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Connect to a remote endpoint using UDP Socket in Xamarin (C#) only works when activating a VPN

I have a UDP Socket connection working between devices with local IPs (192.168.x.x), that I want to switch to a connection with an external server, with the following configuration:
Create socket
private Socket _socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
private const int bufSize = 4096 * 4;
private State state = new State();
private EndPoint epFrom = new IPEndPoint(IPAddress.Any, 0);
private AsyncCallback recv = null;
private static int PORT = 27000;
Openning device as a server to listen through the socket
_socket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.ReuseAddress, true);
_socket.Bind(new IPEndPoint(IPAddress.Any, port));
Receive(); // Function to receive datagrams through the socket, irrelevant to this issue
Connect socket to remote endpoint
// {address:port} of an external server
_socket.Connect(new IPEndPoint(IPAddress.Parse(address), port));
Send data to socket
_socket.BeginSend(data, 0, data.Length, SocketFlags.None, (ar) =>
{
try
{
_socket.EndSend(ar);
}
catch (Exception ex)
{
Console.WriteLine("[UDP] Error Send: " + ex.ToString());
}
}, state);
The code above works fine for local IP addresses, the problem came when trying to use as a remote ip endpoint a public IP of a remote server.
The datagram seemed to be sent but it never arrived to the server. I tried to connect the phone to a VPN and the local IP changed from a 192.168.x.x to a 10.0.x.x, and then the connection to the external server worked.
Therefore, connection with an external IP only worked when the device's IP was a private IP of class A, but it didn't work with a class C range (https://www.meridianoutpost.com/resources/articles/IP-classes.php).
I want to find a solution to be able to connect to external IP address without having to connect a third party VPN software.

If someone has the same question, I have already solved it.
To access external IPs with local class C IPs and no VPN you should do UDP hole punching instead of trying to establish a connection.
Erase the .Connect() and the .Bind() methods, and, instead of using BeginSend/EndSend, use the SendTo or SendToAsync methods with the server's public IP as the endpoint destination.
Additionally, I recommend changing:
SocketOptionName.ReuseAddress
to:
SocketOptionName.PacketInformation

C# .Net 10054 UDP Exception? Cause, fix? [duplicate]

I need to obtain UDP datagram from Asynchronous Socket Server but an exception occurred in my application :
Problem appear there :
Byte[] receiveBytes = udpClient.Receive(ref RemoteIpEndPoint);
The full source code:
class Program
{
static void Main(string[] args)
{
const int PORT = 30485;
IPAddress IP;
IPAddress.TryParse("92.56.23.87", out IP);
// This constructor arbitrarily assigns the local port number.
UdpClient udpClient = new UdpClient(PORT);
Socket receiveSocket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
try
{
udpClient.Connect("92.56.23.87", PORT);
if (udpClient.Client.Connected)
Console.WriteLine("Connected.");
// Sends a message to the host to which you have connected.
Byte[] sendBytes = Encoding.ASCII.GetBytes("CONNECT");
udpClient.Send(sendBytes, sendBytes.Length);
//IPEndPoint object will allow us to read datagrams sent from any source.
IPEndPoint RemoteIpEndPoint = new IPEndPoint(IP, PORT);
// Blocks until a message returns on this socket from a remote host.
Byte[] receiveBytes = udpClient.Receive(ref RemoteIpEndPoint);
string returnData = Encoding.ASCII.GetString(receiveBytes);
// Uses the IPEndPoint object to determine which of these two hosts responded.
Console.WriteLine("This is the message you received " + returnData.ToString());
Console.WriteLine("This message was sent from " + RemoteIpEndPoint.Address.ToString() + " on their port number " + RemoteIpEndPoint.Port.ToString());
udpClient.Close();
}
catch (Exception e)
{
Console.WriteLine(e.ToString());
}
}
}
Exception:
Connected.
System.Net.Sockets.SocketException (0x80004005): An existing connection
was forcibly closed by the remote host at System.Net.Sockets.Socket.ReceiveFrom(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags, EndPoint& remoteEP) at ystem.Net.Sockets.UdpClient.Receive(IPEndPoint& remoteEP) at ConsoleApplication7.Program.Main(String[] args) in c:\users\user\documents\visual studio 2010\Projects\ConsoleApplication7\ConsoleApplication7\Program.cs
What can be the problem?
To provide more information, i bought the private socks connection on this page: http://rapidsocks.com/
this services give me a list of IP and port who in really is not a proxy .. just a connection that give me a proxyIP:proxyPort from a pool on server in response...
How to get that answer with proxyIP:proxyPort from the server?

