Building Mobile Nebula

Getting my hands dirty with Android development.

This post is part of a series:
Next: Building gomobile Using Nix

This post is going to be cheating a bit. I want to start working on adding DNS resolver configuration to the mobile nebula app (if you don’t know nebula, check it out, it’s well worth knowing about), but I also need to write a blog post for this week, so I’m combining the two exercises. This post will essentially be my notes from my progress on today’s task.

(Protip: listen to this while following along to achieve the proper open-source programming aesthetic.)

The current mobile nebula app works very well, but it is lacking one major feature: the ability to specify custom DNS resolvers. This is important because I want to be able to access resources on my nebula network by their hostname, not their IP. Android does everything in its power to make DNS configuration impossible, and essentially the only way to actually accomplish this is by specifying the DNS resolvers within the app. I go into more details about why Android is broken here.


Before I can make changes to the app I need to make sure I can correctly build it in the first place, so that’s the major task for today. The first step to doing so is to install the project’s dependencies. As described in the mobile_nebula README, the dependencies are:

It should be noted that as of writing I haven’t used any of these tools ever, and have only done a small amount of android programming, probably 7 or 8 years ago, so I’m going to have to walk the line between figuring out problems on the fly and not having to completely learning these entire ecosystems; there’s only so many hours in a weekend, after all.

I’m running Archlinux so I install android-studio and flutter by doing:

yay -Sy android-studio flutter

And I install gomobile, according to its documentation via:

go get
gomobile init

Now I startup android-studio and go through the setup wizard for it. I choose standard setup because customized setup doesn’t actually offer any interesting options. Next android-studio spends approximately two lifetimes downloading dependencies while my eyesight goes blurry because I’m drinking my coffee too fast.

It’s annoying that I need to install these dependencies, especially android-studio, in order to build this project. A future goal of mine is to nix this whole thing up, and make a build pipeline where you can provide a full nebula configuration file and it outputs a custom APK file for that specific config; zero configuration required at runtime. This will be useful for lazy/non-technical users who want to be part of the nebula network.

Once android-studio starts up I’m not quite done yet: there’s still the NDK which must be enabled. The instructions given by the link in mobile_nebula’s README explain doing this pretty well, but it’s important to install the specific version indicated in the mobile_nebula repo (21.0.6113669 at time of writing). Only another 1GB of dependency downloading to go….

While waiting for the NDK to download I run flutter doctor to make sure flutter is working, and it gives me some permissions errors. This blog post gives some tips on setting up, and after running the following…

sudo groupadd flutterusers
sudo gpasswd -a $USER flutterusers
sudo chown -R :flutterusers /opt/flutter
sudo chmod -R g+w /opt/flutter/
newgrp flutterusers

… I’m able to run flutter doctor. It gives the following output:

[✓] Flutter (Channel stable, 1.22.6, on Linux, locale en_US.UTF-8)
[!] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    ✗ Android licenses not accepted.  To resolve this, run: flutter doctor --android-licenses
[!] Android Studio
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
[!] Connected device
    ! No devices available

! Doctor found issues in 3 categories.

The first issue is easily solved as per the instructions given. The second is solved by finding the plugin manager in android-studio and installing the flutter plugin (which installs the dart plugin as a dependency, we call that a twofer).

After installing the plugin the doctor command still complains about not finding the plugins, but the above mentioned blog post indicates to me that this is expected. It’s comforting to know that the problems indicated by the doctor may or may not be real problems.

The blog post also indicates that I need openjdk-8 installed, so I do:

yay -S jdk8-openjdk

And use the archlinux-java command to confirm that that is indeed the default version for my shell. The mobile_nebula helpfully expects an file to exist in the root, so if openjdk-8 wasn’t already the default I could make it so within that file.


At this point I think I’m ready to try actually building an APK. Thoughts and prayers required. I run the following in a terminal, since for some reason the Build > Flutter > Build APK dropdown button in android-studio did nothing.

flutter build apk

It takes quite a while to run, but in the end it errors with:

make: 'mobileNebula.aar' is up to date.
cp: cannot create regular file '../android/app/src/main/libs/mobileNebula.aar': No such file or directory

FAILURE: Build failed with an exception.

