The vast majority of my S9s stopped hashing overnight.  They all seemed ok, green lights flashing, hash board LEDs lit red, all communicating with the network just fine.  The temps were all ok.  When I go back and check my error logs, the ones that stopped hashing, all stopped at the same moment.  Whatever hit them, hit them simultaneously.  Restarting the stopped ones corrected the problem.  At this time, everything is operating normally.  Most of them did have the "127.0.0.1 auth.minerlink.com" entry in their /etc/hosts file, so I don't think it was that issue either.  I would sure like to know what caused this.