In UDP land, one way this can occur is when you send a UDP packet to a host, and the remote host doesn't have a listener on that port, and bounces an ICMP host unreachable message in response.
In plain English, what this exception tells you that no process is listening on the far-end on that port.
Update: You should be able to avoid that behavior with the following code:
var udpClient = new UdpClient();
uint IOC_IN = 0x80000000;
uint IOC_VENDOR = 0x18000000;
uint SIO_UDP_CONNRESET = IOC_IN | IOC_VENDOR | 12;
udpClient.Client.IOControl((int)SIO_UDP_CONNRESET, new byte[] { Convert.ToByte(false) }, null);
Microsoft Article 263823 said this on the subject: [hard to find as of 2019]
SYMPTOMS
In Windows 2000, a User Datagram Protocol (UDP) program may
not work and may generate a WSAECONNRESET response.
CAUSE
If sending a datagram using the sendto function results in an
"ICMP port unreachable" response and the select function is set for
readfds, the program returns 1 and the subsequent call to the recvfrom
function does not work with a WSAECONNRESET (10054) error response. In
Microsoft Windows NT 4.0, this situation causes the select function to
block or time out.
RESOLUTION
A new sockets IOCTL called "SIO_UDP_CONNRESET" has been
introduced in Windows 2000. When this IOCTL is used, the program must
be rewritten specifically for Windows 2000 to obtain the original
Windows NT 4.0 behavior. Windows NT 4.0, Microsoft Windows 95, and
Microsoft Windows 98 have no support for this new IOCTL. In addition
to rewriting your application, you will need the hotfix referenced
further down in this article.

This really is a generic error message that could mean anything. Time to get the low level network traffic sniffers to filter what is actually going wrong. Adding extra error handling try catch blocks on the server with decent logging is always a great place to start.

.NET UDP Data Reception

I have a simple UDP listener that I am trying to collect datagrams from. My datagrams can be in one of two data formats. With the first data format, I am receiving data in my program as expected. With the second, there is absolutely no indication that data is ever received from my program, even though I can verify that the UDP data is passing onto the the network interface via Wireshark. I thought that maybe these were malformed UDP packets that Windows was rejecting but Wireshark does label them as UDP. My code is below:
static void Main(string[] args)
{
Thread thdUdpServer = new Thread(new ThreadStart(serverThread));
thdUdpServer.Start();
}
static void serverThread()
{
Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
socket.Bind(new IPEndPoint(new IPAddress(0), 2000));
while (true)
{
byte[] responseData = new byte[128];
socket.Receive(responseData);
string returnData = Encoding.ASCII.GetString(responseData);
Console.WriteLine(DateTime.Now + " " + returnData);
}
The missing packets are all 29 byte datagrams that look something like this (translated to ASCII).
#01RdFFFF...?...... ........F
Why would Wireshark indicate their presence but .NET not seem to see them?

If the bytes contain non-printable ASCII characters, you may not see them on the Console.

There's something missing in your code. It should be throwing a socket exception when calling ReceiveFrom (at least according to MSDN, haven't tried your code.)
You should bind your socket to the address:port you want to listen on (or use 0.0.0.0 as the address to listen on any local address):
socket.Bind(new IPEndPoint(new IPAddress(0), 2000);
The EndPoint in ReceiveFrom is not the listening port for the server. It's the address you want to receive packets from. You can use an Endpoint of 0.0.0.0:0 to receive from any host.
After returning from the method the Endpoint will be filled with the address of the host that sent the packet (client).
You can use Receive instead of ReceiveFrom if you don't care about the client end point.
Likely your client is not sending packets from 192.168.1.100:2000, and that is why you are not receiving them; thought why you're not getting an exception when calling ReceiveFrom is beyond me.
Also:
There is no need to call Convert.ToInt32 in:
new IPEndPoint(IPAddress.Parse("192.168.1.100"), Convert.ToInt32(2000));
2000 is already an int.

