background

After the hardware engineer completed the PCB and schematic design, the routine was modified according to the tutorial of Zhengdian Atom to verify the correctness of the hardware. 
STM32F407 and LAN8720 debugging records . 
Zhengdian Atom network communication experiment

The next step is to implement the function on the product board. 
The original product uses the W5500 hard TCP/IP protocol solution. 
The debugging route attempts to start with UDP loopback.

In the introduction of ICMP protocol 
, you can see that the protocol points out that the error type belongs to port unreachable. 
I tested it with other programs. Sure enough, when the IP of the target device is correct and there is no corresponding port on the target device, it will reply to the ICMP message. 
It can be seen that the problem lies in the port initialization.

3. Factory routine test

I had no choice but to compare it with the original routine of Atom. It can send messages and receive messages normally. 
During the test, I accidentally intercepted such a message: 
 
It can be observed that only when the source address and the destination address are both 8089, ICMP will not return an error message. 
When either of the two is not 8089, an error will be reported. 
Going back to check the code carefully, there are indeed two places where UDP_DEMO_PORT is used. 
They are

err = udp_connect(udppcb, &rmtipaddr, UDP_DEMO_PORT);

and

err = udp_bind(udppcb, IP_ADDR_ANY, UDP_DEMO_PORT);

One is bound to the local port and IP, and the other is bound to the remote device's IP and port. 

The udp_connect function has not been used before. The LWIP manual describes this function as follows: 

LwIP Application Developers Manual

Connected pcbs only receive data from the connected remote address, while unconected pcbs receive datagrams from any remote address.

Therefore, the UDP port connected will only receive packets with the bound source address and source port. 
Try to comment out the udp_connect function, and all packets can be received successfully.