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Chrooted DropBear :: HowTo


This tutorial is being written to help you install DropBear to a chroot environment.
It covers the below sections:


* Installation of DropBear
* Setup DropBear
* Setup Chroot Enviroment
* Debug Chrooted DropBear


DropBear


Dropbear is a relatively small SSH 2 server and client.
It is an alternative lightweight program of openssh and it is designed for environments with low memory and processor resources, such as embedded systems.


http://matt.ucc.asn.au/dropbear/dropbear.html


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Installation

Download


wget -c http://matt.ucc.asn.au/dropbear/releases/dropbear-0.52.tar.bz2

Extract


tar jxf dropbear-0.52.tar.bz2

Configuration


In our installation we choose: /chroot/dropbear as the root path of our chroot environment
And for educational purpose only, we change the default TCP port of ssh to 2222:


cd dropbear-0.52
./configure --prefix=/chroot/dropbear
sed -i 's/22/2222/g' options.h

Compilation


Simple as that:


make

Installation


The default installation process:


make install


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Keys


The next step is to create dss & rsa keys for dropbear ssh server.


We must create the dropbear's key folder first:


mkdir -pv /chroot/dropbear/etc/dropbear


and then:

/chroot/dropbear/bin/dropbearkey -t dss -f /chroot/dropbear/etc/dropbear/dropbear.dss
/chroot/dropbear/bin/dropbearkey -t rsa -s 4096 -f /chroot/dropbear/etc/dropbear/dropbear.rsa


As you can see, we used the chroot environment path without the need of our distribution path hierarchy.
The DropBear's keys are already installed to our chroot environment at once.


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Shared Libraries


We now have to check all the necessaries shared libraries that dropbear needs to run inside a chroot environment


ldd /chroot/dropbear/sbin/dropbear


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Chroot Environment

Structure


cd /chroot/dropbear/
mkdir -pv dev/pts proc etc lib usr/lib var/run var/log


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Libraries


cp /lib/libutil.so.1 lib/
cp /usr/lib/libz.so.1 usr/lib/
cp /lib/libcrypt.so.1 lib
cp /lib/libc.so.6 lib/
cp /lib/ld-linux.so.2 lib/


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Extra Libraries


This libraries are mostly for the authentication process.


cp /lib/libnss_dns.so.2 lib/
cp /lib/libnss_files.so.2 lib/


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Files


Copy necessaries files from root to chroot


cp /etc/localtime etc/
cp /etc/nsswitch.conf etc/
cp /etc/resolv.conf etc/
cp /etc/host.conf etc/
cp /etc/hosts etc/
touch var/log/lastlog
touch var/run/utmp
touch var/log/wtmp


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Devices


We now must be very careful with the next step of our process.
We have to create all the necessaries devices for dropbear to run


(Remember, we are always on the chroot path – eg. /chroot/dropbear)


mknod dev/urandom c 1 9
chmod 0666 dev/urandom

mknod dev/ptmx c 5 2
chmod 0666 dev/ptmx

mknod dev/tty c 5 0
chmod 0666 dev/tty


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Users


Of course we need to add users to our chroot dropbear setup.
You can choose to add an existence user or you can create a new one.
I prefer to add an existence user (eg. ebal):


grep ^ebal /etc/passwd > etc/passwd
grep ^ebal /etc/group > etc/group
grep ^ebal /etc/shadow > etc/shadow
mkdir home/ebal
chown ebal.ebal !$


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Shell


Every user needs a shell!
But we dont need to install bash, we can simple use busybox.
Busybox is a lightweight shell and combines a lot of common unix utils into a small executable binary file.


cp /etc/shells etc/
sed -i 's/bash/sh/' etc/passwd
cd bin
wget -c http://busybox.net/downloads/binaries/1.16.0/busybox-i686
mv busybox-i686 busybox
chmod 0755 busybox
ln -s busybox sh
cd ../


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Mount Points


This is the most important thing that we (you) have to do properly.
The new environment needs access to terminals (this is necessary for a user to login) and to proc filesystem.


mount -o bind /dev/pts dev/pts/
mount -o bind /proc proc/


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Run 


Finally we are ready to run DropBear from a chroot enviroment:


chroot /chroot/dropbear/ \
	/sbin/dropbear \
	-b /etc/dropbear/dropbear.banner \
	-d /etc/dropbear/dropbear.dss \
	-r /etc/dropbear/dropbear.rsa \
	-m -w -g


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Debug


But if something goes wrong, we can always debug the running process with strace:


strace -f chroot /chroot/dropbear/ \
	/sbin/dropbear \
	-b /etc/dropbear/dropbear.banner \
	-d /etc/dropbear/dropbear.dss \
	-r /etc/dropbear/dropbear.rsa \
	-F -E -m -w -g


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