Introduzione

Foremost è uno strumento da riga di comando utile per recuperare file basandosi sui loro header, footers e struttura interna dei dati. Questo procedimento è detto data carving. Foremost può operare sia su file immagine, sia su supporti quali unità flash, hard ddisk, schede di memoria ecc. Supporta diversi tipi di filesystem, inclusi Fat16/32, Ext3/4 e NTFS.

Sviluppato origariamente da enti governativi statunitensi quali United States Air Force Office of Special Investigations and The Center for Information Systems Security Studies and Research, è stato successivamente reso di pubblico dominio.

Installazione

Installare il pacchetto foremost.

Utilizzo di base

The most basic way to use foremost is by providing a source to scan for deleted files (it can be either a partition or an image file, as those generated with dd). Let’s see an example. Imagine we want to scan the /dev/sdb1 partition: before we begin, a very important thing to remember is to never store retrieved data on the same partition we are retrieving the data from, to avoid overwriting delete files still present on the block device. The command we would run is:

$ sudo foremost -i /dev/sdb1

By default, the program creates a directory called output inside the directory we launched it from and uses it as destination. Inside this directory, a subdirectory for each supported file type we are attempting to retrieve is created. Each directory will hold the corresponding file type obtained from the data carving process:

output ├── audit.txt ├── avi ├── bmp ├── dll ├── doc ├── docx ├── exe ├── gif ├── htm ├── jar ├── jpg ├── mbd ├── mov ├── mp4 ├── mpg ├── ole ├── pdf ├── png ├── ppt ├── pptx ├── rar ├── rif ├── sdw ├── sx ├── sxc ├── sxi ├── sxw ├── vis ├── wav ├── wmv ├── xls ├── xlsx └── zip

When foremost completes its job, empty directories are removed. Only the ones containing files are left on the filesystem: this let us immediately know what type of files were successfully retrieved. By default the program tries to retrieve all the supported file types; to restrict our search, we can, however, use the -t option and provide a list of the file types we want to retrieve, separated by a comma. In the example below, we restrict the search only to gif and pdf files:

$ sudo foremost -t gif,pdf -i /dev/sdb1

Recupero su disco separato

Negli esempi seguenti si ipotizza che i file da recuperare siano sul disco /dev/sda, che verranno salvati su un altro disco /dev/sdb, ove sarà necessario montare la cartella scrivibile creata a questo scopo, in cui salvare i file recuperati.

1. Creare sul disco /dev/sda la cartella /recovery/foremost:

   sudo mkdir -p /recovery/foremost

2. Montare il disco /dev/sdb nella cartella /recovery:

   sudo mount /dev/sdb /recovery

3. Avviare foremost digitando il comando:

   sudo foremost -i /dev/sda -o /recovery/foremost

   Per avviare foremost su un file immagine sostituire il disco /dev/sda col nome dell'immagine.

4. I file recuperati saranno di proprietà dell'utente root. Per cambiare i permessi digitare il seguente comando:

   sudo chown -R nomeutente:nomegruppo /recovery/foremost

   «nomeutente:nomegruppo» rappresentano rispettivamente il nome dell'utente e il nome del gruppo di appartenenza (generalmente utente e gruppo hanno lo stesso identificativo).

5. Terminato il recupero, smontare il disco esterno:

   sudo umount /dev/sdb

Specifying an alternative destination

As we already said, if a destination is not explicitly declared, foremost creates an output directory inside our cwd. What if we want to specify an alternative path? All we have to do is to use the -o option and provide said path as argument. If the specified directory doesn’t exist, it is created; if it exists but it’s not empty, the program throws a complain:

ERROR: /home/egdoc/data is not empty

• Please specify another directory or run with -T.

To solve the problem, as suggested by the program itself, we can either use another directory or re-launch the command with the -T option. If we use the -T option, the output directory specified with the -o option is timestamped. This makes possible to run the program multiple times with the same destination. In our case the directory that would be used to store the retrieved files would be:

/home/egdoc/data_Thu_Sep_12_16_32_38_2019

File di configurazione

The foremost configuration file can be used to specify file formats not natively supported by the program. Inside the file we can find several commented examples showing the syntax that should be used to accomplish the task. Here is an example involving the png type (the lines are commented since the file type is supported by default):

# PNG (used in web pages) # (NOTE THIS FORMAT HAS A BUILTIN EXTRACTION FUNCTION) # png y 200000 \x50\x4e\x47? \xff\xfc\xfd\xfe

The information to provide in order to add support for a file type, are, from left to right, separated by a tab character: the file extension (png in this case), whether the header and footer are case sensitive (y), the maximum file size in Bytes (200000), the header (\x50\x4e\x47?) and and the footer (\xff\xfc\xfd\xfe). Only the latter is optional and can be omitted.

