Introduction to Forensics
There are certain aspects to system administration that you can learn only from experience. Computer forensics (among other things the ability to piece together clues from a system to determine how an intruder broke in) can take years or even decades to master. If you have never conducted a forensics analysis on a computer, you might not even know exactly where to start. In this guide, I cover how to use the set of forensics tools in Sleuthkit with its Web front end, Autopsy, to organize your first forensics case.
One of the most common scenarios in which you might want to use forensics tools on a system is the case of a break-in. If your system has been compromised, you must figure out how the attacker broke in so you can patch that security hole. Before you do anything, you need to make an important decision—do you plan to involve law enforcement and prosecute the attacker? If the answer is yes, you should leave the compromised system alone and make no changes to it. Any changes you make post-attack could complicate and taint the evidence, and because of that, many people have a policy of unplugging a system once they detect an attack and leaving it off until law enforcement arrives. Investigators likely will want the complete system, or at least the drives, so they can store it safely; thus, your forensics analysis might end here until your system is returned.
If you do not plan to prosecute the attacker, you still need to set up some policies beforehand on how to respond to an attack. The first policy you should create concerns whether to pull the power from a compromised server immediately. Two main schools of thought exist on this. One school of thought says that because a live server contains valuable data in RAM, such as running processes, logged in users and so forth, that you should try to collect all of that live data first and then power off the server. The opposing school says that once a system is compromised, all parts of the system are potentially compromised and cannot be trusted, including any tools you might use to grab live system data, so you should pull the power from the server immediately. Otherwise, attackers also could have compromised shutdown scripts to remove their tracks. I personally lean more toward the second school of thought and believe that no commands should be run and no changes made to a system once a break-in is discovered.
The second policy you should create beforehand concerns how and whether to image the hard drives on the system and how and when to bring the system back into service. If you cannot tolerate much downtime on the system, you probably will want to create an exact image of the drives to examine elsewhere, and then re-install your operating system on the original drives. Remember, once a system has been compromised, you can no longer trust the system. There could very well be a back door that you missed. It's worth saying again that if you plan to prosecute, you will not be able to bring the system back into service, at least not with the original drives as investigators will need them. If you have the extra space, I recommend creating images of your drives to work from and leaving the originals alone. If you accidentally write to the images, you always can create a fresh image from the original drives. Autopsy can manage raw disk or partition images, so any imaging tool, from dd to Ghost, will work.
For the purposes of this guide, I assume you have created an image of any drives on the system and have stored them on a separate machine that you will use for the forensics analysis. This new machine needs to have both Sleuthkit and Autopsy installed. Some distributions have both Sleuthkit and Autopsy available as precompiled packages, so you can use your distribution's package manager to install them. Otherwise, you can download and compile the tools from the tarballs available on the main project site, sleuthkit.org.
Autopsy works as a Web-based front end to all of the Sleuthkit tools and makes it easy to examine a filesystem without learning each of the different command-line tools. Autopsy also makes it easy to organize multiple forensics analyses into different cases, so you can reference them later. Once Autopsy is installed, get root privileges, and type autopsy into a terminal to start the program. Instructions on Autopsy's settings appear in the terminal, including the default location for evidence (/var/lib/autopsy) and the default port on which it listens (9999). Open a Web browser and type in http://localhost:9999/autopsy to view the default Autopsy page and start your investigation.
From the main Autopsy page, click Open Case to open a case you already have created, or for this example, click New Case. In the New Case page, you can name and describe your case, and you also can provide a list of investigators who will work on the case. Once your case is named and created, you will see the case gallery—a page that simply lists all the cases you have created. If this is your first case, simply click OK to proceed to the Host Gallery. The Host Gallery lists all the servers you are investigating for this case. In our example, only one host was compromised, but often an attacker will move from one compromised host to another, so include as many hosts as you need to investigate in this gallery. As with the Case Gallery, click Add Host to fill out information about the host you are adding.
You will see some interesting fields on the Add Host page relating to time. If the host was set to a time zone different from your local time zone, be sure to put its time zone in the Time Zone field. When you piece together a chain of events, especially across multiple hosts, having correctly synced time is valuable. The Timeskew Adjustment field lets you account for a server with out-of-sync time, and Autopsy automatically adjusts the times to reflect any skew you put in this field.
When you add the host and go back to the Host Gallery, select the host to analyze and click OK to go to the Host Manager page. If this is a new host, the first thing you should do is click Add Image File to add the image you created previously. The image page has only three fields: Location, Type and Import Method. Autopsy expects that the image is available somewhere on the local computer—either actually on the local disk or via an NFS or SMB mount. Type the full file path to the image file in the Location field. The Type field lets you inform Autopsy of the type of image you created. If you imaged the entire drive, select Disk; otherwise, select Partition. If you select Disk, Autopsy scans the partition table for you and lists all the image's partitions.
Autopsy needs the image file to be in its evidence locker in some form, and the Import Method field lets you choose how to put the image file there. If you store all your Autopsy evidence on a separate USB drive, you may want to select Copy, so that a copy of the image stays with the rest of the evidence. If your evidence locker is on your local disk along with the image (which is likely under the default settings), select Symlink or Move, depending on whether you want the image to stay in its original location. Repeat these steps to add any additional images for your host.
Kyle Rankin is VP of engineering operations at Final, Inc., the author of many books including Linux Hardening in Hostile Networks, DevOps Troubleshooting and The Official Ubuntu Server Book, and a columnist for Linux Journal. Follow him @kylerankin
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