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A hardware watchdog can reboot a Linux home server after the operating system stops servicing a timer. Set it up by confirming that the motherboard or watchdog card has a Linux-supported driver, identifying the correct /dev/watchdog* device, enabling systemd’s RuntimeWatchdogSec=, and testing only with a safe recovery plan.
The exact device name, timeout range, boot behavior, and even whether the watchdog can be stopped depend on the host’s hardware, kernel, driver, and systemd version. Treat the procedure below as a decision path, not a universal copy-and-paste configuration.
What a hardware watchdog does
The Linux kernel defines a watchdog timer as a hardware circuit that can reset a computer after a software fault. A userspace process periodically writes to the watchdog device. If those notifications stop because of a hung service, kernel failure, memory problem, or similar fault, the hardware timer eventually expires and resets the machine. See the Linux Watchdog driver API for the underlying model.
This is different from a software-only reboot script. A script that runs inside a frozen kernel cannot recover that kernel; a hardware timer can continue counting independently until it asserts reset. It also is not a backup system, an uninterruptible power supply, or a substitute for monitoring and backups. Its job is to force a reboot when servicing stops.
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Before you enable it: requirements and risks
- A motherboard, chipset, or add-in watchdog card with a Linux driver. Support is hardware- and driver-specific; two boards with similar specifications may expose different features or none at all.
- A Linux watchdog device exposed by the kernel, commonly
/dev/watchdogor a numbered node such as/dev/watchdog0. The actual path must be checked on the target machine. - A system manager capable of servicing the device. On a systemd host, this is controlled by
RuntimeWatchdogSec=and, when necessary,WatchdogDevice=. - A recovery path: local console access or tested out-of-band access, current backups, and a maintenance window for any intentional expiry test.
- A plan for writes in progress. A watchdog reset is abrupt; it can interrupt database, filesystem, or array operations just as a power interruption would.
Do not enable an unknown watchdog remotely and then experiment with short values. First establish which device is present, whether it starts during boot, what timeout values it accepts, and whether the driver supports stopping it.
Step 1: confirm the real hardware and Linux driver
Read the board or server documentation
Look for terms such as hardware watchdog, BMC watchdog, Super I/O watchdog, chipset watchdog, or watchdog timer in the motherboard or server manual. Record the exact board revision and firmware settings. A feature listed for a related model does not prove that your model implements it.
Then check whether the running kernel has a driver for that exact controller. The Linux watchdog framework is driver-mediated, and drivers can expose different or incompatible operations. The kernel’s Watchdog Support index is a useful starting point for identifying supported families, but it does not certify compatibility with an unspecified board.
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Inspect device nodes without assuming one command works everywhere
Distributions and drivers do not provide one universal detection command. Use the inspection tools available on your distribution and compare their results with the board documentation. For example, you can list likely device nodes with:
ls -l /dev/watchdog*If a node exists, inspect the kernel and system logs for the driver that created it. Commands such as dmesg or your distribution’s journal viewer may help, but their output and permissions vary. A node by itself is not proof that the desired onboard controller is selected; a machine can expose more than one watchdog.
Write down:
- Every watchdog node that exists, including numbered nodes.
- The driver or controller name reported by the kernel.
- Whether the node appears early in boot or only after a module loads.
- Any documented minimum, maximum, or discrete timeout values.
- Whether the controller is enabled in firmware and whether another program already opens it.
If no suitable node appears, stop here. Reinstalling systemd settings cannot create hardware support. Recheck firmware options, kernel configuration, and the driver for the exact controller before considering an add-in card.
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When an add-in card is appropriate
An add-in card can provide a watchdog when the motherboard lacks one, but it still requires a supported interface and Linux driver. The kernel support index names a Berkshire Products PC Watchdog Card; that reference does not establish current availability, a particular model’s compatibility, or a recommended purchase for your server. Select a card only after matching its model and interface to the host and the kernel version you will run.
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The timeout is the interval between the last successful service and a forced reset. Choose it from the machine’s real behavior rather than from a round number copied from another system.
