qubes-doc/user/advanced-topics/bind-dirs.rst

===========================================
How to make any file persistent (bind-dirs)
===========================================


What are bind-dirs?
-------------------


With
`bind-dirs <https://github.com/QubesOS/qubes-core-agent-linux/blob/master/vm-systemd/bind-dirs.sh>`__
any arbitrary files or folders can be made persistent in app qubes.

What is it useful for?
----------------------


In an app qube all of the file system comes from the template except
``/home``, ``/usr/local``, and ``/rw``. This means that changes in the
rest of the filesystem are lost when the app qube is shutdown. bind-dirs
provides a mechanism whereby files usually taken from the template can
be persisted across reboots.

For example, in Whonix, `Tor’s data dir /var/lib/tor has been made
persistent in the TemplateBased ProxyVM
sys-whonix <https://github.com/Whonix/qubes-whonix/blob/8438d13d75822e9ea800b9eb6024063f476636ff/usr/lib/qubes-bind-dirs.d/40_qubes-whonix.conf#L5>`__
In this way sys-whonix can benefit from the Tor anonymity feature
‘persistent Tor entry guards’ but does not have to be a standalone.

How to use bind-dirs.sh?
------------------------


In this example, we want to make ``/var/lib/tor`` persistent. Enter all
of the following commands in your app qube.

1. Make sure the directory ``/rw/config/qubes-bind-dirs.d`` exists.

   .. code:: bash

         sudo mkdir -p /rw/config/qubes-bind-dirs.d



2. Create the file ``/rw/config/qubes-bind-dirs.d/50_user.conf`` with
   root permissions, if it doesn’t already exist.

   .. code:: bash

         sudo touch /rw/config/qubes-bind-dirs.d/50_user.conf



3. Add a line to ``/rw/config/qubes-bind-dirs.d/50_user.conf`` that
   appends a folder or file to the ``binds`` variable.

   .. code:: bash

         binds+=( '/var/lib/tor' )



4. Save.

5. If the directory you wish to make persistent doesn’t exist in the
   template on which the app qube is based, you’ll need to create the
   directory (with its full path) under ``/rw/bind-dirs`` in the app
   qube. For example, if ``/var/lib/tor`` didn’t exist in the template,
   then you would execute the following command in your app qube:

   .. code:: bash

         sudo mkdir -p /rw/bind-dirs/var/lib/tor



6. (optional) If the directory you want to persist across reboots
   (``/var/lib/tor`` in this case) needs special ownership and
   permissions, make sure the directory you created just under
   ``/rw/bind-dirs/`` has the same ones (using the commands ``chown``
   and ``chmod``, respectively).

7. Reboot the app qube.

8. Done.



From now on, all files in the ``/var/lib/tor`` directory will persist
across reboots.

You can make as many files or folders persist as you want simply by
making multiple entries in the ``50_user.conf`` file, each on a separate
line. For example, if you added the file ``/etc/tor/torrc`` to the
``binds`` variable, any modifications to *that* file would also persist
across reboots.

.. code:: bash

      binds+=( '/var/lib/tor' )
      binds+=( '/etc/tor/torrc' )



Other Configuration Folders
---------------------------


- ``/usr/lib/qubes-bind-dirs.d`` (lowest priority, for packages)

- ``/etc/qubes-bind-dirs.d`` (intermediate priority, for template wide
  configuration)

- ``/rw/config/qubes-bind-dirs.d`` (highest priority, for per VM
  configuration)



How does it work?
-----------------


bind-dirs.sh is called at startup of an app qube, and configuration
files in the above configuration folders are parsed to build a bash
array. Files or folders identified in the array are copied to
``/rw/bind-dirs`` if they do not already exist there, and are then bind
mounted over the original files/folders.

Creation of the files and folders in ``/rw/bind-dirs`` should be
automatic the first time the app qube is restarted after configuration.

If you want to circumvent this process, you can create the relevant file
structure under ``/rw/bind-dirs`` and make any changes at the same time
that you perform the configuration, before reboot. Note that you must
create the full folder structure under ``/rw/bind-dirs`` - e.g you would
have to create ``/rw/bind-dirs/var/lib/tor``

Limitations
-----------


- Files that exist in the template root image cannot be deleted in the
  app qubes root image using bind-dirs.sh.

- Re-running ``sudo /usr/lib/qubes/init/bind-dirs.sh`` without a
  previous ``sudo /usr/lib/qubes/init/bind-dirs.sh umount`` does not
  work.

- Running ``sudo /usr/lib/qubes/init/bind-dirs.sh umount`` after boot
  (before shutdown) is probably not sane and nothing can be done about
  that.

- Many editors create a temporary file and copy it over the original
  file. If you have bind mounted an individual file this will break the
  mount. Any changes you make will not survive a reboot. If you think
  it likely you will want to edit a file, then either include the
  parent directory in bind-dirs rather than the file, or perform the
  file operation on the file in ``/rw/bind-dirs``.

- Some files are altered when a qube boots - e.g. ``/etc/hosts``. If
  you try to use bind-dirs on such files you may break your qube in
  unpredictable ways. You can add persistent rules to ``/etc/hosts``
  using :doc:`rw/config/rc.local </user/advanced-topics/config-files>`



How to remove binds from bind-dirs.sh?
--------------------------------------


``binds`` is actually just a bash variable (an array) and the
bind-dirs.sh configuration folders are sourced as bash snippets in
lexical order. Therefore if you wanted to remove an existing entry from
the ``binds`` array, you could do that by using a lexically higher
configuration file. For example, if you wanted to make ``/var/lib/tor``
non-persistent in ``sys-whonix`` without manually editing
``/usr/lib/qubes-bind-dirs.d/40_qubes-whonix.conf``, you could use the
following in:

``/rw/config/qubes-bind-dirs.d/50_user.conf``

.. code:: bash

      binds=( "${binds[@]/'/var/lib/tor'}" )



(Editing ``/usr/lib/qubes-bind-dirs.d/40_qubes-whonix.conf`` directly is
strongly discouraged, since such changes get lost when that file is
changed in the package on upgrades.)

Discussion
----------


`app qubes: make selected files and folders located in the root image persistent- review bind-dirs.sh <https://groups.google.com/forum/#!topic/qubes-devel/tcYQ4eV-XX4/discussion>`__