usrtax / dropbear-android

Dropbear version 2018.76 cross-compiled to run on Android with password authentication

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-bit binaries using the Android NDK toolchain.

Generated binares will all be PIE (position indepedent executable) binaries as it is required on Android 5 (L/ollipop).

If building for android < 4.1 then before building, issue:

export DISABLE_PIE=1

Building Dropbear for Android

The process consists of just four parts:

1. Specify the version of Dropbear you'd like to download and crosscompile. Open build-android-dropbear.sh and change the value of VERSION at the top, which defaults to 2018.76
2. Build your standalone android toolchain.
   See the android developer site for more info: https://developer.android.com/ndk/guides/standalone_toolchain.html
3. Export your toolchain's location:

export TOOLCHAIN=/path/to/standalone/toolchain

4. Run the build script:

./build-dropbear-android.sh

Generated binaries will be outputted to {android dropbear 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.