Tiny Image: Dropbear with Busybox

Docker Image

Initialize Ubuntu

Ubuntu 18.04.

Use cloud-init:

packages:
  - git
  - build-essential
  - curl
  - wget
  - nano
  - vim
  - ltrace
  - strace
  - ca-certificates
  - socat

Install requirements

apt install mpc binutils libncurses-dev linux-headers-generic libssl-dev musl musl-tools musl-dev zlib1g-dev libtomcrypt-dev libc6-dev

Reboot.

Clone SRC

Here, we are building rootfs, not bootloader, nor kernel image.

• https://busybox.net/downloads/busybox-1.35.0.tar.bz2
• https://github.com/mkj/dropbear/archive/refs/tags/DROPBEAR_2020.81.tar.gz

Build with non-root user.

Dropbear

$ autoconf; autoheader
$ ./configure --enable-static --with-zlib=/usr/lib/x86_64-linux-gnu
$ cp ./default_options.h localoptions.h # if not exist
$ vim ./localoptions.h
$ CFLAGS="-I/usr/include -ffunction-sections -fdata-sections" LDFLAGS="/usr/lib/x86_64-linux-gnu,-Wl,--gc-sections" make PROGRAMS="dropbear dbclient dropbearkey dropbearconvert scp" MULTI=1 STATIC=1 strip
$ cp dropbearmulti ../

Link script:

mkdir -p /usr/bin
cd /usr/bin
dbmtoollets=(scp dropbearkey dropbearconvert dropbear ssh dbclient)
for i in "${dbmtoollets[@]}"
do 
	ln -s dropbearmulti $i
done

Busybox

Download source code.

Run:

$ make menuconfig
$ LDFLAGS="--static" make -j 2

Link Script:

cd /usr/bin
busybox install -s /usr/bin

Docker

You should be aware that dropbear SSH server still relies on /etc/passwd, /etc/shadow, /etc/group, so you still need a distro as backend.