While OpenWrt and embedded system platforms that have been discussed previously are advantageous because of their size, it is clear that their functionality is limited due to their constrained and limited platforms. Another platform that has gained popularity in recent years is the Raspberry Pi. It is an inexpensive, credit card-sized computer that is modular and flexible. It has a built-in wired Ethernet port and four USBs that allow for wireless adapters, 3G or Bluetooth interfaces, and more. The beauty of this platform is that the storage is based on a MicroSD card, so multiple gigs of dumps or rainbow tables are not an issue.

The specifications for the current shipping Raspberry Pi 2 are as follows:

You may have noticed that the Raspberry Pi does not have built-in wireless capabilities specific to our application. You can use a USB wireless adapter to add this functionality. Here, you have several options that will fit the bill; however, you'll want to choose one that is compatible with Kali Linux and meets your size requirements. A discussion of these adapters and their compatibility can be found in the first chapter.

For this example, we have used the popular Alfa AWUS036NH wireless adapter (http://www.alfa.com.tw/products_show.php?pc=34&ps=21), which is an 802.11b/g/n High Gain 2000mW adapter. This allows it to hear more distant RF signals; however, the size of the adapter is nearly the same size as the Raspberry Pi itself, which might not work for your scenario.

The other option is to use a mini USB Wi-Fi adapter. This is barely larger than the USB port itself and provides basic capabilities to your Raspberry Pi.

Ranz Mini USB 150 Mbps 802.11n Wireless Adapter (http://www.ranzindia.net/)

After your hardware has been selected, you can proceed to downloading and installing an image to your MicroSD card for use in the Raspberry Pi. Offensive Security has built an image specific for either the Raspberry Pi (A, B, or B+) or Raspberry Pi 2. They are distributed from their Custom Images Download Page located at https://www.offensive-security.com/kali-linux-vmware-arm-image-download/. Select the version that corresponds to your Raspberry Pi version and carry out the following steps:

The image will now be copied to the raw disk device rather than the block device. This is significantly faster on OSX:

After Kali is successfully installed, you will want to generate new SSH keys as all of the instances that are imaged using the file provided from Offensive Security will have the same keys. To do this, you will run the following commands:

#rm /etc/ssh/ssh_host_*
#dpkg-reconfigure openssh-server
#service ssh restart

This will delete the existing keys, regenerate new keys, and then restart the SSHD server to activate the newly generated keys, as shown in the following screenshot:

You're now ready to proceed!

Throughout this book, we've covered what you can do from here with Kali running on the device. The unique thing about the Raspberry Pi is its portability and size (in that it can be concealed and left on the site while you test from elsewhere). The Raspberry Pi will need to get connected to the Internet in order to enable you to control it remotely. One way would be to get the device plugged into an Ethernet port at the target that you will be testing. This might or might not be possible depending on the environment and the wired network controls, such as port security or MAC authentication that could be enabled on the target's network. Ideally, removing this variable from the equation would be your best course of action. Both the Raspberry Pi and some devices running OpenWrt allow for a USB 3G interface for communication. Though not a topic covered by this book, it is recommended that those pursuing this as long-term tool for your pen-testing toolbox pursue mobile broadband connectivity by either a locally connected 3G/4G interface or by combining it with a personal hotspot to bridge Wi-Fi to the mobile network.