How to configure a listening-only NTP client?

[xpector]

I'm creating an advertisement display. It will be behind a firewall, and even within its own network segment it should never make an outgoing connection, but only receive stuff, like a picture to display (via VNC listening client) or current datetime. It is not known in advance, what host(s) will send stuff; luckily, everything from own segment is okay to accept.

So polling pool.ntp.org is obviously not an option. According to man, "ntpd can operate in any of several modes, including symmetric active/passive".

• Is it possible to start ntpd in way that it sends no requests but accepts (S)NTP packets?
• If not ntpd, anything else? (possibly from RaspberryPi / busybox repertoire)
• Hosts sending stuff to my RaspberryPi are Windows10 PCs. I cannot install programms there, but only run portable stuff. My plan is to write a script to write the (S)NTP packet content into a file and then send it with packetsender.com. Any simpler ways?

Disclaimer: this is a cross-post from superuser.com

[xpector]

A bit dirty and not that quick at all. Anyway:
A peer PC calls wget and includes the timestamp into the URI, like this: http://myraspi:8888/201803201234.56
On RasPi, netcat listens port 8888, greps requests for timestamps and pipes those to xargs date --set

Inspired by https://jonlabelle.com/snippets/view/shell/netcat-commands