clock

Clock of a Linux, by SSH

Returns the offset of the machine's clock from a time server, with and without its sign. Positive, the clock is ahead; negative, it is behind.

How it works

The collector runs date +%s on the machine (the seconds since 1 January 1970 UTC), then asks the time server named by protocol:ntp:server (pool.ntp.org when empty) with one NTP client request.

• The server's time is brought back to the middle of the SSH call, measured by the poller's clock: neither the SSH round trip nor the time between the two readings counts in the offset, nor the poller's own clock error.
• date truncates to the second: half a second is added, the middle of the second the true time lies in.
• Portable: date +%s is known by BusyBox and GNU coreutils, and gives UTC seconds with no time zone to guess. Nothing is written on the target.

Prerequisites

• an SSH account for monitoring, without write rights;
• the fingerprint of the server's host key, set in protocol:ssh:fingerprint (not needed when the Security settings trust the first SSH fingerprint presented);
• the poller reaching the time server on UDP port 123.

Outputs

Watches

Limits

• The offset is read to about one second: the SSH call, connection included, is a second long at worst. The default thresholds stay far above it.
• A time server that does not answer stops the collector unreachable.