The Linux kernel can reset the system if serious problems are detected.
This can be implemented via special watchdog hardware, or via a slightly
less reliable software-only watchdog inside the kernel. Either way, there
needs to be a daemon that tells the kernel the system is working fine. If the
daemon stops doing that, the system is reset.
is such a daemon. It opens
and keeps writing to it often enough to keep the kernel from resetting,
at least once per minute. Each write delays the reboot
time another minute. After a minute of inactivity the watchdog hardware will
cause the reset. In the case of the software watchdog the ability to
reboot will depend on the state of the machines and interrupts.
The watchdog daemon can be stopped without causing a reboot if the device
is closed correctly, unless your kernel is compiled with the
The watchdog daemon does several tests to check the system status:
Is the process table full?
Is there enough free memory?
Are some files accessible?
Have some files changed within a given interval?
Is the average work load too high?
Has a file table overflow occurred?
Is a process still running? The process is specified by a pid file.
Do some IP addresses answer to ping?
Do network interfaces receive traffic?
Is the temperature too high? (Temperature data not always available.)
Execute a user defined command to do arbitrary tests.
If any of these checks fail watchdog will cause a shutdown. Should any of
these tests except the user defined binary last longer than one minute the
machine will be rebooted, too.
Available command line options are the following:
Set verbose mode. Only implemented if compiled with
mode will log each several infos in
This is useful if you want to see exactly what happened until the watchdog rebooted
the system. Currently it logs the temperature (if available), the load
average, the change date of the files it checks and how often it went to sleep.
Try to synchronize the filesystem every time the process is awake. Note that
the system is rebooted if for any reason the synchronizing lasts longer
than a minute.
Soft-boot the system if an error occurs during the main loop, e.g. if a
given file is not accessible via the
call. Note that
this does not apply to the opening of
which are opened before the main loop starts.
Force the usage of the interval given or the maximal load average given
in the config file.
as the configuration file instead of the default
Do not reboot or halt the machine. This is for testing purposes. All checks
are executed and the results are logged as usual, but no action is taken.
Also your hardware card or the kernel software watchdog driver is not
enabled. Temperature checking is also disabled since this triggers
the hardware watchdog on some cards.
starts, it puts itself into the background and then tries all checks
specified in its configuration file in turn. Between each two tests it will write to
the kernel device to prevent a reset.
After finishing all tests watchdog goes to sleep for some
time. The kernel drivers expects a write to the watchdog device every minute.
Otherwise the system will be reset. As a default
will sleep for
only 1 second so it triggers the device early enough.
Under high system load
might be swapped out of memory and may fail
to make it back in in time. Under these circumstances the Linux kernel will
reset the machine. To make sure you won't get unnecessary reboots make
sure you have the variable
in the configuration file
This adds real time support to
it will lock itself into memory and there should be no problem even under the
highest of loads.
On system running out of memory the kernel will try to free enough memory by killing process. The
daemon itself is exempted from this so-called out-of-memory killer.
Also you can specify a maximal allowed load average. Once this load average
is reached the system is rebooted. You may specify maximal load averages for
1 minute, 5 minutes or 15 minutes. The default values is to disable this
test. Be careful not to set this parameter too low. To set a value less then
the predefined minimal value of 2, you have to use the
You can also specify a minimal amount of virtual memory you want to have
available as free. As soon as more virtual memory is used action is taken by
Note, however, that watchdog does not distinguish between
different types of memory usage. It just checks for free virtual memory.
If you have a watchdog card with temperature sensor you can specify
the maximal allowed temperature. Once this temperature is reached the
system is halted. The default value is 120. There is no unit conversion so make
sure you use the same unit as your hardware.
will issue warnings
once the temperature increases 90%, 95% and 98% of this temperature.
When using file mode
will try to
the given files. Errors returned
by stat will
cause a reboot. For a reboot the stat call has to last at least one minute.
This may happen if the file is located on an NFS mounted filesystem. If your
system relies on an NFS mounted filesystem you might try this option.
However, in such a case the
option may not work if the NFS server is
can read the pid from a pid file and
see whether the process still exists. If not, action is taken
So you can for instance restart the server from your
will try periodically to fork itself to see whether the process
table is full. This process will leave a zombie process until watchdog wakes
up again and catches it; this is harmless, don't worry about it.
In ping mode
tries to ping the given IP addresses. These addresses do
not have to be a single machine. It is possible to ping to a broadcast
address instead to see if at least one machine in a subnet is still living.
Do not use this broadcast ping unless your MIS person a) knows about it andb) has given you explicit permission to use it!
will send out three ping packages and wait up to <interval> seconds
for the reply with <interval> being the time it goes to sleep between two
times triggering the watchdog device. Thus a unreachable network will not
cause a hard reset but a soft reboot.
You can also test passively for an unreachable network by just monitoring
a given interface for traffic. If no traffic arrives the network is
considered unreachable causing a soft reboot or action from the
watchdog can run an external command for user-defined tests. A return code
not equal 0 means an error occured and watchdog should react. If the external
command is killed by an uncaught signal this is considered an error by watchdog
The command may take longer than the time slice defined for the kernel device
without a problem. However, error messages are
generated into the syslog facility. If you have enabled softboot on error
the machine will be rebooted if the binary doesn't exit in half the time
sleeps between two tries triggering the kernel device.
If you specify a repair binary it will be started instead of shutting down
the system. If this binary is not able to fix the problem
will still cause a reboot afterwards.
If the machine is halted an email is sent to notify a human that
the machine is going down. Starting with version 4.4
will also notify the human in charge if the machine is rebooted.
A soft reboot (i.e. controlled shutdown and reboot) is initiated for every
error that is found. Since there might be no more processes available,
watchdog does it all by himself. That means:
Kill all processes with SIGTERM.
After a short pause kill all remaining processes with SIGKILL.
Record a shutdown entry in wtmp.
Save the random seed from
If the device is non-existant or
there is no filename for saving this step is skipped.
Turn off accounting.
Turn off quota and swap.
Unmount all partitions except the root partition.
Remount the root partition read-only.
Shut down all network interfaces.
If the return code of the check binary is not zero
will assume an
error and reboot the system. Be careful with this if you are using the
real-time properties of watchdog since
will wait for the return of
this binary before proceeding. An positive exit code is interpreted as an
system error code (see
for details). Negative values are special to
Reboot the system. This is not exactly an error message but a command to
If the return code is -1
will not try to run a shutdown
Reset the system. This is not exactly an error message but a command to
If the return code is -2
watchdog will simply refuse to write the
kernel device again.
Maximum load average exceeded.
The temperature inside is too high.
contains no (or not enough) data.
The given file was not changed in the given interval.
contains invalid data.
Child process was killed by a signal.
Child process did not return in time.
Free for personal use.
The repair binary is started with one parameter: the error number that
to initiate the boot process. After trying to repair the
system the binary should exit with 0 if the system was successfully repaired
and thus there is no need to boot anymore. A return value not equal 0 tells
to reboot. The return code of the repair binary should be the error
number of the error causing
to reboot. Be careful with this if you
are using the real-time properties since
will wait for
the return of this binary before proceeding.
None known so far.
The original code is an example written by Alan Cox
<firstname.lastname@example.org>, the author of the kernel driver. All
additions were written by Michael Meskes <email@example.com>. Johnie Ingram
<firstname.lastname@example.org> had the idea of testing the load average. He also took
over the Debian specific work. Dave Cinege <email@example.com> brought
up some hardware watchdog issues and helped testing this stuff.