ESP32 cannot send and receive UDP packets to itself

It seems you can a UDP packet on a PC to itself, but you can't send a UDP packet from an ESP32 to itself:

import network, socket

# Create access point for the phone to log onto
si = network.WLAN(network.AP_IF) 
si.active(True)       
print("Wifi is: ESP_%X%X%X" % tuple(si.config("mac")[-3:]))
print(si.ifconfig())

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.settimeout(3)
port = 9099
ipnum = "192.168.4.1"
s.bind((ipnum, port))   

s.sendto(b"An ESP32 UDP Packet", (ipnum, port))
print(s.recvfrom(100))  # <--- this times out

response is:

Wifi is: ESP_8C1FB2
('192.168.4.1', '255.255.255.0', '192.168.4.1', '0.0.0.0')

Traceback (most recent call last):
File "", line 16, in
OSError: [Errno 110] ETIMEDOUT

However, when I connect my PC to the ESP_8C1FB2 hotspot then I can run the following:

import socket

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.settimeout(3)
port = 9099
ipnum = "192.168.4.2"
s.bind((ipnum, port))   

s.sendto(b"A PC UDP Packet", (ipnum, port))
print(s.recvfrom(100))

response is:

(b'A PC UDP Packet', ('192.168.4.2', 9099))

I am able to send and receive UDP packets between the PC and the ESP32

• PC: s.sendto(b"A PC UDP Packet", ("192.168.4.1", port)); print(s.recvfrom(100)))
• ESP32: s.sendto(b"An ESP UDP Packet", ("192.168.4.2", port)); print(s.recvfrom(100)))

The socket.bind function seems to have to be to the device's own IPnumber. Fortunately this is quite predictable when working from the ESP's hotspot.

The report provides a reproducible ESP32 access-point script using network.WLAN and UDP socket calls, plus PC comparison cases. Start by reproducing the self-send timeout and compare it with the PC and PC-to-ESP cases; done means an ESP32 can send a datagram to its own bound address and receive it.