Getting Access to the Affected Drives

Again, you have a choice to make: You can leave the affected hard drives in the system and boot off of a CD-ROM or USB memory stick, or you can remove the hard drives and attach them to another system in order to image them.

If you decide to leave the hard drives in their original system, ensuring that you configure the BIOS to boot off of CD-ROM or memory stick and not the hard drive is imperative.

Also, note that you cannot guarantee you will not write to the hard drives, for example, by making a mistake during drive imaging.

Booting with BackTrack

After you shut down the system, one popular alternative is to begin the investigation by booting to a Live Linux CD. A Live system will reveal the evidence disk simply as data – you won't have to worry about any scripts or trojaned utilities the attackers might have left on the system to cover their tracks.

Like many software choices in Linux, the choice of a Live distro leaves you both blessed and cursed by an abundance of options. Almost all bootable installation media (Red Hat, Debian, etc.) have a recovery or emergency mode you can use to access the system. Live Linux distributions – on CD or USB media – are also a possibility.

I recommend the security-oriented BackTrack Live CD distribution (Figure 1) [4]. Some of the advantages of BackTrack are:

• BackTrack supports multiple file system types, including EXT2, EXT3, VFAT, NTFS, and more.
• BackTrack is under active development and is specifically aimed at penetration testing.
• BackTrack includes forensics utilities such as the enhanced disk copy tool, dcfldd [5].

Figure 1: The BackTrack Live Linux distribution specializes in forensics.

To download BackTrack, you're going to need a BitTorrent client (the software is distributed via BitTorrent); I used the current beta version of BackTrack with no problems. Simply go to the BackTrack download page, hit the torrent, and wait for it to download. Then you can use any standard CD burning software to burn BackTrack to a CD-ROM.

Alternatively, you can download a USB version and copy it onto a USB memory stick; of course, this only works if the system you are interacting with is recent enough to support booting off of a USB memory stick.

After you have a copy, you can start BackTrack by simply booting off the CD-ROM or USB stick.

Creating a Drive Image

Creating a complete image of a partition or a drive is relatively simple in Linux. The dd tool, which is included on the BackTrack CD, can image a partition or an entire hard drive.

To pipe the contents of the image to another system and reduce the need to disassemble systems to get the hard drives out, you can combine dd with tools such as nc or SSH. Simply run the command

dd if=/dev/hda1 bs=2k | nc 192.168.0.1 9000

which will image the first partition on the first IDE hard drive (hda1) and send the data to TCP port 9000 on the host 192.168.0.1. On the host 192.168.0.1, simply run nc in listening mode and dump the output to a file:

nc -l 9000 > driveimage.dd

dcfldd includes a number of capabilities useful for digital forensics work. Unlike dd, dcfldd has the ability to create MD5 and SHA256 hashes of the data, thus allowing you to verify the data more easily, and the ability to split files up easily by specifying a split size. (Working with a single 250GB file may prove unwieldy for some people.) For example, the following

dcfldd if=/dev/hda1 hash=md5,sha256 hashwindow=10G md5log=md5.txt sha256log=sha256.txt hashconv=after bs=512 conv=noerror,sync split=10G splitformat=aa of=driveimage.dd

will image hda1, create an MD5 and a SHA256 hash of each 10GB block of data (and write them to a log), continue reading if it encounters errors, and pad input blocks if needed. Finally, the command splits the data into 10GB files with filenames ending in aa, ab, and so on.

One thing to remember is that unless you are sending the data over a secure, trusted network, you're going to need to be able to prove it hasn't been altered in transit. This can be done by making cryptographic hashes (MD5 and SHA256) and safely copying them off the system, for example, via memory stick, or by encrypting the data in transit by using a tool such as OpenSSH to create a tunnel.