- Measure or document the slowest normal boot, including storage checks, encrypted-volume prompts, and network mounts.
- Allow time for expected maintenance pauses and transient load. A busy home server should not reset merely because a backup temporarily delays userspace.
- Keep the interval short enough to recover from a genuine hang without creating an endless outage.
- Confirm the hardware’s supported values. Some devices accept only fixed intervals; others round requests.
Systemd can select the closest timeout the device supports. Its manager services the hardware at least once during each half of the configured timeout, so the requested value is not a promise that the hardware will count with arbitrary precision. These behaviors are documented in the systemd-system.conf manual.
For an example only, a setting of 120s asks for a two-minute runtime timeout. It is not a recommendation for every home server; use the value supported by your device and justified by your boot and workload measurements.
Step 3: enable systemd servicing
Set the manager options
On a systemd installation, edit the system manager configuration, normally /etc/systemd/system.conf, and set a nonzero runtime timeout:
RuntimeWatchdogSec=120sIf the intended device is not the default, select it explicitly:
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WatchdogDevice=/dev/watchdog1Systemd’s documented default for WatchdogDevice= is /dev/watchdog0. Do not copy that path unless it is the node you identified. If you set RuntimeWatchdogSec=default, systemd opens and services the watchdog without changing its default timeout; that can be useful when the hardware’s own default is known and appropriate, but it leaves the effective interval to the device.
Remove a leading comment character from the settings if the file contains commented examples. Keep one unambiguous value for each option. Because systemd versions and distribution packaging differ, apply the manager configuration using the re-exec or reload procedure documented for your release, commonly a systemd manager re-exec, or reboot the host during a maintenance window. Verify the result after the manager has reread the file; editing the file alone does not prove that the running manager used it.
Confirm that the requested value was accepted
After applying the setting, inspect the system manager’s boot or service journal for watchdog activation and any message showing the selected device or an adjusted timeout. A driver may round the request to its nearest supported value, so record the effective value rather than assuming it equals the text in the configuration file.
If systemd reports that the node cannot be opened, check the path, driver load order, permissions, and whether another watchdog client already owns the device. If it reports an unsupported timeout, choose a value from the hardware’s documented range instead of repeatedly retrying the same request.
Step 4: understand boot handoff
A watchdog may begin running before userspace opens its device. The kernel watchdog parameters include handle_boot_enabled and open_timeout; they govern how an already-enabled watchdog is handled and how long the kernel continues servicing it while waiting for userspace to open the device. See the WatchDog Module Parameters documentation for the parameters supported by the relevant kernel and driver.
This matters on a server with a long or variable boot. If the kernel stops servicing a running timer before systemd takes ownership, a healthy machine can reset during startup. Conversely, an overly short handoff period can make normal module loading look like a hardware fault. Check the kernel command line, module parameters, firmware watchdog state, and boot logs on the actual host; there is no safe universal value.
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Step 5: treat stop behavior and nowayout as a design decision
The Linux watchdog API allows closing a device to disable it in many drivers, but drivers may implement a magic-close mechanism, and a watchdog configured with nowayout cannot be stopped by an ordinary close. Some hardware cannot be stopped at all. The API and parameter documentation describe these differences at watchdog-api.html and watchdog-parameters.html.
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Document the result in the server’s runbook: device path, driver, effective timeout, firmware state, boot handoff settings, whether nowayout is enabled, and the procedure for recovering from a timer that cannot be stopped.
Step 6: validate without risking an accidental reset
Perform a non-destructive validation first
- Confirm that the selected node exists and belongs to the intended controller.
- Confirm that the system manager accepted
RuntimeWatchdogSec=andWatchdogDevice=, including any timeout rounding. - Check that the watchdog is serviced throughout ordinary operation and that no boot or shutdown messages report a handoff failure.
- Verify local console or out-of-band access and make sure recent backups can be restored.
- Record the expected reboot evidence, including the previous boot’s journal and the next boot’s kernel log.
