Android Dropbear

A patch set and script to download and cross-compile Dropbear SSH server for use on Android with password authentication. As the 64-bit binaries don't seem to work reliably, this project is configured to compile 32-biti ARM binaries using the Android NDK toolchain.

Generated binares will all be PIE (position indepedent executable) binaries as it is required since Android 5 (Lollipop).

If you want to build dropbear for Android 4.1 - 4.4, set the ANDROID_API variable at the top to 16.

Building Dropbear for Android

The process consists of just four parts:

1. (Optionally) specify the ANDROID_API value as described above, which defaults to 21
2. (Optionally) specify the version of Dropbear you'd like to download and crosscompile. Open build-android-dropbear.sh and change the value of DROPBEAR_VERSION at the top, which defaults to 2018.76
3. (Optionally) specify the binaries you want to build with the PROGRAMS variable, by default, the server, dropbearkey and dropbearconvert binary will be built
4. Run the build script:

./build-dropbear-android.sh

It will then download the appropriate NDK, Dropbear sources, and compile the binaries. Generated binaries will be outputted to {dropbear-android repo directory}/target/arm

Customizations

Much of the project is pre-configured with sane defaults, but if you'd like to customize the behavior of Dropbear for Android, here are a few tips.

1. To change/configure most options, look in and modify the following files as appropriate:
   a) default_options.h
   b) sysoptions.h
   c) config.h

For instance, to change the port Dropbear runs on or to change the default location in which Dropbear tries to generate keys, edit default_options.h and modify the respective values.

Basic usage

Dropbear for Android adds a few special flags to Dropbear:

• A: signifies Android mode and allows for password authentication in the absence of the crypt() lib
• G: allows us to specify the GID dropbear should run as
• U: allows us to specify the UID dropbear should run as
• N: specify the login username for the session
• T: specify the authentication key for the session

A typical usecase would be:

./dropbear -d /path/to/dropbear_dss_host_key -r /path/to/dropbear_rsa_hostkey -p 10022 -P /path/to/dropbear.pid -R -A -N user -C password -U u0_aXX -G u0_aXX

The above command will run the Dropbear server with password authentication enabled for the user 'user' with password 'password' and will attempt to run as u0_aXX and in that group. More information can be found by issuing:

./dropbear --help

Credits

Big thanks to mkj who has been maintaining Dropbear:
https://github.com/mkj/dropbear

Thanks to NHellfire who's work made the process of getting 2018.76 up and running much easier:
https://github.com/NHellFire/dropbear-android

Thanks to jmfoy for the config.sub and config.guess files:
https://github.com/jfmoy/android-dropbear

Another thank you to the various other repositories out there whose various approches helped lead to this completed project.