Penetration Testing with Raspberry Pi - Second Edition

When researching for cheap and portable computing devices, there are many out there to choose from. This can make the process a little daunting if we don't know what we are looking for. Before we made our purchase, we started with a list of requirements that we felt were important. Some of the requirements we had when purchasing our option was the following:

There are very popular options out there such as, Arduino, Banana Pi, and even Intel with its newly announced Joule board. Each vendor out there is certainly a great option to use. The Intel platform is very powerful, but came in at too high a cost to justify. The Arduino certainly has a lot of community support and hardware options, but lacked some of the power we were looking for. Based on our requirements mentioned earlier, we decided to use a Raspberry Pi. It's the perfect small computer that offers a ton of flexibility, well-loved by the community with a lot of support, and definitely priced right.

Raspberry Pi has been around for some time. So if you do any research, we can see that there are multiple options out there. Here are the models to choose from:

Let's not forget about the littlest Raspberry Pi out there, the Raspberry Pi Zero. This is a very inexpensive computer, typically available for $5. The Zero was released shortly after the Raspberry Pi 2 Model B. It is great for a lot of different projects we may want to build, offering a single-core 1 GHz processor and 512 MB SDRAM, and a cheaper alternative than the Pi 2 or Pi 3 models.

We should keep in mind that the Raspberry Pi Zero is a low-powered device compared to the Pi 3, so our mileage may vary. It's definitely not a direct replacement for the Pi 3 model, especially if we're looking for more hardware resources for our project.

As the models advanced, so did the hardware. The Pi 1 and Pi 2 models are great units, and they are still perfect for embedded projects. But due to the hardware enhancements and the power of the Raspberry Pi 3, Model B was our choice for this project. We wanted to get as much power in this little form factor as possible. The more power we have at our disposal, the better we should expect the pen-testing tools will perform.

Some of the key advantages the Raspberry Pi 3 has over the Raspberry Pi 2 are as follows:

These new additions can definitely help us in our quest to create the perfect portable pen-testing platform, but as we'll see a little later, some of these features are still not quite ready for prime time.

So with all these options to choose from, we selected the Raspberry Pi 3. The power was there with the Quad cores running at 1.2 GHz as well as SDRAM. But that wasn't just it, the Raspberry Pi 3 also offered the flexibility to use new pen-testing tools with the built-in wireless, as well as Bluetooth. We were very excited to see these new options compared with the past versions.

One of the first things we notice about the operating systems we can run on Raspberry Pi is that the list is pretty extensive. There is a lot of support for the hardware. That is yet another reason why we chose the Raspberry Pi hardware versus the other platforms that are available. For the penetration testing software, we chose to use Kali Linux (https://www.kali.org) for our Pi pen-testing box. Kali Linux comes with a ton of security tools already installed, and it is the successor to BackTrack, a well-respected, security-oriented Linux distribution we've used in the past. The Raspberry Pi custom images for Kali Linux are maintained by Offensive Security (https://www.offensive-security.com/).

Kali Linux is not the only great distro (or distribution) the specific blend of Linux operating system and applications) out there. Other great pen-testing distros are available for penetration testing. These other distros include PwnPi, Raspberry Pwn, and PwnBerry Pi. We will talk about these distros a little later in the book, specifically in Chapter 6, Finishing the Attack - Report and Withdraw. But for now, we are going to focus on Kali Linux as our distro of choice because of its huge community and support for most projects we targeted to include in this book.

Purchasing a Raspberry Pi can be a daunting task. There are lots of kits on the Internet to choose from, as well as a ton of accessories available. We went to the CanaKit website (http://www.canakit.com) to look over some of the options. For beginners to the Raspberry Pi, we definitely suggest getting one of many available kits rather than piecing together the platform. Most, such as the CanaKit we selected, come with a lot of the things we will need right away, and will save us some money by buying the bundle versus purchasing the individual components a la carte.

The two main CanaKit offerings for Raspberry Pi 3 are the Ultimate Starter Kit and the Complete Starter Kit. The Ultimate Starter Kit comes with quite a few more accessories than the Complete Starter Kit. These additions include breadboards, a ribbon cable, a General-purpose input/output (GPIO) to Breadboard interface card, just to name a few. The price is only $15.00 more for all the additional stuff, so we went with the Ultimate Starter Kit because we not only found it to be the best deal overall, but also were not sure what future projects we may need the additional hardware for. We ordered ours through Amazon for about $89. Shop around, there are other sites out there as well to order from, and if we were in education, there are sites that provide these kits at significant discounts.

