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How to Make Any File Persistent (bind-dirs)

What are bind-dirs?

With bind-dirs 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 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.

Inside the app qube.

Make sure folder /rw/config/qubes-bind-dirs.d exists.
```
sudo mkdir -p /rw/config/qubes-bind-dirs.d
```
Create a file /rw/config/qubes-bind-dirs.d/50_user.conf with root rights.
Edit the file 50_user.conf to append a folder or file name to the binds variable.
```
binds+=( '/var/lib/tor' )
```
Save.
Reboot the app qube.
Done.

From now on any files within the /var/lib/tor folder will persist across reboots.

You can make make many files or folders persist, 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 will persist across reboots.

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 /rw/config/rc.local

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

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

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