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It seems that after a factory-reset, whenever a new Google account is enabled on a device and potentially after some software updates sync will be enabled again even if it was turned off in the Android settings before any personal data was loaded onto the device.

How can this be prevented?

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The approach(es) I'm using:

  • don't use a Google account at all (if really needed, Playstore can be accessed via e.g. YalpStore – and instead of GMS I use microG anyway). As that's not practicable for some folks:
  • before adding the Google account, make sure you've prepared a different "account storage", made it the default and have all your data (contacts, calendars) in there. Examples for that:
    • DAVDroid with some self-hosted server (e.g. ownCloud, Nextcloud, Baïkal …)
    • "Device storage". Either your device/ROM supports that out-of-the-box, or you've got to use some helpers like Offline Calendar for calendars or MyLocalAccount for contacts.

I've personally made very good experiences with DAVDroid. Google Sync never touched its data.

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Some things I thought about were:

  • Going into offline mode before doing any of the above: but enabling Internet could be done by default after these.
  • Installing the NetGuard firewall and configure it to start automatically: but the app could be disabled or take too long to start up. And it doesn't help with factory-resets.
  • Going into a remote area where there's no mobile Internet: impractical.
  • Somehow encasing or modifying the phone to be able to prevent it from Internet access and going into Offline-mode afterwards: likely impractical.

But I guess the best solution is the following:

  • Removing the SIM card before doing any of these and turning off one's WLAN router (or going to a place without WLAN access) before doing any of these: somewhat impractical but probably the best solution.

    • It might not be possible to remove the SIM card during any of these as one would need to remove the battery or SD card during phone operation.
      I think one should also remove encrypted SD cards during any of these.

After doing this one needs to go into offline mode, disable "synchronization" settings (including in all apps such as messengers) and install the apparently only non-root open source firewall for Android: NetGuard.

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