The following image is from CanaKits of the Complete Starter Kit, which is a good option if we were looking for all the major components needed in this book at the lowest price:

As for the Ultimate Starter Kit, the following image from CanaKits shows just how much more is included. This is one of the kits we purchased, just so we had more project options in the future:

Putting together Raspberry Pi 3 for basic operation is a pretty straightforward process. There are a few items that need to be assembled before the initial use. Depending on package we get, we may have some additional parts that can be put together. The first thing we did was installed the heat sinks onto both the Broadcom chip, as well as the LAN chip:

Next, we put the board in the case for protection, since we don't want anything to happen to our Raspberry Pi. There are different case options, and depending on the kit we get, we may get a different color or type. For example, there may be a need to hide our Pi from others. So stealth is sometimes a need or requirement, and the Pi can be hidden in objects or placed in a plain white case to look like it belongs to something else, such as a power adapter. In situations such as this, we may want to consider using a USB power supply to power our Raspberry Pi so as not to draw attention to the power cable running from the hidden Pi to the wall. This is ideal for a true plant scenario. Based on our tests, powering our Pi with the USB power stick gives us about 1 week or so, but our mileage will vary depending on the size of the power stick, as well as how heavily we consume the resources of the Pi over that time period.

After that, the Raspberry Pi 3 was fully assembled, we merely hooked up our monitor via the High-Definition Multimedia Interface (HDMI) interface using the cable provided,which is plugged in our USB mouse and keyboard, and start preparing for the SD card for the operating system.

The first step in installing Kali Linux onto our Raspberry Pi 3 is to prep the microSD card. For Kali Linux, we need to have at least 8 GB of capacity. For best performance, we'll try to make sure that the microSD card is a class 10. We want to make sure that with all of that new power and speed from the Quad core CPU, we won't get slowed down by a slow microSD card. It also helps to ensure that any separately purchased SD cards we may be considering are compatible or suitable, as some SD card brands and product lines work better than others. A great resource for checking this is the eLinux website (http://elinux.org/RPi_SD_cards).

The SD card that comes with our Raspberry Pi may have software on it already. Ours came with NOOBS on it, which is handy if we are not sure what distro we are looking for, as we can choose from several options in the menu within NOOBS. Because we knew we wanted Kali Linux on our Raspberry Pi, we formatted the microSD card to start fresh and installed our own operating system on it. It is always a good idea to copy the existing content of the microSD card to another place before blowing it away. This way, we have the initial version of NOOBS in case we need to use that in the future. With the Ultimate Starter Kit, we received a USB-based microSD adapter. This is a very handy adapter, as most computers do not have a microSD card slot on them, including Apple devices. We plugged in our 32 GB microSD card into the adapter and then into our computer; then, we were ready to rock.

The following image shows the USB-based microSD adapter that we used in our lab:

Getting the right image of Kali is important for proper operation. When we browse https://www.kali.org/, we can find all the options available for Kali Linux. Since we are using an ARM processor on the Raspberry Pi, we will need to install the Raspberry Pi-specific image. The link will redirect us to the Offensive Security site (https://www.offensive-security.com/kali-linux-arm-images/) for a custom Kali image. We should note that there are lots of different ARM options depending on the hardware platform we are using. Since we are using Raspberry Pi 3, we will choose the version that works with that platform. We'll make sure that we note where our image gets downloaded to, so we don't have to go searching later. The ARM image is specifically designed for the Raspberry Pi hardware versus the full-blown image. Again, let's verify that we download the correct image.

Now that we have the image downloaded and ready to install, we need to write it to the microSD card. How we do this will depend on the operating system that we are using. For Windows, we can use the Win32DiskImager. This utility is available at the following URL:

https://sourceforge.net/projects/win32diskimager/

Once the utility is downloaded and opened, we are ready to proceed with imaging the SD card. We will first need to unzip the Kali image. We can use a program such as 7-Zip to unzip the image. When we unzip the file, we will be left with a folder, where we will find the .img file. We then need to select the image file in the Win32DiskImager utility, as well as the correct drive letter for the microSD card we want the image to go on. Let's click on Write, and let it do its job. This process can take some time, so be patient. When it's complete, press the Exit button.

The following screenshot is of the Win32DiskImager utility. It's a great little utility that is very easy to use:

If we are using a Mac or Linux machine, we can use the built-in dd utility to do the writing of the image.

