Develop Reference

How to set up TCP communication between two computers

I am trying to use the TCP protocol with the System.Net.Sockets library to have one computer send a string to the other. Here is part of my host code. Is the listener object supposed to be declared with the client IP address or the host IP address (like it is in the code below)?
IPAddress ipAddr = Dns.GetHostEntry(Dns.GetHostName()).AddressList[0]; //Automatically retrieves IPAddress.
int port = 135; //specify port number.
TcpListener listener = new TcpListener(ipAddr, port);
Also, here is a piece of my client code. Which constructor do I use?
TcpClient client = new TcpClient();

The listener object listens on the host's IP. The constructor takes it as an argument because you could have several IP's on the computer and it doesn't know which one you want to listen on.
As far as the client goes, you can pick whichever constructor you want. By using the parameterized constructors, you can connect right away, whereas the default requires a call to one of the overloads of Connect. It mostly depends on if you actually want to connect at instantiation time.
See MSDN for the overloads you can use.

Here is a TCP server sample code:
int port = 135; //specify port number.
TcpListener listener = new TcpListener(IPAddress.Any, port);
// Start listening for client requests
listen.Start();
And then you can select to use polling method to create a simple sync TCP server.
The client (PowerShell):
$Address = [System.Net.Dns]::GetHostAddresses("<hostName>")
$Port = 135
$client = New-Object System.Net.Sorckets.TcpClient
$Client.Connect($Address, $Port)

UDP networking with multiple network

I'm currently developing a network application on my machine with 2 network interfaces with the following setup -
I want to send UDP message by using the first configuration with IP 192.168.1.2.
using (var udpClient = new UdpClient(5556))
{
udpClient.Connect(IPAddress.Parse("192.168.1.2"), 5556);
// DO STUFF
}
When I try this I get the following error -
No connection could be made because the target machine actively
refused it
Strange this is that when I disable my other network that this works perfectly but with 2 connections (LAN & WiFi) it doesn't work anymore...
I guess that it is sending on wrong adapter?
Is this because my Default gateway is the same or what am I doing wrong?
I'm new to developing network-based applications...

You have to specify an IPEndPoint with networking card IP like this:
var endpoint = new IPEndPoint(IPAddress.Parse("192.168.1.2"), portNum);
UdpClient client = new UdpClient(endpoint);

You are not telling UdpClient which IP to use.
UdpClient has a constructor that can take an IPEndPoint.
const string ip = "192.168.1.2";
const int port = 5556;
var listenEndpoint = new IPEndPoint(IPAddress.Parse(ip), port);
var udpClient = new UdpClient(listenEndpoint);

I'm not that good in C# but I think I understand the problem. You are binding your udpClient to port 5556 but you're not specifying the IP address. The OS will select the IP address for you. I suppose it could be either 192.168.1.2 or 192.168.1.5, you cannot tell which one.
Then with the same udpClient you are trying to connect to a remote host listening on 192.168.1.2, port 5556. And probably nobody is listening there. This is why you receive the error.
I would do the following (sorry for my bad C# skills):
Have two udpClient's, in one thread or program/process:
var udpClient1 = new UdpClient("192.168.1.2", 5556);
This one will listen on IP addres 192.168.1.2, port 5556.
in second thread or program/process:
var udpClient2 = new UdpClient(); //doesn't matter which (IP, port) the OS will bind it to.
udpClient2.Connect(IPAddress.Parse("192.168.1.2"), 5556); //connect to peer/server listening in (IP=192.168.1.2, port=5556)
You could handle both udpClient1 and udpClient2 in the same thread also.
The important is that if you connect to and (IP, port) there has to be some application/process listening there. Otherwise, the remote host will send an ICMP message indicating the datagram cannot be delivered. The connection is refused.

2 instances of the same program broadcasting via UDPClient

I'm using UdpClient to broadcast my program location so a centralize computer can find it and start working with it.
It works great, until 2 users on the same computer at the same time try to broadcast. Since this is a valid use case (think Windows Terminal Server), I'm trying to figure out the best way to address it:
Here is the code I'm currently trying:
m_UDPClients = new List<UdpClient>();
IPAddress[] localIPs = Dns.GetHostAddresses(Dns.GetHostName());
foreach (IPAddress ip in localIPs)
{
string ipStr = ip.ToString();
if (!ipStr.Contains(".")) // I only want readable addresses
continue;
IPEndPoint ipLocalEndPoint = new IPEndPoint(ip, port);
try
{
UdpClient udpClient = new UdpClient();
udpClient.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
udpClient.Client.Bind(ipLocalEndPoint);
m_UDPClients.Add(udpClient);
}
catch (Exception e)
{
BSSLog.Error(e);
}
}
if there is another instance of the program running already, the bind will throw this exception:
An attempt was made to access a socket in a way forbidden by its access permissions
This broadcast is just the program 'advertising' some details about how to connect to it's services; The broadcast happens every few seconds and it's payload is probably < 1k.
I did try to create and destroy the UDPclients as needed and just accept the collision, but it was ugly.
So...the question is: How do I handle 30 instances of the same program all attempting to advertise themselves via the same UDP port (no listening, just post the payload and move on) on a single computer?
I know I could build a Windows service and play traffic cop, etc... but I'm trying to keep this simple and 'limited user rights' friendly.
Before I run off and attempt to use a predefined system wide named pipe or something, I thought I was ask for help. :)

There isn't anything wrong with the code. In our test environment we had a monitoring program that wasn't opening the port correctly.
Good thing we wasted 2 days chasing a red herring.
thank you everyone.

