10.4. Investigating the Incident

Investigating a computer breach is like investigating a crime scene. Detectives collect evidence, note any strange clues, and take inventory on loss and damage. An analysis of a computer compromise can either be done as the attack is happening or post-mortem.
Although it is unwise to trust any system log files on an exploited system, there are other forensic utilities to aid in the analysis. The purpose and features of these tools vary, but they commonly create bit-image copies of media, correlate events and processes, show low level file system information, and recover deleted files whenever possible.
It is also a good idea to record of all of the investigatory actions executed on a compromised system by using the script command, as in the following example: 
 script -q <file-name>
Replace <file-name> with file name for the script log. Always save the log file on media other than the hard drive of the compromised system — a floppy disk or CD-ROM works particularly well for this purpose.
By recording all your actions, an audit trail is created that may prove valuable if the attacker is ever caught.

10.4.1. Collecting an Evidential Image

Creating a bit-image copy of media is a feasible first step. If performing data forensic work, it is a requirement. It is recommended to make two copies: one for analysis and investigation, and a second to be stored along with the original for evidence in any legal proceedings.
You can use the dd command that is part of the coreutils package in Red Hat Enterprise Linux to create a monolithic image of an exploited system as evidence in an investigation or for comparison with trusted images. Suppose there is a single hard drive from a system you want to image. Attach that drive as a slave to the system and then use dd to create the image file, such as the following: 
 dd if=/dev/hdd bs=1k conv=noerror,sync of=/home/evidence/image1
This command creates a single file named image1 using a 1k block size for speed. The conv=noerror,sync options force dd to continue reading and dumping data even if bad sectors are encountered on the suspect drive. It is now possible to study the resulting image file or even attempt to recover deleted files.