Packet Data Sniffing from ASP.NET

I would like to sniff packet data using C#
I need it to sniff all types of data not only HTTP
When I try to do it I get this error:
System.Net.Sockets.SocketException: An attempt was made to access a socket in a way forbidden by its access permissions at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress) at System.Net.Sockets.Socket.Bind(EndPoint localEP)
public void StartCapture(string InterfaceIp)
{
try
{
//Start capturing the packets...
//For sniffing the socket to capture the packets has to be a raw socket, with the
//address family being of type internetwork, and protocol being IP
mainSocket = new System.Net.Sockets.Socket(System.Net.Sockets.AddressFamily.InterNetwork,
System.Net.Sockets.SocketType.Raw, System.Net.Sockets.ProtocolType.IP);
//Bind the socket to the selected IP address
mainSocket.Bind(new System.Net.IPEndPoint(System.Net.IPAddress.Parse(InterfaceIp), 0));
//Set the socket options
mainSocket.SetSocketOption(System.Net.Sockets.SocketOptionLevel.IP, //Applies only to IP packets
System.Net.Sockets.SocketOptionName.HeaderIncluded, //Set the include the header
true); //option to true
byte[] byTrue = new byte[4] { 1, 0, 0, 0 };
byte[] byOut = new byte[4] { 1, 0, 0, 0 }; //Capture outgoing packets
//Socket.IOControl is analogous to the WSAIoctl method of Winsock 2
mainSocket.IOControl(System.Net.Sockets.IOControlCode.ReceiveAll, //Equivalent to SIO_RCVALL constant
//of Winsock 2
byTrue,
byOut);
//Start receiving the packets asynchronously
mainSocket.BeginReceive(byteData, 0, byteData.Length, System.Net.Sockets.SocketFlags.None,
new AsyncCallback(OnReceive), null);
}
catch (Exception ex)
{
//Report Exception
}
}
Note:I am using ASP.Net & it is really important to sniff the data and not only the headers of the packet
I think the problem is related to not having the right privileges to access to socket, although using impersonation didn't solve the problem
I found the solution to my problem, it's related to a recent implementation that limits binding to a socket that is protected and limited to a single ip address
Here are some interesting links
http://msdn.microsoft.com/en-us/library/cc150667(v=vs.85).aspx
http://msdn.microsoft.com/en-us/library/ms740668%28v=VS.85%29.aspx

If you are OK with using a library, SharpPcap is a nice one and it's very simple to use.

I'm experimenting with raw sockets.
I fixed this error,
System.Net.Sockets.SocketException: 'An attempt was made to access a
socket in a way forbidden by its access permissions'
simply by running visual studio in administrator mode.

How do I implement Traceroute using UDP?

Apparently ICMP isn't the only way to create a Traceroute. This and this answer indicates it's possible to send a UDP packet (or any other) with a low TTL and wait for the ICMP message.
How would I go about implementing this in C#? System.IO.Sockets? The TCP objects? Anyone know of an easy/best way?
Update 1:
The following code seems to correctly throw an exception when the TTL is hit. How do I extract information from the returned UDP Packet?
How do I know that the UDP packet I'm receiving is intended for me (and not some other application on my host?)
public void PingUDPAsync(IPAddress _destination, short ttl)
{
// This constructor arbitrarily assigns the local port number.
UdpClient udpClient = new UdpClient(21000);
udpClient.Ttl = ttl;
// udpClient.DontFragment = true;
try
{
udpClient.Connect(_destination, 21000);
// Sends a message to the host to which you have connected.
Byte[] sendBytes = Encoding.ASCII.GetBytes("Is anybody there?");
udpClient.Send(sendBytes, sendBytes.Length);
//IPEndPoint object will allow us to read datagrams sent from any source.
IPEndPoint RemoteIpEndPoint = new IPEndPoint(IPAddress.Any, 0);
// Blocks until a message returns on this socket from a remote host.
Byte[] receiveBytes = udpClient.Receive(ref RemoteIpEndPoint);
string returnData = Encoding.ASCII.GetString(receiveBytes);
// Uses the IPEndPoint object to determine which of these two hosts responded.
Console.WriteLine("This is the message you received " +
returnData.ToString());
Console.WriteLine("This message was sent from " +
RemoteIpEndPoint.Address.ToString() +
" on their port number " +
RemoteIpEndPoint.Port.ToString());
udpClient.Close();
}
catch (SocketException socketException)
{
Console.WriteLine(socketException.ToString());
}
catch (Exception e)
{
Console.WriteLine(e.ToString());
}
}

Yes, System.Net.Sockets should provide you all the primitive objects you would need to send/receive UDP/TCP packets. Plenty of documentation and samples online, the two articles you included in your question are very interesting and a good starting point :)

https://learningnetwork.cisco.com/thread/87497
You can check out the answer here that goes into detail on Cisco's UPD traceroute implementation. It is rather comprehensive and can easily adapted to target a specific UDP port. You do not get a UDP packet back from the target. Rather, you get an ICMP reply to indicate the traffic was not received. The UDP packet that you originate has a random response port number included and your host tracks what ports are used by what applications. When the ICMP response is sent back, it is sent to the host IP and the response port included in the UDP header. Your host will then see the port and know it is bound to your application. It then delivers the packet to your application.