Installing Debian GNU/Linux in a chroot on a Samsung Galaxy S II

(Last updated 2011-11-25)

There are numerous how-tos on the web for installing a Debian chroot onto a smartphone running Android, but most of them either give you an image to install on your phone (which doesn't teach you anything) or have you set up the Debian system on your desktop and copy it to the phone. Neither of these options appeal to me, so I've done it my own way and written this document to light the way for others, too.

Assumptions made:

• You have an Android smartphone with access to a root shell via SSH. I run CyanogenMod 7.1 on my Galaxy S II, but any Android system with root SSH access will do.
• You have a basic understanding of Debian and its packaging system. This procedure does require manual invocation of dpkg if you run into problems -- if you don't know what dpkg is or how to use it, this method probably isn't for you.

If not all of these assumptions are true, please don't contact me if something doesn't go right for you when following this how-to.

Preparation

The first thing you need to do is install cdebootstrap on your phone. cdebootstrap is a tool for bootstrapping a Debian system, usually from another Debian system, but in this case we'll be doing it from Android.

So, to install cdebootstrap:

• Download the cdebootstrap-static package for your architecture. Both armel and armhf packages should work on recent Android phones; if you have an older phone, you may need to stick with armel. I'll be using armhf in this tutorial, but you are free to choose whichever architecture suits you best.
• Now, copy the files needed for cdebootstrap to work across to the phone. It doesn't matter where you put them, as long as you can execute the cdebootstrap binary -- so don't put it in /mnt/sdcard, as this is mounted with the noexec mount option. These files are nicely ready for us in a tarball located at ./usr/lib/cdebootstrap/cdebootstrap_0.5.8+b1_armhf.tar.gz inside the package, so you can just pipe the contents of this tarball to ssh and extract it on the phone. I'm going to put it into /data/local/bin/, because that's already in the default $PATH so I don't have to care about it again. $ mkdir /tmp/cdebootstrap && cd /tmp/cdebootstrap $ ar x /path/to/cdebootstrap-static_0.5.8+b1_armhf.deb data.tar.gz $ tar xzOf data.tar.gz ./usr/lib/cdebootstrap/cdebootstrap_0.5.8+b1_armhf.tar.gz | ssh root@$PHONE 'tar xz -C /data/local/bin/' Obviously, replace $PHONE with the hostname of your phone.

Bootstrapping the Debian system

Now that you have a working cdebootstrap on your phone, you can use it to bootstrap Debian. First, however, you will need to create a file system for Debian to live on. It may be possible to create such a file system directly on the SD card, but as I don't really understand Android partitioning, I decided not to try this and risk losing data. (If someone knows of a way to do this, please contact me and let me know!) The reason you need a separate file system for Debian is that Android mounts its own file systems with options like noexec, which would disallow executing all binaries placed on them (thus defeating the purpose of an operating system).

The easiest way to set up a file system is to simply create a file in /mnt/sdcard and format that -- but beware, if you make this file larger than 2 GiB, the FAT file system it lives on will complain and you will lose the image next time you reboot. This happened to me the first time I tried it, and I hadn't made a backup, so I had to do it all over again. :(

So, ssh to your phone, and run the following commands:

# dd if=/dev/zero of=/mnt/sdcard/debian.img bs=1024 count=2000000 # mke2fs -Fj /mnt/sdcard/debian.img # mkdir /debian # mount -t ext3 -o noatime,errors=remount-ro /mnt/sdcard/debian.img /debian