If the path of the configuration file it’s not explicitly provided with the -c option, a file named foremost.conf is searched and used, if present, in the current working directory. If it is not found the default configuration file, /etc/foremost.conf is used instead.

Aggiungere altri formati di file

By reading the examples provided in the configuration file, we can easily add support for a new file type. In this example we will add support for flac audio files. Flac (Free Lossless Audio Coded) is a non-proprietary lossless audio format which is able to provide compressed audio without quality loss. First of all, we know that the header of this file type in hexadecimal form is 66 4C 61 43 00 00 00 22 (fLaC in ASCII), and we can verify it by using a program like hexdump on a flac file:

$ hexdump -C blind_guardian_war_of_wrath.flac|head 00000000 66 4c 61 43 00 00 00 22 12 00 12 00 00 00 0e 00 |fLaC..."........| 00000010 36 f2 0a c4 42 f0 00 4d 04 60 6d 0b 64 36 d7 bd |6...B..M.`m.d6..| 00000020 3e 4c 0d 8b c1 46 b6 fe cd 42 04 00 03 db 20 00 |>L...F...B.... .| 00000030 00 00 72 65 66 65 72 65 6e 63 65 20 6c 69 62 46 |..reference libF| 00000040 4c 41 43 20 31 2e 33 2e 31 20 32 30 31 34 31 31 |LAC 1.3.1 201411| 00000050 32 35 21 00 00 00 12 00 00 00 54 49 54 4c 45 3d |25!.......TITLE=| 00000060 57 61 72 20 6f 66 20 57 72 61 74 68 11 00 00 00 |War of Wrath....| 00000070 52 45 4c 45 41 53 45 43 4f 55 4e 54 52 59 3d 44 |RELEASECOUNTRY=D| 00000080 45 0c 00 00 00 54 4f 54 41 4c 44 49 53 43 53 3d |E....TOTALDISCS=| 00000090 32 0c 00 00 00 4c 41 42 45 4c 3d 56 69 72 67 69 |2....LABEL=Virgi|

As you can see the file signature is indeed what we expected. Here we will assume a maximum file size of 30 MB, or 30000000 Bytes. Let’s add the entry to the file:

flac y 30000000 \x66\x4c\x61\x43\x00\x00\x00\x22

The footer signature is optional so here we didn’t provide it. The program should now be able to recover deleted flac files. Let’s verify it. To test that everything works as expected I previously placed, and then removed, a flac file from the /dev/sdb1 partition, and then proceeded to run the command:

$ sudo foremost -i /dev/sdb1 -o $HOME/Documents/output

As expected, the program was able to retrieve the deleted flac file (it was the only file on the device, on purpose), although it renamed it with a random string. The original filename cannot be retrieved because, as we know, files metadata is contained in the filesystem, and not in the file itself:

/home/egdoc/Documents └── output

• ├── audit.txt └── flac
  • └── 00020482.flac

The audit.txt file contains information about the actions performed by the program, in this case:

Foremost version 1.5.7 by Jesse Kornblum, Kris Kendall, and Nick Mikus Audit File

Foremost started at Thu Sep 12 23:47:04 2019 Invocation: foremost -i /dev/sdb1 -o /home/egdoc/Documents/output Output directory: /home/egdoc/Documents/output Configuration file: /etc/foremost.conf

File: /dev/sdb1 Start: Thu Sep 12 23:47:04 2019 Length: 200 MB (209715200 bytes)

Num Name (bs=512) Size File Offset Comment

0: 00020482.flac 28 MB 10486784 Finish: Thu Sep 12 23:47:04 2019

1 FILES EXTRACTED

flac:= 1

Foremost finished at Thu Sep 12 23:47:04 2019

Ulteriori risorse

• Pagina ufficiale del progetto

• Voce enciclopedica (in inglese)

• Pagina man

• Guida originae (in inglese)

• Portale recupero dati

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