C# How to send UDP packets between 2 computer with dedicated NICs (no router)

Question

C# How to send UDP packets between 2 computer with dedicated NICs (no router)

Steve Cobb 0

I have 2 computers connected via built in NICs to a router connected to the web. Each computer has a second dedicated NIC on a different sub-net - connected directly to each other (no router). Device A (Sender) IPv4 Address. . . . . . . . . . . : 192.168.1.21 built in NIC - connected to internet IPv4 Address. . . . . . . . . . . : 192.168.0.121 SFP NIC card connected directly to device B Device B (Receiver) IPv4 Address. . . . . . . . . . . : 192.168.1.173 built in NIC - connected to internet IPv4 Address. . . . . . . . . . . : 192.168.0.122 SFP NIC card connected directly to device A

I have a sending app on Device A and receiving app on Device B. I can ping from Device A to B just fine using either 192.168.1.173 or 192.168.0.122 - so the connection is fine.

// Sending side
Socket socket = 
byte
IPAddress receiver_address = 
IPEndPoint receiver = 
int
IPEndPoint local = (IPEndPoint)
Console

// Receiving side
Socket socket = 
IPEndPoint receiver = 
socket

byte
IPEndPoint peer = 
EndPoint peer_ref = (EndPoint)

int
string
peer
IPEndPoint local = (IPEndPoint)
Console

So, Running the sender code to 192.168.1.173 works fine. Ping works both ways between 192.168.0.121 and 192.168.0.122. So the connection is good. However, Running the sender code to 192.168.0.122 does not get received by the receiver program. Wireshark does show the packets arriving at 192.168.0.122 - so the packets are getting there. Why does the receiver not see the packets when sent from 192.168.0.121 to 192.168.0.122? It seems the sender is set correctly as the path works and WireShark sees the packets. It seems the receiver is configured incorrectly or looking only at the wrong interface. I’m sure it is something simple. Do I need to bind the receiver to the sub-net or NIC interface?

Steve Cobb 0 Reputation points

2024-02-18T23:04:00.91+00:00
It looks like the code got mangled - as well as the initial text. Will try again Device A (Sender) IPv4 Address. . . . . . . . . . . : 192.168.1.21 built in NIC - connected to internet IPv4 Address. . . . . . . . . . . : 192.168.0.121 SFP NIC card connected directly to device B Device B (Receiver) IPv4 Address. . . . . . . . . . . : 192.168.1.173 built in NIC - connected to internet IPv4 Address. . . . . . . . . . . : 192.168.0.122 SFP NIC card connected directly to device A

I have a sending app on Device A and receiving app on Device B. I can ping from Device A to B just fine using either 192.168.1.173 or 192.168.0.122 - so the connection is fine.

// Sending side Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp); byte[] buffer = Encoding.ASCII.GetBytes(msg); IPAddress receiver_address = IPAddress.Parse(RECEIVER_ADDRESS); // 192.168.1.173 works - 192.168.0.122 doesn’t IPEndPoint receiver = new IPEndPoint(receiver_address, 8288); int sent_length = socket.SendTo(buffer, receiver); IPEndPoint local = (IPEndPoint)socket.LocalEndPoint; Console.WriteLine("- MSG {0}B of {1}B sent to {2}:{3} on port {4}", sent_length, msg.Length, receiver_address, 8288, local.Port);

// Receiving side Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp); IPEndPoint receiver = new IPEndPoint(IPAddress.Any, 8288); socket.Bind(receiver); byte[] buffer = new byte[BUFFER_SIZE]; IPEndPoint peer = new IPEndPoint(IPAddress.Any, 0); EndPoint peer_ref = (EndPoint)peer; int received_length = socket.ReceiveFrom(buffer, ref peer_ref); string msg = Encoding.ASCII.GetString(buffer, 0, received_length); peer = (IPEndPoint)peer_ref; IPEndPoint local = (IPEndPoint)socket.LocalEndPoint; Console.WriteLine("- MSG {0}B of {1}B from {2}:{3} on port {4}", msg.Length, received_length, peer.Address, peer.Port, local.Port);

So, Running the sender to 192.168.1.173 works fine. Ping works both ways between 192.168.0.121 and 192.168.0.122. So the connection is good. However, Running the sender to 192.168.0.122 does not get received by the receiver program. Wireshark does show the packets arriving at 192.168.0.122 - so the packets are getting there. Why does the receiver not see the packets when sent from 192.168.0.121 to 192.168.0.122? It seems the sender is set correctly as the path works and WireShark sees the packets. It seems the receiver is configured incorrectly or looking only at the wrong interface. I’m sure it is something simple.

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Answer 1

Hi @Steve Cobb ,

Here are some suggestions for checks that may be useful.

Check Receiving Application Configuration: Revisit the configuration of the receiving application on Device B. Ensure that it is listening on the correct IP address (192.168.0.122) and port number. Verify that there are no restrictions or filters in the application code that may prevent it from processing incoming packets.
Verify Packet Contents: Use Wireshark to inspect the contents of the received packets. Ensure that the packets contain the expected data and are formatted correctly. If there are any discrepancies, it could indicate issues with the sending application or the packet content itself.
Network Interface Configuration: Double-check the network interface configuration on Device B. Ensure that the network interface corresponding to 192.168.0.122 is properly configured and active. Verify that there are no conflicting network settings or issues with the network adapter driver.
Firewall and Security Software: Review the firewall and security software settings on Device B. Ensure that they are not blocking or interfering with the incoming UDP traffic on port 192.168.0.122. Temporarily disable any firewall or security software to see if it resolves the issue.
Debugging the Receiving Application: Add debugging statements or logging to the receiving application code to trace the flow of incoming packets and identify any errors or issues in the processing logic. This can help pinpoint where the problem is occurring within the application.

Best Regards.

Jiachen Li

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Steve Cobb 0 Reputation points

2024-03-02T19:11:49.99+00:00

Thank you for the response @Jiachen Li-MSFT I'm currently looking at testing your suggestions. Overall, the software and Ports aren't blocked - as I can send packets just fine - as long as I use the addresses of the built-in NIC with the router. And in both cases, I'm using the same port (8288). Also Wire Shark does see the packets come in when sent via the SFP NIC. I'll let you know what I find. Thanks again, Steve
Jiachen Li-MSFT 34,221 Reputation points Microsoft External Staff

2024-03-06T08:35:25.4733333+00:00

Remember to bind the listener to a specific receiving IP address and port to avoid messing up or having problems with packet processing.

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C# How to send UDP packets between 2 computer with dedicated NICs (no router)

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