Netcat作为UDP客户端不接收来自Arduino Wifi Shield的响应数据包。 简单的C UDP客户端。 为什么？

Question

I am testing out the WiFi UDP Send and Receive String example Arduino code with the Arduino Wifi Shield (newest firmware update Mar. 2013), and I'm getting behaviour I cannot explain.

Expected Operation

The Arduino receives UDP packets on its own local arbitrary port from another computer on the router - remoteIP:remotePort. It acknowledges to the client that it has received the packet by sending a UDP packet of its own back to remoteIP:remotePort.

I thought I would test by using netcat. netcat will send the UDP packets and Arduino receives, but netcat is not receiving the packets sent by Arduino. The blame lies in netcat, because I used a Simple C UDP Client and that client WOULD receive the Arduino server UDP responses.

Does anyone know what extra cruft is in Netcat that would be causing this? I've noticed similar issues when using netcat on TCP to interact with servers using unknown protocols. The interaction was not "raw," things were not showing up that seemingly should be, like these UDP packets from the UDP server. Is it something about the makeup of the UDP packets that does not satisfy netcat?

Arduino UDP Server Code

Simple C UDP Client Code

Answer 1

The main difference between your code and nc is, that you use an unconnected socket and specify the recipient within the sendto call, while nc will use a connected socket. This means, that your socket will accept replies from any address, while nc will only accept replies from the same IP and port where it send the request to. I don't know about the Arduino udp library you use, but from a short look at the code I don't see anything like using a udp connection, so I assume that my theory is correct. If you use tcpdump/wireshark this is what I expect you to see with your client at IP cip and port cport and Arduino at IP cip and port 32000 (matching your example C-code):

--> request from cip:cport to   aip:32000
<-- reply to     cip:cport from aip:random_port

But what you need if you want it to work with nc is

--> request from cip:cport to   aip:32000
<-- reply to     cip:cport from aip:32000