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+---
+title: "Android 'hosts'-based<br> adblocking without root"
+date: 2023-07-11
+layout: "blog"
+toc: true
+---
+
+I hate how obnoxious tracking and advertising are online.
+In Firefox I can just use an adblocker,
+but for mobile apps it isn't so simple.
+There, you basically have two options:
+
+1.  Run a "local VPN" that filters all requests,
+    but then you can't use a real VPN at the same time,
+    and you need to trust whoever wrote the app (use open-source!).
+    When I tried this, I found it also often turned itself off,
+    so... not ideal.
+
+2.  Root your phone to enable more powerful methods;
+    the easiest approach is to replace your
+    [`hosts` file](https://en.wikipedia.org/wiki/Hosts_(file)#Extended_applications)
+    with one that contains all domains you want to avoid.
+    However, rooting a phone can be tricky, and if you succeed,
+    all the authentication and banking apps you've collected over the years
+    will refuse to work unless you jump through even more complicated hoops...
+    Ah, I love it when apps decide what I can and can't do with my phone.
+
+
+On Android, I recommend you take a look
+at [AdAway](https://adaway.org/), which can do both methods.
+But it turns out there's a better way under certain circumstances...
+
+Are you running a custom Android ROM, like [LineageOS](https://lineageos.org/)?
+Then you might be in luck!
+Open your ***Settings*** app and look for the ***Build number***
+in a section called ***About phone*** or similar.
+Does the number contain the string ***"userdebug"*** (see below)?
+If yes, congratulations, read on!
+
+{% include image.html file="build-number-full.png" width="42%"
+    alt="'Build number' at the bottom of the 'About phone' section" %}
+
+
+
+## Part 1: preparation
+
+First, we need to enable developer settings on your phone.
+You're already in the right place:
+tap ***Build number*** several times
+until it asks for your password as confirmation.
+Once enabled, you can find a new entry ***Developer options***
+in the ***System*** section of the ***Settings*** app.
+There, in the ***Debugging*** section, enable both
+***USB debugging*** and ***Rooted debugging***:
+
+{% include image.html file="dev-settings-full.png" width="42%"
+    alt="Debugging settings in the 'Developer options' menu" %}
+
+Next, you'll need a trusted PC to plug the device into
+(yes, *trusted*, I mean it: you're about to give it root access to your phone).
+Make sure it has the `adb` program installed;
+read [this guide](https://www.xda-developers.com/install-adb-windows-macos-linux/)
+to get it (or on Linux you can use the package manager).
+Check it's working by running:
+
+```sh
+adb devices
+```
+
+Your device should show up, although it may have a weird name.
+It'll say it's *"unauthorized"* until you accept
+the pop-up that will appear on the phone.
+
+Finally, prepare the `hosts` file you want to upload to the device.
+I recommend [Steven Black's files](https://github.com/StevenBlack/hosts),
+but you can also find others or even write one yourself.
+
+
+
+## Part 2: pushing the file
+
+If you have a *"userdebug"* build,
+you can give `adb` root privileges by running:
+
+```sh
+adb root
+```
+
+This is required for the following commands to work
+(if you don't trust me, try it).
+Next, we need to get read/write access to Android's system files.
+To do so, execute these commands:
+
+```sh
+# Turn off `dm-verity' system integrity checking.
+# Note: it'll tell you to reboot, but in my experience
+# this isn't necessary. It won't do any harm either.
+adb disable-verity
+# Request read/write access to the system
+adb remount
+```
+
+Now we're ready to copy the `hosts` file you prepared to the device:
+
+```sh
+adb push path-to-desired-hosts-file /system/etc/hosts
+```
+
+If you want, you can check it succeeded
+by reading the file from the phone:
+
+```sh
+# Enter a shell on the phone
+adb shell
+# Read its current `hosts' file
+more /system/etc/hosts
+# Leave the shell, back to your PC
+exit
+```
+
+
+## Part 3: cleaning up
+
+When you're satisfied, turn integrity checking back on and reboot the device:
+```sh
+# Re-enable `dm-verity' for security
+adb enable-verity
+# Reboot so the new `hosts' takes effect
+adb reboot
+```
+
+Once rebooted, go back to ***Developer options***
+and disable ***Root debugging***
+and ***USB debugging*** for security reasons.
+Then, if you wish, you can turn off ***Developers options*** entirely.
+
+Congratulations, you've now made the domains listed in `hosts`
+inaccessible for all apps on your phone, enjoy!
+But beware: Android system updates may reset the `hosts` file,
+although they don't always in my experience.
+You can always easily check the file's contents after an update.