I sat down scratching my head… that ntp client was syncing perfectly in unicast, and didn't create any association once configured in multicast. "Dah, the same old problem", I told to myself, "it's not getting the packets, setting a multicast route will fix it".

So I prepared the usual debugging set: one window running tcpdump 'dst port ntp', one window on the client running watch ntpq -c pe -c as, another one with tail -f /var/log/syslog | grep ntp and a free shell window. To my surprise, as soon as I fired up tcpdump, multicast ntp packets showed up. "What the…?!" I said. … Continue reading →