# Chrony Knowledge Patch > chrony - Skill: `nevaberry/chrony-knowledge-patch` (Agent Skill, multi-file: 7 files) - Install (CLI): `npx skillmds@latest add nevaberry/chrony-knowledge-patch` - Raw SKILL.md: https://api.skillmd.com/api/skills/nevaberry/chrony-knowledge-patch/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: MIT - Author: nevaberry (https://skillmd.com/u/nevaberry) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/nevaberry/chrony-knowledge-patch --- # chrony 4.7–4.8 Knowledge Patch ## Reference index | Reference | Topics | | --- | --- | | [core-configuration.md](references/core-configuration.md) | Upgrade requirements, daemon and client behavior, source options, packet controls, logging, service integration | | [reference-clocks.md](references/reference-clocks.md) | RTC, PHC, PPS, GPS, gpsd, reachability, locking, startup, and time scales | | [hardware-timestamping-and-ptp.md](references/hardware-timestamping-and-ptp.md) | Hardware timestamping, interleaved NTP, NTP-over-PTP, transparent clocks, PHCs, and virtualization | | [network-time-security.md](references/network-time-security.md) | NTS client state, authentication diagnostics, server setup, policy compatibility, and verification | | [monitoring-and-troubleshooting.md](references/monitoring-and-troubleshooting.md) | `chronyc`, exporter metrics, source selection, transport errors, client records, and network recovery | ## Using this patch - Check the upgrade notes before carrying an older configuration or build setup forward. - Use the quick references below for common decisions, then load the indexed topic file for constraints and edge cases. - Treat examples as fragments: retain the access controls, source diversity, measured offsets, and platform-specific paths required by the deployment. ## Breaking changes and upgrade checks ### Configuration and build compatibility - Since 4.7, integer configuration values receive sanity checks. Values that were accepted despite being nonsensical can now fail validation. - NTS builds require Nettle 3.6 or newer and GnuTLS 3.6.14 or newer. - Building without POSIX threads is no longer supported. - On RHEL, the FIPS and OSPP profiles are incompatible with NTS and can make an NTS-configured daemon exit fatally. A service-only exception can be set in `/etc/sysconfig/chronyd`: ```sh GNUTLS_FORCE_FIPS_MODE=0 ``` ### Security and compatibility behavior - In 4.8, root-run `chronyc -u` drops privileges to the configured client user. - The `chronyc` socket is hidden to prevent unsafe permission changes against it. - Keep both endpoints of an NTP-over-PTP connection on the same chrony version. - Do not discipline the physical PHC used for hardware-timestamped NTP-over-PTP; use a virtual clock if PTP must share the interface. ## Core 4.7–4.8 configuration ### Source retention and identity The `maxunreach` source option, added in batch `4.8-configuration`, limits the number of polls for which an unreachable NTP source or refclock can remain selected. It defaults to 100000, and retention also requires a sample newer than the oldest sample from any reachable source. ```conf server ntp.example.net maxunreach 8 refclock PPS /dev/pps0 maxunreach 4 ``` The `copy` option lets a closely related client assume an upstream server's reference ID and stratum. It is useful when one daemon disciplines the system clock and a second `chronyd -x` serves it. Prevent synchronization loops; `peer` does not support `copy`. ```conf server 127.0.0.1 copy ``` ### Traffic and remote-command controls - `opencommands` selects which monitoring commands are remotely available. Expose only the required command surface. - `dscp` marks transmitted NTP packets with a DSCP value from 0 through 63; the default is 0. For example, `dscp 46` selects Expedited Forwarding. - `ptpdomain` filters NTP-over-PTP traffic by PTP domain. Its range is 0 through 255 and its default is 123. ```conf dscp 46 ptpdomain 42 ``` ### Persistence and lifecycle - `driftfile` accepts an `interval` option to control drift-data write frequency. - `local` accepts `waitsynced` and `waitunsynced` for explicit synchronized and unsynchronized wait behavior. - `chronyd` supports systemd `Type=notify` readiness signaling. - `cyclelogs` attempts to reopen the message log selected by `-l`. ## Selection and reachability `chronyc sources` prints `Reach` as an eight-bit poll history in octal. A set bit records a valid response or refclock sample; it does not prove the measurement passed every synchronization test. In `chronyc selectdata`, uppercase `S` means that the source has only measurements older than reachable sources or has exceeded its `maxunreach` limit. ```sh chronyc sources -v chronyc selectdata -v ``` For a persistent network-delay increase, NTP test C can reject measurements for hours while older low-delay samples define its expectation. Inspect with `ntpdata`; if the change is permanent, clear retained samples. ```sh chronyc ntpdata chronyc reset sources ``` Repeated cases can be mitigated with a larger `maxclockerror`, or a very large `maxdelaydevratio` that effectively disables the test at the cost of weaker delay filtering. ## Reference clocks and PPS/GPS ### New and changed drivers - Batch `4.7` adds an RTC refclock driver. - A PHC refclock can use a network-interface name instead of a `/dev/ptpN` path. - Refclocks no longer need multiple samples per poll and remain reachable when excessive-delay samples are