I'm creating an advertisement display, based on RaspberryPi (no real time clock present) and piCore, which is busybox-based tinycorelinux. 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 ntpd(8), "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. Any simpler ways?
  • That's an odd security policy. Outgoing packets even within subnet are bad, but accepting any garbage from the same subnet is okay? – grawity Mar 7 '18 at 17:23
  • @grawity, that's not really a policy. All devices in the segement trust each other. However, at development time, nothing is known about the deployment environment and I do not want that users have to configure anything on RasPi. They will just add RasPis to listeners list on the host which sends pictures. – Xpector Mar 8 '18 at 9:49
  • Perhaps multicast-NTP would be suitable then. – grawity Mar 8 '18 at 10:02

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