Hooks for specific system management

Custom event scripts

Beginning with libvirt 0.8.0, specific events on a host system will trigger custom scripts.

These custom hook scripts are executed when any of the following actions occur:

Script location

The libvirt hook scripts are located in the directory $SYSCONFDIR/libvirt/hooks/.

To use hook scripts, you will need to create this hooks directory manually, place the desired hook scripts inside, then make them executable.

Script names

At present, there are three hook scripts that can be called:

Script structure

The hook scripts are executed using standard Linux process creation functions. Therefore, they must begin with the declaration of the command interpreter to use.

For example:




Other command interpreters are equally valid, as is any executable binary, so you are welcome to use your favourite languages.

Script arguments

The hook scripts are called with specific command line arguments, depending upon the script, and the operation being performed.

The guest hook scripts, qemu and lxc, are also given the full XML description for the domain on their stdin. This includes items such the UUID of the domain and its storage information, and is intended to provide all the libvirt information the script needs.

The command line arguments take this approach:

  1. The first argument is the name of the object involved in the operation, or '-' if there is none.

    For example, the name of a guest being started.

  2. The second argument is the name of the operation being performed.

    For example, "start" if a guest is being started.

  3. The third argument is a sub-operation indication, or '-' if there is none.

  4. The last argument is an extra argument string, or '-' if there is none.


This translates to the following specifics for each hook script:


Please note that when the libvirt daemon is restarted, the daemon hook script is called once with the "shutdown" operation, and then once with the "start" operation. There is no specific operation to indicate a "restart" is occurring.


Script execution

QEMU guest migration

Migration of a QEMU guest involves running hook scripts on both the source and destination hosts:

  1. At the beginning of the migration, the qemu hook script on the destination host is executed with the "migrate" operation.
  2. Before QEMU process is spawned, the two operations ("prepare" and "start") called for domain start are executed on destination host.
  3. If both of these hook script executions exit successfully (exit status 0), the migration continues. Any other exit code indicates failure, and the migration is aborted.
  4. The QEMU guest is then migrated to the destination host.
  5. Unless an error occurs during the migration process, the qemu hook script on the source host is then executed with the "stopped" and "release" operations to indicate it is no longer running on this host. Regardless of the return codes, the migration is not aborted as it has already been performed.

Calling libvirt functions from within a hook script


A hook script must not call back into libvirt, as the libvirt daemon is already waiting for the script to exit.

A deadlock is likely to occur.

Return codes and logging

If a hook script returns with an exit code of 0, the libvirt daemon regards this as successful and performs no logging of it.

However, if a hook script returns with a non zero exit code, the libvirt daemon regards this as a failure, logs its return code, and additionally logs anything on stderr the hook script returns.

For example, a hook script might use this code to indicate failure, and send a text string to stderr:

echo "Could not find required XYZZY" >&2
exit 1

The resulting entry in the libvirt log will appear as:

20:02:40.297: error : virHookCall:285 : Hook script execution failed: internal error Child process (LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
                       HOME=/root USER=root LOGNAME=root /etc/libvirt/hooks/qemu qemu prepare begin -) unexpected exit status 1: Could not find required XYZZY