* Where:
Build file '/tmp/src/mobile_nebula/android/app/build.gradle' line: 95

* What went wrong:
A problem occurred evaluating project ':app'.
> Process 'command './'' finished with non-zero exit value 1

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at

Running Gradle task 'bundleRelease'...
Running Gradle task 'bundleRelease'... Done                         1.7s
Gradle task bundleRelease failed with exit code 1

I narrow down the problem to the ./ script in the repo’s root, which takes in either android or ios as an argument. Running it directly as ./ android results in the same error:

make: 'mobileNebula.aar' is up to date.
cp: cannot create regular file '../android/app/src/main/libs/mobileNebula.aar': No such file or directory

So now I gotta figure out wtf that mobileNebula.aar file is. The first thing I note is that not only is that file not there, but the libs directory it’s supposed to be present in is also not there. So I suspect that there’s a missing build step somewhere.

I search for the string mobileNebula.aar within the project using ag and find that it’s built by nebula/Makefile as follows:

mobileNebula.aar: *.go
	gomobile bind -trimpath -v --target=android

So that file is made by gomobile, good to know! Additionally the file is actually there in the nebula directory, so I suspect there’s just a missing build step to move it into android/app/src/main/libs. Via some more ag-ing I find that the code which is supposed to move the mobileNebula.aar file is in the script, but that script doesn’t create the libs folder as it ought to. I apply the following diff:

diff --git a/ b/
index 601ed7b..4f73b4c 100755
--- a/
+++ b/
@@ -16,7 +16,7 @@ if [ "$1" = "ios" ]; then
 elif [ "$1" = "android" ]; then
   # Build nebula for android
   make mobileNebula.aar
-  rm -rf ../android/app/src/main/libs/mobileNebula.aar
+  mkdir -p ../android/app/src/main/libs
   cp mobileNebula.aar ../android/app/src/main/libs/mobileNebula.aar


(The rm -rf isn’t necessary, since a) that file is about to be overwritten by the subsequent cp whether or not it’s there, and b) it’s just deleting a single file so the -rf is an unnecessary risk).

At this point I re-run flutter build apk and receive a new error. Progress!

A problem occurred evaluating root project 'android'.
> A problem occurred configuring project ':app'.
   > Removing unused resources requires unused code shrinking to be turned on. See for more information.

I recall that in the original mobile_nebula README it mentions to run the flutter build command with the --no-shrink option, so I try:

flutter build apk --no-shrink

Finally we really get somewhere. The command takes a very long time to run as it downloads yet more dependencies (mostly android SDK stuff from the looks of it), but unfortunately still errors out:

Execution failed for task ':app:processReleaseResources'.
> Could not resolve all files for configuration ':app:releaseRuntimeClasspath'.
   > Failed to transform mobileNebula-.aar (:mobileNebula:) to match attributes {artifactType=android-compiled-dependencies-resources, org.gradle.status=integration}.
      > Execution failed for AarResourcesCompilerTransform: /home/mediocregopher/.gradle/caches/transforms-2/files-2.1/735fc805916d942f5311063c106e7363/jetified-mobileNebula.
         > /home/mediocregopher/.gradle/caches/transforms-2/files-2.1/735fc805916d942f5311063c106e7363/jetified-mobileNebula/AndroidManifest.xml

Time for more ag-ing. I find the file android/app/build.gradle, which has the following block:

    implementation (name:'mobileNebula', ext:'aar') {
        exec {
            workingDir '../../'
            environment("ANDROID_NDK_HOME", android.ndkDirectory)
            environment("ANDROID_HOME", android.sdkDirectory)
            commandLine './', 'android'

I never set up the ANDROID_HOME or ANDROID_NDK_HOME environment variables, and I suppose that if I’m running the flutter command outside of android-studio there wouldn’t be a way for flutter to know those values, so I try setting them within my

export ANDROID_HOME=~/Android/Sdk
export ANDROID_NDK_HOME=~/Android/Sdk/ndk/21.0.6113669

Re-running the build command still results in the same error. But it occurs to me that I probably had built the mobileNebula.aar without those set previously, so maybe it was built with the wrong NDK version or something. I tried deleting nebula/mobileNebula.aar and try building again. This time… new errors! Lots of them! Big ones and small ones!

