Android Dropbear 2018.76

A script & patch to cross-compile Dropbear SSH server/client for use on Android with password authentication. Since the 64-bit binaries don't seem to work reliably, this project is configured to compile a single muti-purpose 32-bit binary using a standalone Android r11c NDK toolchain.

Generated binary will be PIE (position indepedent executable) as required on Android 5 (L/ollipop) and above.

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

export DISABLE_PIE=1

Building Dropbear for Android

The process consists of just 3 parts:

1. Git clone this repo:

git clone https://github.com/Geofferey/dropbear-android.git

2. Change to the direcotry:

cd android-dropbear

3. Run the build script:

./build.sh

Generated binary will be outputted to {android dropbear repo directory}/target/arm/dropbearmulti

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.

2. It is also possible to change behavior of build script by exporting vars before execution.
   a) to build different version export VERSION=
   b) to build multiple binaries export MULTI=1
   c) to build non static binaries export STATIC=0
   d) to select programs to output export PROGRAMS=
   available programs dbclient dropbear dropbearconvert dropbearkey scp
   e) to use another toolchain export TOOLCHAIN=/path/to/tc

Build Time VARs

Here is a list of variables to be exported before running the build in order to customize the outputted bin(s) and behavior of script. The values below are the defaults when unspecified.

• Starts build from unpatched unmodified copy of source
  CLEAN=0

• Defines the default listening port for the Dropbear server
  DEFAULT_PORT=10022

• Define the PATH(s) to binary executables on Android
  DEFAULT_PATH

• Disables build of PIE binary
  DISABLE_PIE

• Specifies the directory dropbear stores and loads host keys from
  HOSTKEYS_DIR=./

• Run the build in interactive mode to allow for modifications (Press Return to Continue...)
  INTERACTIVE=1

• Default directory to start in upon successful login
  LOGIN_DIR=/data/local

• Runs 'make clean' before compilation
  MAKE_CLEAN=1

• Outputs multiple binaries instead of combined linkable binary
  MULTI=0

• Path to store process ID
  PID_PATH=

• Programs to build
  PROGRAMS=dropbear dbclient dropbearconvert dropbearkey scp

• Path to sftp-server
  SFTPSERVER_PATH=/usr/libexec/sftp-server

• Path to the ssh client
  SSHCLI_PATH=/usr/bin/dbclient

• Build statically linked binary
  STATIC=1

• Path to the toolchain
  TOOLCHAIN=/dropbear-android/android-rc11-standalone-toolchain

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

Contributions

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

Thanks to wolfdude & serasihay @XDA for netbsd_getpass.c implementation:
https://forum.xda-developers.com/nexus-7-2013/general/guide-compiling-dropbear-2015-67-t3142412/page2
https://forum.xda-developers.com/nexus-7-2013/general/guide-compiling-dropbear-2016-73-t3351671

Thanks to yoshinrt for openpty.patch fix:
https://github.com/yoshinrt/dropbear-android

Another thank you to the various repositories out there whose contributions helped lead to the completion of this project.