Ask HN: How to maintain an identical "hot standby" Android phone | Hacker News Reader

Ask HN: How to maintain an identical "hot standby" Android phone

55 pointsrkagerer26 comments

I'd like to buy two identical devices (eg. latest Pixel) and set one up to mirror the other - so if I accidentally drop one down a volcano I can grab the other and keep on rolling.

I want to keep everything on my own infrastructure, without relying on Google backup or the like. Sync needn't be realtime, nightly is fine. The devices should be kept as similar as possible - settings, apps, data etc. I'll only use one at a time (one "master"). In a perfect world my service provider, appstore platform, app publishers, etc. wouldn't be able to tell the difference (although I realize that might be asking too much). I don't mind using an alternate flavour of Android if needed eg. LineageOS, GrapheneOS, etc.

Has anyone accomplished this and gotten it working smoothly in practice? Any tooling you can recommend to help?

View on HN

Not generally possible.

Any application that stores any data in the Secure Element of the phone is, by design, not capable of being backed up, even with root. Examples include various bank apps that serve as 2-factor authenticators.

Without that, one of the possible solutions is to use NeoBackup on a rooted phone so it backs up everything daily to some external server on your infrastructure. It can be, for example, a NextCloud server.

> Any application that stores any data in the Secure Element of the phone is, by design, not capable of being backed up, even with root. Examples include various bank apps that serve as 2-factor authenticators.

This probably depends a lot on what bank app you have/use. My (large co.) bank app works perfectly fine across my pixel, iPhone and iPad. The “app data” isn’t replicated but since everything comes from the server it’s identical. I can verify and make transactions on either device.

Even 2fa apps like MS Authenticator allow you to use multiple devices by storing/syncing the seed via the cloud.

Is it 100% identical in every way perceivable by the user, or only close to identical?

it's 100% identical to the user.

TBH the app doesn't do much. Open it -> see balance and charges and credit/debit etc.

Trigger opening it from payment portal -> "verify transaction with biometrics/PIN" -> confirm

In my case, it's different.

Open the app the first time, accept the license, it contacts the server (anonymously, as it doesn't have any user data at this point) and establishes a random TOTP seed and a hex ID (which is an unchanging hex string visible in the UI along with the generated codes). There is no need to log in, and the app does not even have a login form. The bank has a database table that maps the hex ID to the corresponding TOTP seed. From that point on, the app never uses the network.

When you open the app the next time, it functions as a regular TOTP generator that keeps the seed in some protected area that cannot be backed up even if you have root (ignoring the license concerns) - except that you can't use the generated codes anywhere.

This also means that reinstalling the application, or installing it on a second phone, will result in a new ID and a new TOTP seed, so the two phones will have different hex IDs and different seeds, and thus will not agree on the codes generated.

To make the application useful, you need to physically go to the bank and present the running app together with your passport. They will inspect your phone for signs of rooting, dangerous settings or unwanted apps, associate the hex ID displayed by their app with your account (so that the generated TOTP codes start working on their online banking website), and disable the previous one (so you can't use two phones).

That sounds oddly intense for a bank app. I understand them not wanting a rooted phone and a few of the measures but everything sounds quite interesting/heavy. Would you be okay saying which bank this is or which country you are in?

I still wonder if with something like a root hiding thing like grapheneOS’s software this could be “overcome”.

Metrobank (for business customers) in the Philippines. Regular customers of the same bank have a different app with less strictness.

And yes, root hiding works on a test phone (which has a copy of the app not linked to my bank account), and the TOTP seed can be extracted from memory using Fridump and imported into a different app. But obviously, this won't pass the "manual inspection of the phone" step, because they will likely find the renamed Magisk app.

Thanks, that's very interesting that a business account in the Philippines has so much security. I think if a bank tried that here in Europe (inspecting phones) folks wouldn't like the privacy intrusion. Though, I think apps can be hidden from the launcher (and perhaps also the settings app with some more effort)?

I'm OK if there are one or two exceptions, like if my banking app needs to be manually reinstalled after 'failover'.

Out of curiosity, do open source phones like the Librem 5 or Pinephone Pro have a secure element and if so do they provide you with tools to provision and manage it yourself?

Most app data you cannot back up easily without root anymore. I used to use Titanium but stuff stopped working consistently even with root.

Settings and some apps aside, I otherwise basically do this myself. I keep my last model phone as the fallback and use Syncthing to copy pictures, SMS and call log exports, TOTP exports, Signal backups, and any other useful files automatically to it. Contacts and everything else is cloud already. Would take about 10 minutes to reimport and start using.

swift backup replaced it for me

Thanks - I haven't tried Swift Backup before. Does it work reasonably well when not tied to a Google account?

