How to make any file in a TemplateBasedVM persistent using bind-dirs

What is bind-dirs?

With bind-dirs any arbitrary files or folders can be made persistent in TemplateBasedVMs.

What is it useful for?

In a TemplateBasedVM all of the file system comes from the template except /home and /usr/local. This means that changes in the rest of the filesystem are lost when the TemplateBasedVM 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 StandaloneVM.

Minimum Qubes Version

bind-dirs.sh works with Qubes R3.2 and above.

How to use bind-dirs.sh?

Inside your TemplateBasedVM.

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 inside a TemplateBasedVM.
Edit the file 50_user.conf to append a folder or file name to the binds variable. (In the following example we are using folder /var/lib/tor. You can replace that name with a folder or file name of your choice.)
```
binds+=( '/var/lib/tor' )
binds+=( '/etc/tor/torrc' )
```

Multiple entries are possible, each on a separate line.

Save.
Reboot the TemplateBasedVM.
Done.

If you added for example folder /var/lib/tor to the binds variable, from now any files within that folder would persist reboots. If you added for example file /etc/tor/torrc to the binds variable, from now any modifications to that file would persist reboots.

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 on startup of a TemplateBasedVM, and configuration files in the configuration folders above 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 file and folders in /rw/bind-dirs should be automatic the first time the TemplateBasedVM is restarted after configuration.

If you want to circumvent this process, you can create the relevant filestructure under /rw/bind-dirs and make any changes at the same time that you perform the configuration, before reboot.

Limitations

Files that exist in the TemplateVM root image cannot be deleted in the TemplateBasedVMs root image using bind-dirs.sh.
Re-running sudo /usr/lib/qubes/bind-dirs.sh without a previous sudo /usr/lib/qubes/bind-dirs.sh umount does not work.
Running sudo /usr/lib/qubes/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.

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-persistant in sys-whonix without manually editing /usr/lib/qubes-bind-dirs.d/40_qubes-whonix.conf, you could use the following.

/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 recommended against, since such changes get lost when that file is changed in the package on upgrades.)

Discussion

TemplateBasedVMs: make selected files and folders located in the root image persistent- review bind-dirs.sh

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