At this point I’m a bit fed up, and want to try a completely fresh build. I back up my modified and files, delete the mobile_nebula repo, re-clone it, reinstall those files, and try building again. This time just a single error:

Execution failed for task ':app:lintVitalRelease'.
> Could not resolve all artifacts for configuration ':app:debugRuntimeClasspath'.
   > Failed to transform libs.jar to match attributes {artifactType=processed-jar, org.gradle.libraryelements=jar, org.gradle.usage=java-runtime}.
      > Execution failed for JetifyTransform: /tmp/src/mobile_nebula/build/app/intermediates/flutter/debug/libs.jar.
         > Failed to transform '/tmp/src/mobile_nebula/build/app/intermediates/flutter/debug/libs.jar' using Jetifier. Reason: FileNotFoundException, message: /tmp/src/mobile_nebula/build/app/intermediates/flutter/debug/libs.jar (No such file or directory). (Run with --stacktrace for more details.)
           Please file a bug at

So that’s cool, apparently there’s a bug with flutter and I should file a support ticket? Well, probably not. It seems that while build/app/intermediates/flutter/debug/libs.jar indeed doesn’t exist in the repo, build/app/intermediates/flutter/release/libs.jar does, so this appears to possibly be an issue in declaring which build environment is being used.

After some googling I found this flutter issue related to the error. Tldr: gradle’s not playing nicely with flutter. Downgrading could help, but apparently building with the --debug flag also works. I don’t want to build a release version anyway, so this sits fine with me. I run…

flutter build apk --no-shrink --debug

And would you look at that, I got a result!

✓ Built build/app/outputs/flutter-apk/app-debug.apk.


Building was probably the hard part, but I’m not totally out of the woods yet. Theoretically I could email this apk to my phone or something, but I’d like something with a faster turnover time; I need adb.

I install adb via the android-tools package:

yay -S android-tools

Before adb will work, however, I need to turn on USB debugging on my phone, which I do by following this article. Once connected I confirm that adb can talk to my phone by doing:

adb devices

And then, finally, I can install the apk:

adb install build/app/outputs/flutter-apk/app-debug.apk


adb: failed to install build/app/outputs/flutter-apk/app-debug.apk: Failure [INSTALL_FAILED_UPDATE_INCOMPATIBLE: Package net.defined.mobile_nebula signatures do not match previously installed version; ignoring!]

I’m guessing this is because I already have the real nebula app installed. I uninstall it and try again.


Performing Streamed Install

I can open the nebula app on my phone and it works… fine. There’s some pre-existing networks already installed, which isn’t the case for the Play Store version as far as I can remember, so I suspect those are only there in the debugging build. Unfortunately the presence of these test networks causes the app the throw a bunch of errors because it can’t contact those networks. Oh well.

The presence of those test networks, in a way, is actually a good thing, as it means there’s probably already a starting point for what I want to do: building a per-device nebula app with a config preloaded into it.

Further Steps

Beyond continuing on towards my actual goal of adding DNS resolvers to this app, there’s a couple of other paths I could potentially go down at this point.

  • As mentioned, nixify the whole thing. I’m 99% sure the android-studio GUI isn’t actually needed at all, and I only used it for installing the CMake and NDK plugins because I didn’t bother to look up how to do it on the CLI.

  • Figuring out how to do a proper release build would be great, just for my own education. Based on the flutter issue it’s possible that all that’s needed is to downgrade gradle, but maybe that’s not so easy.

  • Get an android emulator working so that I don’t have to install to my phone everytime I want to test the app out. I’m not sure if that will also work for the VPN aspect of the app, but it will at least help me iterate on UI changes faster.

But at this point I’m done for the day, I’ll continue on this project some other time.

If you liked this post, consider checking out other posts in the series:
Next: Building gomobile Using Nix