Setup cloud services it connects to on your own infrastructure and you can just sync your contacts and get 99% of everything that way on any Android phone. I keep all my media online in a database and it's super convenient for any usage.

How do you have duplicate SIM cards?

T-Mobile offers 'Data with Paired Digits' lines; $10/mo for 600kbps max data and $20/mo for high-speed data. The second sim receives calls and most texts same as the first. The possible dealbreaker is short-code SMS never arrives to the Paired Digits line. Technically, the second sim has its own number, and T-mobile routes calls and texts to both lines.

You can replicate a lot of the data and apps as others have explained. But once you get to SIM cards it’s a different game.

The advice would change if you’re domestic (staying in the same country as your service provider) or international. If you’re domestic then eSIM is the way to go here, you can have a new one issued and activated fairly quickly. If you’re international you can either move to a VOIP provider or use some service (like a Matrix bridge) to share SMS messages across. VOIP would be your only real option for calls as well - both phones would just be SIP trunks and can be hot swapped or used simultaneously.

So if you don't need to make calls (but use WhatsApp or similar instead), or have call forwarding options, consider using a backup phone with your SIM, which uses MacroDroid to forward your SMS to your email. Now your SIM stays safely at home and you can use burner SIMs on the road. Keep a 3rd phone as your backup ready to go.

I've successfully used MyPhoneExplorer[1] and immich[2] to perform "some" backup tasks (contacts + media).

There also was a free software used by phone repair shops that could clone the entire device... If I find out it's name again, I'm gonna post it here.

[1]: https://www.fjsoft.at/en/ [2]: https://github.com/immich-app/immich

No different than a hot-hot replication so just follow that pattern

I feel like this is better accomplished with a device snapshot-image that is regularly loaded onto a new device (daily? Hourly?)

Updating a device makes automation non trivial as you have to do something physical most likely and can’t just do it via API

You could even just skip acquiring the device until you need it given the idea that it may be years before you have this volcano situation. However that doesn’t meet your spec precicely

I would better define the idempotency interval you’re ok with losing data between and then using the method that updates the simplest at that frequency

Thanks for responding. Hopefully I'm correctly interpreting "idempotency interval" - I'm OK with loosing a day's worth of data.

Basically the period(s) for which you will block perfect replications to your save state

Unlock bootloader, root with patched img, disable automatic updates, when an update comes out go through the steps of creating a rooted image and flash upgrade.

With root, some app to do backups and restores of system/app data (which is why you'd need root), + syncthing, you can probably get 99% of the way there.

Buuuut... You now have the problems of root, which means a lot of apps won't work out of the box, you'll have to do some per app config to hide root/unlocked bootloader from them, and that's only getting harder.

So tl;dr don't use smart features, and only deal with syncing calls/sms/photos. The rest you do via the browser.

I agree with the posts above, though I will add that sandboxing apps like Insular work around some apps misbehaving when rooted where things like even Magisk hide fall short. I would unlock the bootloader, write TWRP to recovery and go that route somehow (rsync or something? I'm envisioning maybe plugging your phones to a usb ethernet adapter here, I don't know if it's even possible to WiFi or network from TWRP though so .tar.xz?).

Devices are a bit more complicated then they were just a few years ago, a cryptographic salt is stored somewhere, and the trim area (TA) plays a role, so you would run into trouble simply writing (dd) the /data partition to your second phone, but all you really need to do is keep the /data partition and /sdcard synced. Surely a script leveraging the features of TWRP seems like it should just work though?

Cheers, lovely question! - threader

LineageOS, CalyxOS and GrapheneOS all come with Seedvault which allows nightly remote backups. They're anything but complete, though. For folders, Syncthing might be interesting. I don't think it's possible without rooting your phone. Then, NeoBackup could do it.

Thanks. I looked into Seedvault and gather one of its challenges is a reliance on API's that are deprecated and (in the meantime) becoming intentionally degraded. Eg. Apps can exclude themselves via android:allowBackup=false.

There have been calls to modify Seedvault to impersonate D2D transfer in order to bypass the restriction. Or I wonder if I can patch the OS to ignore/override that app manifest flag? (I gather some people are patching their APK's).

I wrote a little more about this here: https://news.ycombinator.com/item?id=37774254

I believe the big tech will grow even bigger in the future.

So all in TQQQ.

Reply on news.ycombinator.com