File Carving with the Foremost tool

File carving is an important subject area in computer forensics. It can be used by a forensics expert to recover hidden files and verify that those files were present on a suspected computer.

The purpose of this article is to explain what file carving is, what it is used for and how it is processed.

We will focus on the Foremost tool, what can be done with this tool and how to get it.

Keywords: file carving ; foremost ; traces

File carving is a field of computer forensics in which we are searching for hidden or deleted files based on raw data from a disk or flash memory.

File systems all have metadata describing the hierarchy of stored documents (folders tree structure, files…) but file carving does not rely on these metadata but rather on raw data to identify and recover files.

A file can be identified by its header and footer for example a jpeg files raw data alays starts with 0xFFD8 and always ends with 0xFFD9.

We call these headers “magic numbers” or “file signatures”. A list of known file signatures can be found here: https://en.wikipedia.org/wiki/List_of_file_signatures .

Foremost is a software designed for data recovering and running on Linux machine in command line, there's no graphical user interface.

It was originally designed by Kris Kendall and Jesse Kornblum (special agents of the U.S. Air Force Office of Special Investigations ) in March 2001.

The tool read and copy parts of the drive into the memory of the computer running the programm. Foremost then process file carving to search in the memory for a file header. When a file header is found the tool writes it in an output file and also writes the data following this header until a footer is found or until the possible size limit of the file is achieved. All the output files are written in an output directory allowing users to consult them.

A nice fact about foremost is that it is desinged for ignoring the file system so he can process file carving whatever the file system used on the investigated disk.

You can find the tool by following this link : http://foremost.sourceforge.net/ .

You can also get it by typing : “apt-get install foremost”in a terminal.

If you have Kali Linux , this tool is already installed and you can use it via command lines in the terminal.

To use foremost you have to follow this syntax :

foremost [-h] [-v] [-d] [-vqWQT] [-b <blocksize>] [-o <dir>] [-t <type>] [-s <num>] [-i<file>]

Here is the list of the possible options (you can access it by typing “man foremost ” in a terminal) :

-h : Displays a help screen and exit

-V : Show copyrhight information and exit

-d : Turn on indirect block detection, this works well for Unix file systems

-T : Time stamp the output directory so you don't have to delete the output dir when running ultiple times.

-v : Enables verbose mode. This causes more information regardig the current state of the program to be displayed on the screen, and is highly recommended

-q : Enables quick mode. In quick mode, only the start of each sector is searched for matching headers. That is, the header is searcherd only up to the length of the longest header. The rest of the sector, usually about 500 bytes, is ignored. This mode makes foremost run considerably faster, but it may cause you to miss files that are embedded in other files. For example, using quick mode you will not be able to find JPEG images embedded in Microsoft Word documents.Quick mode should not be used when examining NTFS file systems. Because NTFS will store small files inside the Master File Table, these files will be missed during quick mode.

-Q : Enables Quiet mode. most error messages will be suppressed.

-w : Enables write audit only mode. No files will be extracted.

-a : Enables rite all headers, perform no error detection in terms of corrupted files.

-b number : Allows you to specify the block size used in foremost. This is relevant for file naming and quick searches. The default is 512. ie. foremost -b 1024 image.dd

-k number : Allows you to specify the cunk size used in foremost. This can imporve speed if you have enough RAM to fit the image in. It reduces the checking that occurs between chunks of the buffer. For examle if you had > 500MB of RAM. ie. foremost - k 500 image.dd

-i file : The file is used as the input file. If no input file is specified or the input file cannot be read then stdin is used.

-o directory : Recovered files are written to the directory directory .

-c file : Sets the configuration file to use If non is specified, the file “foremost.conf” from the current directory is used, if that doesn't exist then “/etc/foremost.conf” is used. The format for the configuration file is described in the default configuration file inculded with this program.

-s number : Skips number blocks in the input file before beginning the search for headers. ie. foremost -s 512 -t jpeg -i /dev/hda1