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On an Orange Pi Zero running a Raspbian server, it's possible to use the watchdog very easily just by running the command echo 1 > /dev/watchdog as root. The idea is that the system will certainly reboot after some time that this command is executed, so I need to keep repeating this command in a regular interval of time to keep the system on. We can implement a watchdog using cron as root and making it execute the following script on boot:

#!/bin/bash

while [ true ]; do

    echo 1 > /dev/watchdog

    sleep 5

done

This script works fine on the Orange Pi Zero... However, on my desktop computer running Ubuntu 18.04 the command echo 1 > /dev/watchdog doesn't work at all. Is it possible to activate the watchdog on any device running Linux? Or is it one parameter that needs to be specified on the Kernel while it's compiled?

There are two types of watchdog; hardware and software. On the Orange Pi the SOC chip provides a hardware watchdog. If initialised then it needs to be pinged every so often, otherwise it performs a board reset.

However not many desktops have hardware watchdogs, so the kernel provides a software version. Now the kernel will try and keep track, and force a reboot. This isn't as good as a hardware watchdog because if the kernel, itself, breaks then nothing will trigger the reset. But it works.

The software watchdog can be initialised by loading the softdog module

% modprobe softdog

% dmesg | tail -1

[  120.573945] softdog: Software Watchdog Timer: 0.08 initialized. soft_noboot=0 soft_margin=60 sec soft_panic=0 (nowayout=0)

We can see this has a 60 second timeout by default.

If I then do

% echo > /dev/watchdog

% dmesg | tail -1

[  154.514976] watchdog: watchdog0: watchdog did not stop!

We can see the watchdog hadn't timed out.

I then leave the machine idle for a minute and on the console I see

[  214.624112] softdog: Initiating system reboot

and the OS reboots.

The I/O redirection closes the watchdog file handle after writing the 1. Depending on how the watchdog device is configured, closing the file handle can also disable the watchdog.

Try

exec 3>/dev/watchdog

echo 1 >&3

This will keep the watchdog device open in the current shell, so the timer will not be stopped.

Most people run a dedicated watchdog daemon rather than using cron; this daemon runs a list of checks before resetting the timer, so the machine also reboots if tests fail. This could be used to verify that a database service actually processes queries, while regular service monitoring would only verify that the process is running.