// send data to server
Socket sending_socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
IPAddress send_to_address = IPAddress.Parse(ipOfServer);
IPEndPoint sending_end_point = new IPEndPoint(send_to_address, portServerIsListeningOn);
sending_socket.SendTo(Encoding.ASCII.GetBytes("Test"), sending_end_point);
// get info
var port = sending_socket.LocalEndPoint.ToString().Split(':')[1];
// now wait for server to send data back
IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, int.Parse(port));
byte[] buffer = new byte[1024];
sending_socket.Receive(buffer); // <----- we can receive data now!!!!!

Have you considered using UPnP on the client to configure NAT traversal to allow incoming packets on a particular port? The client would then only need to communicate the inbound IP and port to the server, and wait for the server to send packets.
http://en.wikipedia.org/wiki/Universal_Plug_and_Play

Seems you are able to connect with the server first time.After successful connection you need to close and disconnect the connection each time.Please check this sample code
http://codeidol.com/csharp/csharp-network/Connectionless-Sockets/A-Simple-UDP-Application/

Test if Socket is connected in C#

Hi i am writing a simple server program that is listening for connections. My Question is, How can I test if the socket is connected or not. Here is my code
using System;
using System.Net;
using System.Net.Sockets;
class server
{
static int port = 0;
static String hostName = Dns.GetHostName();
static IPAddress ipAddress;
static bool listening = true;
public static void Main(String[] args)
{
IPHostEntry ipEntry = Dns.GetHostByName(hostName);
//Get a list of possible ip addresses
IPAddress[] addr = ipEntry.AddressList;
//The first one in the array is the ip address of the hostname
ipAddress = addr[0];
TcpListener server = new TcpListener(ipAddress,port);
Console.Write("Listening for Connections on " + hostName + "...");
do
{
//start listening for connections
server.Start();
} while (listening);
//Accept the connection from the client, you are now connected
Socket connection = server.AcceptSocket();
Console.Write("You are now connected to the server");
connection.Close();
}
}

I think you got your beans messed up here. Underneath, at the OS level, there are two distinct notions: a listening socket - that's the TcpListener, and a connected socket - that's what you get after successful accept().
Now, the listening TCP socket is not connected, but bound to the port (and possibly address) on the local machine. That's where a server waits for connection requests from clients. Once such request arrives the OS creates a new socket, which is connected in a sense that it has all four parts that are required for communication - local IP address and port, and remote address and port - filled in.
Start with some introductory text, something like this one. Even better - start with a real one.

server.Start()should be outside the loop. It's only called once and the listening socket will remain open until Stop is called.
AcceptSocket will block until a client have connected. If you want to be able to accept multiple sockets, then keep looping it.