discarded. - In batch `4.8`, `extpps` works on Linux 6.15 and newer. - Refclock samples are validated before updating reachability, so invalid samples no longer make a source appear reachable. ### Lock a precise PPS to coarse time of day For one GPS receiver providing NMEA through gpsd SHM and PPS through `/dev/pps0`, lock PPS to the NMEA refid and mark PPS trusted. This allows PPS selection even when the much less accurate correlated NMEA source is a falseticker. ```conf refclock SHM 0 refid NMEA offset 0.000 precision 1e-3 poll 0 filter 3 refclock PPS /dev/pps0 refid PPS lock NMEA poll 3 trust ``` Measure late NMEA delivery using statistics logs. A message arriving 900–1000 ms after its pulse can lock PPS to the wrong whole second; apply the opposite of the measured NMEA estimated offset and improve or reduce serial output if needed. ### gpsd socket choice - gpsd 3.25 adds `/run/chrony.clk.ttyX.sock` for serial date-time; the older `/run/chrony.ttyX.sock` carries PPS only. - Use the `.clk.` socket as a non-selected lock source, or use `SHM 0` with older gpsd. - For a gpsd-backed SOCK refclock, start gpsd after `chronyd` creates the socket and pass `-n` so gpsd polls without waiting for a client. ## Hardware timestamping and NTP-over-PTP Check `ethtool -T` before enabling hardware timestamps. The interface needs hardware transmit and receive timestamping, a matching `/dev/ptpN`, and `HWTSTAMP_FILTER_ALL` for ordinary NTP packets. ```sh ethtool -T eth2 ``` A local hardware-timestamped client can poll both NTP and the hardware clock at 16 Hz. F323 is experimental and improves stability when both endpoints support it. ```conf server 192.168.123.1 minpoll -4 maxpoll -4 xleave extfield F323 filter 5 hwtimestamp * minpoll -4 ``` For one-step end-to-end transparent switches, put NTP in PTP event messages on port 319 and request F324 at the client. Two-step, peer-to-peer, and boundary PTP clocks do not provide useful corrections for this mode. ```conf server 192.168.123.1 xleave extfield F324 port 319 ptpport 319 ``` At nanosecond precision, set a smaller `clockprecision`, at least on the server. Cross timestamping is automatic when the kernel provides it; `nocrossts` disables it. Transparent-clock corrections do not remove asymmetric PCIe latency between the NIC and system clock. Limit `hwtimestamp` to interfaces not used by another timestamping application. Large interleaved-mode servers also need enough `clientloglimit` space for per-client transmit timestamps. ## Network Time Security Require NTS on a source with `nts`, and persist established state with `ntsdumpdir` to avoid a fresh NTS-KE exchange after every reboot. ```conf server time.example.com iburst nts ntsdumpdir /var/lib/chrony ``` Check authentication with `chronyc -N authdata`. A working NTS source reports mode `NTS`, nonzero `KeyID`, `Type`, and `KLen`, no NAKs, and at least one stored cookie. Packet reachability alone does not establish authentication, and failed negotiation does not downgrade silently. An NTS server needs a readable PEM key, a PEM certificate file containing required intermediates, TCP 4460 for NTS-KE, and UDP 123 for NTP. ```conf ntsserverkey /etc/pki/tls/private/time.example.com.key ntsservercert /etc/pki/tls/certs/time.example.com.crt ntsdumpdir /var/lib/chrony ``` Verify from another host without changing its clock, then inspect server counters: ```sh chronyd -Q -t 3 'server time.example.com iburst nts maxsamples 1' chronyc serverstats ``` ## Monitoring and first-response diagnostics With `cmdport 0`, the exporter can use `unix:///run/chrony/chronyd.sock` instead of `[::1]:323`. The extended `ntpdata` source collector requires the socket; run the exporter as root or the chrony user, and add `--collector.chmod-socket` when running as root. ```sh chrony_exporter --chrony.address=unix:///run/chrony/chronyd.sock \ --collector.sources.with-ntpdata --collector.chmod-socket ``` An absolute clock-error bound is: ```promql abs(chrony_tracking_last_offset_seconds) + chrony_tracking_root_dispersion_seconds + (0.5 * chrony_tracking_root_delay_seconds) ``` - `506 Cannot talk to daemon`: check daemon availability, UDP 323, firewall rules, and `cmdallow`/`bindcmdaddress`. - `501 Not authorised`: the command needs the privileged Unix socket, or the socket directory ownership or permissions prevented its creation. - `activity` reporting unknown addresses means DNS resolution is incomplete; `chronyc -N sources -a` still shows configured names. - Windows NTP sources can advertise several seconds of root dispersion. Confirm with `ntpdata`, then raise `maxdistance` only as far as required. - After an unannounced disconnection, `chronyc online` triggers immediate measurements when connectivity returns; pair it with `offline` handling. ## Platform deployment checks - A KVM guest needs `CONFIG_PTP_1588_CLOCK_KVM` for a virtual PHC. - On RHEL, mask `20-chrony-onoffline` if routes managed outside NetworkManager must not leave sources offline. - On Gentoo, direct Linux PPS is a separate non-default `pps` USE flag; verify the other required time, security, and refclock feature gates. - Gentoo OpenRC reads `/etc/conf.d/chronyd`; `-u ntp -F 2` selects the account and level-two seccomp, while `-s -r` restores RTC-derived time and saved samples when configured. - VMware Tools immediately steps a resumed guest to repair the lifecycle-induced offset created while the guest clock and synchronizer were suspended.