Preparing a Kali Linux ARM chroot
Although you can download pre-rolled Kali ARM images from our download area, there may be applications which will require building your own custom bootstrapped Kali rootfs for ARM.
The following procedure shows an example of building a fairly generic Kali armhf rootfs. If you wish to build for armel, use that value rather than “armhf” when you export the architecture environment variable.
You’ll need to have root privileges to do this procedure, or the ability to escalate your privileges with the command “sudo su”.
Some Notes on This Procedure
The intention in this article is more to provide a high-level overview of how the build scripts work than an actual manual procedure (although it’s completely possible to walk through this example at the command line). The style mimics that used in the build scripts used to create the pre-rolled images. Specifically, you’ll see a construct in several places that looks like:
This simply amounts to creating a new file, ~/arm-stuff/rootfs/kali-armhf/etc/apt/sources.list, with the contents:
Real-World Custom Kali Linux Builds for ARM Devices
Before we walk through our example, it’s probably good to see how a custom ARM build would actually be accomplished. Typically, to do a local build of an image for, e.g., Raspberry Pi, the process would be something like the following. As an initial one-time set-up, clone the ARM build scripts repository on GitHub and install the build prerequisites:
To do an ARM build, you must enable cross-compilation for your current shell session:
Then simply invoke the build script for the specific platform. So, for a Raspberry Pi build of Kali Linux 2016.2, execute the commands:
The ARM build scripts are all completely self-contained, aside from the initial one-time installation of the build prerequisites. The first time you run one of the ARM build scripts, it is extremely important that you inspect the output for any errors such as missing tools, etc., correct them, and then re-run the script until you get a clean build. Only at that point can you go ahead to make any customizations you want to the basic build script to create the specific “recipe” you’re after.
An Annotated Example of a Generic ARM Build of Kali Linux
The build described here is both minimal and generic. A small number of basic packages are included, and the fuller configuration needed for a real-world platform is omitted for the sake of clarity. Use this example as a reference for understanding what’s going on in the official ARM build scripts and as a high-level guide for writing your own build scripts. It can be successfully followed as a stand-alone tutorial from the command line, but the image produced is not likely to run on any particular device. Use the pre-rolled build scripts, either as-is or with your own customizations, to produce working images for concrete hardware.
Install Required Tools and Dependencies
This is a general set-up task, and should only ever need to be done once:
Enable Cross-Compilation
In order to enable the Linaro cross-compilation, you will need to set these environment variable at the beginning of every session:
Define Architecture and Custom Packages
This is where you define some environment variables for your required ARM architecture (armel vs armhf) and list the packages to be installed in your image. These will be used throughout this article, so make sure to modify them to your needs:
Build the Kali rootfsSet Up the Base rootfs
As our starting point, we’ll create a standard directory structure and use debootstrap to install a base ARM rootfs from the Kali Linux repositories. We then copy over qemu-arm-static, an ARM emulator, from our host machine into the rootfs in order to initiate the 2nd stage chroot:
2nd Stage chroot
First, we’ll chroot into our newly-created base rootfs, use debootstrap a second time to construct our second-stage rootfs, and configure base image settings such as repositories (in /etc/apt/sources.list), host name (in /etc/hostname), default network interfaces and behavior (in /etc/network/interfaces and /etc/resolv.conf), etc. Change these to suit your requirements:
Now, we’re ready to assemble our third-stage chroot.
3rd Stage chroot
This is where your specific customizations come in. Your $packages list is installed, as are keymaps, a default kali user password of “kali” is set, and other configuration changes and fixes are applied:
Here, we’ll create the script to do the third-stage chroot
Now, we’ll run it from within our second-stage chroot:
Manual Configuration Within the chroot
If you need to make any further modifications in your rootfs environment, you can do so by manually chrooting into it with the following command and making any needed changes:
Once you’ve completed your modifications, leave the chroot’ed rootfs with the command
Cleanup
Lastly, we create and run a cleanup script in our chroot to free up space used by cached files and run any other cleanup jobs we may require, and unmount the directories we were using in our rootfs:
Congratulations! Your custom Kali ARM rootfs is located in the ~/arm-stuff/rootfs/kali-$architecture directory. You can now tar up this directory or convert it to an image file for further work.
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