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Build and run dropbear with sftp support for Android

So now that there's no clean way of mounting your new Jelly Bean Android device as a mass-storage device, you're trying to find a simple way of sharing files from/to your Android device. The SSHFS filesystem is a convenient way of accessing a SSH filesystem, but obviously it has some pre-requisites on the target device:

  • An SSH server
  • sftp-server support

There are some SSH servers available on the Android market, or better said "SSH server GUIs". They are all based on dropbear 0.52, which is pretty old, and they actually provide binaries which we really can't say where and how they were built. Also, none of those provide an out-of-the-box working sftp-server, which is mandatory to use SSHFS.

This document aims at explaining how to build your own dropbear and sftp-server binaries, and how to use them.

SSH access to a SG4

Cross-compiling pre-requisites

  • Assuming you're using Debian (wheezy) GNU/Linux, install the following package
# apt-get install emdebian-archive-keyring
  • Then add this repository to /etc/apt/sources.list.d/emdebian.sources.list
# deb squeeze main
  • Install the cross compiling tools for arm
# apt-get install g++-4.4-arm-linux-gnueabi
# apt-get install xapt
  • And then needed libs for OpenSSH to build sftp-server
# xapt -a armel -m zlib1g-dev
# xapt -a armel -m libssl-dev

Build dropbear

$ wget -O- -q|tar jxvf -
  • Apply the following dropbear patch (based on dropbear/Android patch by Jakob Blomer)
$ cd dropbear-2013.58
$ patch < dropbear-2013-58-android.patch
  • Fire up the configure script with the following options
	$ ./configure --host=arm-linux-gnueabi --disable-zlib --disable-largefile --disable-loginfunc --disable-shadow --disable-utmp --disable-utmpx --disable-wtmp --disable-wtmpx --disable-pututline --disable-pututxline --disable-lastlog --disable-syslog CC=/usr/bin/arm-linux-gnueabi-gcc
  • Tune up the options.h file according to your needs or fetch mine

  • Build dropbear with those variables set

$ STATIC=1 MULTI=1 CC=arm-linux-gnueabi-gcc SCPPROGRESS=0 PROGRAMS="dropbear dropbearkey scp dbclient" make strip
  • Push the dropbearmulti binary to a writable directory of your android device
$ adb push dropbearmulti /sdcard/tmp

Build sftp-server

  • Fetch and uncompress OpenSSH (2013/07 latest version is 6.2p2)
$ wget -O- -q|tar zxvf -
$ cd openssh-6.2p2
$ patch < sftp-server-android.patch
  • Fire up the configure script, note that we disable everything we can and ask for a statically linked binary thanks to --with-ldflags=-static.
$ ./configure --host=arm-linux-gnueabi  --without-shadow --disable-largefile --disable-etc-default-login --disable-lastlog --disable-utmp --disable-utmpx --disable-wtmp --disable-wtmpx --disable-libutil --disable-pututline --disable-pututxline CC=/usr/bin/arm-linux-gnueabi-gcc --with-ldflags=-static
  • Build sftp-server the classic way:
$ make sftp-server

And finally send it to your Android device via adb:

$ adb push sftp-server /sdcard/tmp

Prepare your device for dropbear

  • Create needed directories
$ adb shell
$ su
# mkdir -p /data/dropbear/{bin,etc,var}
# cd /data/dropbear
  • Copy previously pushed binaries to dropbear's bin directory
# cp /sdcard/tmp/{dropbearmulti,sftp-server} bin/
  • dropbearmulti is a multi-call binary, it is required to create the actual programs symlinks
# cd bin
# ln -s dropbearmulti dropbear
# ln -s dropbearmulti dropbearkey
# ln -s dropbearmulti dbclient
# ln -s dropbearmulti scp
  • Create needed private and public keys for this device
# bin/dropbearkey -t rsa -f etc/dropbear_rsa_host_key
# bin/dropbearkey -t dss -f etc/dropbear_dss_host_key
# bin/dropbearkey -t rsa -f etc/id_rsa
# bin/dropbearkey -f etc/id_rsa -y > etc/
  • Populate the authorized_keys file
# cat > etc/authorized_keys
<paste here the authorized id_{rsa,dsa}.pub keys>
  • Try dropbear by launching it as a foreground process
# bin/dropbear -A -N shell -U 1000 -G 1000 -R etc/authorized_keys -F
  • In order to be able to use scp, it must be seen on $PATH
# mount -o remount,rw /system
# ln -s /data/dropbear/bin/dropbearmulti /system/xbin/scp
# mount -o remount,ro /system

Run dropbear as a daemon

  • Once everything works as expected, simply start dropbear without the -F flag and with full path to authorized_keys
# bin/dropbear -A -N shell -U 1000 -G 1000 -R /data/dropbear/etc/authorized_keys
  • From now on, you will be able to access your device through SSH, but also through SFTP, thus making is "mountable" using SSHFS.

Useful URLs