Document existing NTP monitoring programs in nagios/icinga2, and pool

Users will want to monitor their ntpd servers. There are already many good solutions, so ntpsec just needs to document, not reinvent.

Nagios has these two:

spidey ntpsec # /usr/lib64/nagios/plugins/check_ntp_peer

Usage: check_ntp_peer -H [-4|-6] [-w ] [-c ] [-W ] [-C ] [-j ] [-k ] [-v verbose]

spidey ntpsec # /usr/lib64/nagios/plugins/check_ntp_time

Usage: check_ntp_time -H [-4|-6] [-w ] [-c ] [-v verbose] [-o ] /usr/lib64/nagios/plugins/check_ntp_time

I know the pool also has some tools.