The process on the Mac is as follows:

Booting time on Raspberry Pi 3 is pretty quick due to the nice bump in hardware compared with previous versions. Once Kali Linux boots up, it should be at a login prompt within the GUI. Here, we can log in the first time with the following credentials:

User: root

Password: toor

We'll click on Log In, enter our default credentials, and we should be logged into Kali Linux now, ready to get started. This is where the fun begins!

We know that it's tempting to just dive on into the Kali Linux interface and start running some great security tools. But first, there are some important housekeeping items to take care of. These items are as follows:

First on our list is to change our password. Kali Linux ships with the same default credentials, so we'll want to make sure that no one can log into our box except us. How embarrassing would be it if we had our penetration testing box penetrated by another party. Think of the irony there! To start this process, we need to open up a terminal and enter the passwd command. We'll be asked to type the password in twice to make sure that it is correct:

The other very important thing to do is to update Kali as soon as we can. This will ensure that we have the latest and greatest versions of code and applications. The process for updating Kali is pretty straightforward. We'll simply type the following commands into the CLI.

The upgrade will install all the newest version of the packages installed. The dist-upgrade command will install this plus intelligently updates all the dependencies with the new versions of packages. The dist-upgrade command is certainly not required, but we definitely recommend it:

apt-get update
apt-get upgrade
apt-get dist-upgrade

After this, we reboot our Raspberry Pi 3, and when it comes back, we should have a fully updated operating system, ready for playing around with some pen-testing tools.

Next, we want to resize the partition to use all the available space on our 32GB microSD card. We will show two different ways of doing this. The first way will be via the CLI. The second will take advantage of gparted in GUI.

Starting with CLI, if we run the  df -h  command, the following figure shows we don't have a partition that is close to the size of our microSD card. It is currently only at 6.7G:

The process to expand this partition involves a couple of steps. The following steps will help us unlock the full usable size of our microSD card. It is important to have that extra space for log files, command outputs, or tcpdumps.

We need to make sure that we follow the steps very carefully, as we wouldn't want to erase our root partition. This process uses the  fdisk, parted and resize2fs commands. Here is the process that we used:

Now that we've seen the CLI commands to increase that partition size, let's look at another way of using the GUI interface within Kali Linux. These steps will walk through the process:

We recommend backing up the original system software that came with our Raspberry Pi before formatting it for a Kali Linux installation. Most Raspberry Pi microSD cards come with a form of NOOBS that contains various operating system options from which you can select our primary operating system. Hopefully, we took our own advice earlier and copied the default files that came on our microSD card to another location. If so, we can just copy them back to the SD card. But, if we didn't copy those files off, we can download the NOOBS software again from the following URL if needed:

https://www.raspberrypi.org/downloads/

One of the best use cases for cloning is to create a gold image of Kali Linux for our Raspberry Pi. Once we have that image all set, we can clone that image to use on other microSD cards. In our example, we will copy our gold image to a file named raspberrypi.img.

The cloning process for our SD card is very simple. Many Windows utilities, such as Win32 Disk Imager, which was covered earlier in the chapter, will make an exact copy of the SD card. On a Mac, we can open a command prompt to identify our SD card and type the diskutil list command:

In the preceding screenshot, our microSD card is /dev/disk2. On another system, our microSD card might be different than in this example, so we'll need to make sure to verify naming for each setup. We can clone our card by creating a disk image and saving it to the desktop. We will issue the following command to accomplish this task:

sudo dd if=/dev/disk2 of=raspberrypi.img

The following screenshot shows how we had to enter our password before the command would execute:

It can take 30 minutes or more to clone an SD card. The speed of creating the image will depend on the size and speed of the microSD card, the amount of data on it, the speed of the copying computer, and the block size we specify. In other words, we will need to be patient and let it copy.

Here is a list of some of the common problems that we either ran into or have heard others having:

In this chapter, we covered the various hardware options available as well as why we chose the Raspberry Pi 3. We discussed the various kits that are available, the benefits of buying kits, and their differences. We also talked about the various software platforms out there and why we chose Kali Linux as our software platform of choice.

Next, we covered our approach to getting started with Kali Linux and the Raspberry Pi 3. This included getting the hardware prepped, installing software on the microSD card, and the basics of setting up Kali Linux to ensure its security. At this point, we should have a fully functioning and up-to-date Kali Linux installation running on our Raspberry Pi 3, and be ready to start diving into the tools that will turn this computer into a powerful pen-testing platform.

In the next chapter, we will start diving into some of the essential tools that will allow us to access and use the Raspberry Pi 3 as the pen-testing box. This will include several methods we can use to remotely and securely access the Raspberry Pi 3, configuring various types of interfaces, and setting up a command and control server.