FS#3749 - busybox-ntp failing on specific ISP but ntpd works.

[openwrt-bot]

barhom:

• Device: glinet B1300 router
• Running 21.02

Attached are two pcap files you can open in wireshark.

This issue seems to only affect one Danish ISP, however I think it is worth reporting to see if it can be fixed.
The built in busybox-ntpd does not work (see packet in sysntpd.pcap)

However, if I install ntpdate instead then ntp is able to update (using the same ntp server).
You can see difference in the packets by comparing the two pcap files.

My best bet is that the ISP is blocking specific data in NTPv4 Packets that busybox-ntp is sending.