clock

Clock of a Linux host, by SNMP

Returns the offset of the host's clock from a time server, with and without its sign, in seconds. One watch is offered: the clock drift.

How it works

The collector reads the system time, hrSystemDate (HOST-RESOURCES-MIB), by SNMP, and asks a time server for its time by NTP (pool.ntp.org when the "Time server" variable is unset).

• The server's time is brought back to the middle of the SNMP read: neither the SNMP round trip, nor the time between the two reads, nor the error of the poller's own clock counts in the offset.
• A positive offset means the host's clock is ahead; negative, behind.
• An agent that serves the date without its offset from UTC does not say which instant it is: the collector then stops as misconfigured rather than guess. net-snmp serves the offset.
• A time server that does not answer stops the collector as unreachable.

Prerequisites

• snmpd (net-snmp) running on the host, listening on an address the poller reaches (UDP 161), with the firewall open to the poller only.
• The view granted to the poller includes hrSystem (.1.3.6.1.2.1.25.1), which most default configurations already allow.
• Read-only access, SNMPv2c or SNMPv3, set on the host or one of its folders (protocol:snmp:* variables).
• The poller reaches the time server on UDP 123 (protocol:ntp:server, protocol:ntp:timeout).

# /etc/snmp/snmpd.conf
agentaddress udp:161
view   banshee included .1.3.6.1.2.1.25.1
rocommunity <community> 192.0.2.10 -V banshee
# or: rouser <user> priv -V banshee

Outputs

Watches

Limits

• hrSystemDate is only precise to a tenth of a second: an offset below that is noise.
• The collector measures the offset; it does not say whether the host synchronises its clock (chrony, systemd-timesyncd).