TcpClient connect fails with IPv6Any

The problem is, the following code works well if IPAddress.Any was given as a parameter, but throws an error if `IPAddress.IPv6Any is used.
I receive error #10057
Socket is not connected.
A request to send or receive data was disallowed because the socket is
not connected and (when sending on a datagram socket using
sendto) no address was supplied. Any other type of operation might
also return this error—for example, setsockopt setting SO_KEEPALIVE if
the connection has been reset.
Why does it fails to work as IPv6Any? I'm pretty sure it's not the firewall, since the port remains the same and it works with IPv4 (and my firewall should pass any requests made by my application).
To short up the code, it's something like this:
The Listener:
listener = new TcpListener(IPAddress.IPv6Any, portNr);
listener.AllowNatTraversal(true);
listener.Start();
listener.BeginAcceptTcpClient(this.AcceptClient, null);
The Client:
client = new TcpClient();
client.NoDelay = true;
try
{
this.client.Connect(ip, port); //ip = "localhost" when connecting as server
}
catch (Exception ex)
{
FileLogger.LogMessage(ex);
Disconnect();
}
I'm trying to set up the "server-side" of the TCP-connection.
What I do is that I start a listener at localhost, and then connect to it as a client (and allow others to join as clients as well).
What I'm trying to achieve with this is direct addressability of this TCP server, following this article: http://blogs.msdn.com/b/ncl/archive/2009/07/27/end-to-end-connectivity-with-nat-traversal-.aspx
The reason I'm doing this is that I want person A to be able to connect to a person B when they both are behind NAT routers.

I know this answer comes a bit late, but I also had this issue and it was client related. The problem is, that your provided code...
client = new TcpClient();
... creates an IPv4-Instance of the TcpClient that is not capable of interpreting IPv6-Addresses. So if you already have the IP address at the moment of initialization, try to initialize your TcpClient like this:
TcpClient client = new TcpClient(ip.AddressFamily);
If the variable ip is a string, you need to convert it to type IPAddress first:
IPAddress iAddr = IPAddress.Parse(ip);
Actually a IPv6-TcpClient seems to be compatible to IPv4-Addresses as well, so you can also initialize your client as follows:
TcpClient client = new TcpClient(AddressFamily.InterNetworkV6)
Whilst the upper suggestions seem to be the cleanest ones, the bottom suggestion seems to be the more universal one. At the end it's up to your preferences.

I solved a similar issue where the following line would only block connections coming from IPv4 addresses:
listener = new TcpListener(IPAddress.IPv6Any, portNr);
It seems the socket when configured to accept IPv6 connections, by default, accepts ONLY IPv6 connections. To fix this problem i had to update my code to this:
listener.Server.SetSocketOption(SocketOptionLevel.IPv6, SocketOptionName.IPv6Only, false);
listener.Start();

When you use TcpClient.Connect(string, int) you make it possible to break because of DNS resolution.
Though Microsoft documents that IPv6 address will be tried first, the resolution may only return IPv4 addresses.
http://msdn.microsoft.com/en-us/library/8s2yca91.aspx
Therefore, can you try to call TcpClient.Connect(IPAddress.IPv6Loopback, port) on client side to test if it works?

IPAddress.Loopback == FAIL
IPAddress.IPv6Loopback == SUCCESS
Perhaps localhost is mapping to the IPv4 Loopback Address in your case?

UdpClient receive on broadcast address

In c# I am using the UdpClient.Receive function:
public void StartUdpListener(Object state)
{
try
{
udpServer = new UdpClient(new IPEndPoint(IPAddress.Broadcast, 1234));
}
catch (SocketException ex)
{
MessageBox.Show(ex.ErrorCode.ToString());
}
IPEndPoint remoteEndPoint = null;
receivedNotification=udpServer.Receive(ref remoteEndPoint);
...
However I am getting a socket exception saying that the address is not available with error code 10049
What do I do to negate this exception?

Here's the jist of some code I am currently using in a production app that works (we've got a bit extra in there to handle the case where the client are server apps are running on a standalone installation). It's job is to receive udp notifications that messages are ready for processing. As mentioned by Adam Alexander your only problem is that you need to use IPAddress.Any, instead of IPAddress.Broadcast. You would only use IPAddress.Broadcast when you wanted to Send a broadcast UDP packet.
Set up the udp client
this.broadcastAddress = new IPEndPoint(IPAddress.Any, 1234);
this.udpClient = new UdpClient();
this.udpClient.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
this.udpClient.ExclusiveAddressUse = false; // only if you want to send/receive on same machine.
And to trigger the start of an async receive using a callback.
this.udpClient.Client.Bind(this.broadcastAddress);
this.udpClient.BeginReceive(new AsyncCallback(this.ReceiveCallback), null);
Hopefully this helps, you should be able to adapt it to working synchronously without too much issue. Very similar to what you are doing. If you're still getting the error after this then something else must be using the port that you are trying to listen on.
So, to clarify.
IPAddress.Any = Used to receive. I want to listen for a packet arriving on any IP Address.
IPAddress.Broadcast = Used to send. I want to send a packet to anyone who is listening.

for your purposes I believe you will want to use IPAddress.Any instead of IPAddress.Broadcast. Hope this helps!

That error means the protocol cant bind to the selected IP/port combination.
I havent used UDP broadcast in ages, but I do recall you need to use different IP ranges.

There's nothing wrong with the way you have configured your UdpClient. Have you tried a different port number? Perhaps 1234 is already in use on your system by a different app.