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

What are 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, /usr/local, and /rw. 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.

How to use bind-dirs.sh?

In this example, we want to make /var/lib/tor persistent.

Inside the 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.
Edit the file 50_user.conf to append a folder or file name to the binds variable.
```
binds+=( '/var/lib/tor' )
```
Save.
Reboot the TemplateBasedVM.
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 a TemplateBasedVM, 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 TemplateBasedVM 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 TemplateVM root image cannot be deleted in the TemplateBasedVMs 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

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

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