Plan an expiry test only during controlled downtime
An expiry test deliberately allows the timer to run out, and the documented action is a hardware reset. Do not begin by killing processes on a production-like machine with unprotected writes, mounted removable media, or no recovery access. Schedule downtime, stop or quiesce applications cleanly, ensure storage is safe, and keep a local or out-of-band console available.
Use the driver and systemd documentation for the supported test method on your host; do not assume that closing a file descriptor or stopping an arbitrary service will produce a controlled test. After the reset, verify that the machine boots, the watchdog is rearmed as intended, filesystems are clean, and your monitoring distinguishes the planned reset from an unexplained crash.
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| Decision point | Onboard controller | Add-in watchdog card |
|---|---|---|
| Availability | Depends on the motherboard, chipset, firmware, and kernel driver. | Requires a compatible slot, interface, card firmware, and Linux driver. |
| Linux device | May appear as a board-specific watchdog node; the path is not guaranteed. | Uses the node and driver exposed by the card; identify it after installation. |
| Timeout range | Determined by the onboard controller and driver; values may be discrete. | Determined by the card and its driver; verify before configuring systemd. |
| Boot behavior | Can be enabled by firmware or kernel parameters before userspace starts. | May have its own power-on state and handoff behavior. |
| Replacement impact | No additional hardware, but a board change can remove support. | Can move between compatible hosts, but requires a new compatibility check. |
| Best selection rule | Use it when the exact controller is documented and supported by the running kernel. | Use it only when the card model and host interface have confirmed Linux support. |
Troubleshooting common failure modes
No /dev/watchdog* node exists
Check firmware enablement, kernel configuration, module loading, and the controller’s driver support. Inspect the kernel’s watchdog support index and your distribution’s package information. Do not assume that a server-class board automatically includes a usable watchdog.
The wrong device is selected
List all watchdog nodes and match each to its driver. Set WatchdogDevice= to the intended path instead of relying on the documented default of /dev/watchdog0. Recheck after every kernel or hardware change because numbering can differ.
Systemd rejects the timeout
Read the manager’s log for the effective range or rounding behavior, then choose a supported interval. A value that works on one controller may be invalid on another. If the hardware’s default is the only reliable setting, use RuntimeWatchdogSec=default only after confirming that default is suitable.
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The machine resets during boot
Investigate whether the timer was enabled by firmware or an earlier kernel stage and whether userspace opened it before open_timeout elapsed. Increase the handoff margin only within the parameters supported by the kernel and driver, and make sure systemd is selecting the intended node.
Stopping a service causes a reset
That can indicate nowayout, a driver-specific magic-close rule, or hardware that cannot be stopped. Treat the timer as armed until the driver documentation and a safe maintenance test prove otherwise. Keep recovery access available before stopping watchdog-related services.
There is an unexplained reboot
Correlate the reboot time with the previous boot’s journal, kernel messages, storage errors, thermal or power events, and watchdog logs. A watchdog reset proves that servicing stopped; it does not identify whether the original cause was a kernel fault, a stalled userspace manager, a driver problem, or an external interruption. Preserve the logs before the next reboot overwrites useful context.
Keep the configuration maintainable
Store the watchdog settings with the server’s operational documentation and change history. Include the board model and revision, firmware watchdog setting, kernel and systemd versions, device path, driver name, requested and effective timeout, boot handoff parameters, nowayout state, and recovery contacts.
Revalidate after a motherboard replacement, firmware update, kernel change, distribution migration, or systemd upgrade. The Linux API page is valuable background, but it records a historical review date of 10/05/2007 and describes the Linux 2.4.18 state. Use it for the general mechanism and API semantics while checking current behavior against the kernel, driver, and systemd versions actually running on the home server.
Quick Recap
Further reading
- Linux Watchdog driver API — device semantics, servicing, close behavior, and reset mechanism.
- WatchDog Module Parameters — boot handoff and driver parameter details.
- systemd-system.conf manual —
RuntimeWatchdogSec=,WatchdogDevice=, timeout rounding, and servicing cadence. - Linux Watchdog Support — kernel driver support index.
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