[chrony-users] Questions about "holdover" state

["Valera Requena, Juan" <jvalera@xxxxxxxx>]

Hello,

I am trying to better understand how chronyd behaves when all reference sources become unavailable, specifically regarding holdover behavior.

By holdover I mean the situation in which the host was synchronized to one or more NTP sources, and then loses connectivity to all reference servers, continuing to discipline or free-run the clock based on its last known state.

I have two related questions:

1. Identification of holdover state
   Is there a way to explicitly identify that chronyd is operating in a holdover state using chronyc or chronyd status information?
   For example:
• Is there a specific flag, status field, or command output that indicates “synchronized but no reachable sources”?
• Should this be inferred indirectly (e.g., from source reachability, Leap status, or Reference ID)?
2. Transition from holdover to unsynchronized
   How does chronyd internally determine when a system, while in holdover, transitions to a non-synchronized state?
• Is this based on a maximum estimated error, time since last valid measurement, root dispersion growth, or another threshold?
• Are there configuration parameters that influence this decision (e.g., limits on acceptable uncertainty or timeout values)?

In short, I would like to understand:

• How to reliably detect that chronyd is in holdover, and
• What criteria chronyd uses to decide that the clock is no longer considered synchronized once all reference sources remain unavailable for some time.

Any clarification or pointers to relevant documentation or source code would be greatly appreciated.

Thank you!

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