Using local port of 123 for UDP listening

[JGifford]

Using Ethernet.h and NTP.h with an Arduino Uno R3 and an Ethernet Shield R3.

NTP client won't receive responses from remote NTP server when local/source UDP port is 123.

Changing line 54 to use a different local/source UDP port allows it to function.

NTP/NTP.cpp

Line 54 in 564a820

udp->begin(NTP_PORT);

Original, doesn't work with my hardware configuration:

	udp->begin(NTP_PORT);

Changed, works with my hardware configuration:

	udp->begin(8888);

Could we possibly create a mechanism to allow local modification of this local/source UDP port?

Example sketches available upon request.

[JGifford]

As long as I start UDP with "random" (non-port 123) port after starting NTP, it works.
NTPinstance.begin();
Udp.begin(8888);

Would be nice to specify the local/source UDP port

[sstaub]

The port number is fixed by design, see RFC5906. You can change the NTP port also in the ntp.h line 41.

[JGifford]

Thanks for setting me straight about source and destination ports for NTP. I figured the source (client) port would be a random one but you're right--they're both supposed to be 123.

I'm starting to think that my issue is with Ethernet's UDP implementation. I can't seem to communicate to a remote NTP server unless I do something like this:
NTPclient:8888-->NTPserver:123
by putting the following code in my setup()
UDPinstance.begin(8888);