Q:

ESP8266, NodeMCU, soft AP - UDP server-like soft AP, independent access point

I am using NodeMCU (with ESP8266-E) with an upgraded firmware. All basic commands work perfectly but there is one problem.
I wanted to create an independent access point, which could have a behaviour like a UDP server. That means without direct connection to any other access points. A simple UDP server like soft AP.
I followed these steps:

I have uploaded a new firmware to NodeMCU.
I have downloaded ESPlorer for better work with NodeMCU.
I have uploaded the source code below.
I have connected to the NodeMCU access point on my desktop.
I have sent some strings to the NodeMCU using a Java UDP client program.
I have looked at the messages on ESPlorer.
NodeMCU has not received any such strings.

-- 
print("ESP8266 Server")
wifi.setmode(wifi.STATIONAP);
wifi.ap.config({ssid="test",pwd="12345678"});
print("Server IP Address:",wifi.ap.getip())

-- 30s timeout for an inactive client
srv = net.createServer(net.UDP, 30)
-- server listens on 5000, if data received, print data to console
srv:listen(5000, function(sk)
  sk:on("receive", function(sck, data) 
    print("received: " .. data)
  end)
  sk:on("connection", function(s)
    print("connection established")
  end)
end)

When I tried to send a message using a Java application, there was no change in ESPlorer. Not even when I tried to send a message using the Hercules program (great program for TCP, UDP communication).
I guess that maybe it will be the wrong IP address. I am using the IP address of the AP and not the IP address of the station.
In other words I am using this address: wifi.ap.getip() and not this address wifi.sta.getip() for connections to the UDP server. But sta.getip() returns a nil object. Really I don't know.
I will be glad for any advice.
Thank you very much.

A:

Ok, let's restart this since you updated the question. I should have switched on my brain before I gave you the first hints, sorry about this.
UDP is connectionless and, therefore, there's of course no s:on("connection"). As a consequence you can't register your callbacks on a socket but on the server itself. It is in the documentation but it's easy to miss.
This should get you going:
wifi.setmode(wifi.STATIONAP)
wifi.ap.config({ ssid = "test", pwd = "12345678" })
print("Server IP Address:", wifi.ap.getip())

srv = net.createServer(net.UDP)
srv:listen(5000)
srv:on("receive", function(s, data)
    print("received: " .. data)
    s:send("echo: " .. data)
end)

I ran this against a firmware from the dev branch and tested from the command line like so 
$ echo "foo" | nc -w1 -u 192.168.4.1 5000
echo: foo

ESPlorer then also correctly printed "received: foo".

