From ForensicsWiki
Revision as of 06:43, 28 November 2013 by Joachim Metz (Talk | contribs) (Flags)

Jump to: navigation, search

Information icon.png

Please help to improve this article by expanding it.
Further information might be found on the discussion page.

File format

The gzip file (.gz) format consists of:

  • a file header
  • optional extra headers, such as the original file name,
  • a body, containing a DEFLATE-compressed payload
  • an 8-byte footer, containing a CRC-32 checksum and the length of the original uncompressed data.

File header

The file header is 10 bytes in size and contains:

Offset Size Value Description
0 2 0x1f 0x8b Signature (or identification byte 1 and 2)
2 1 Compression Method
3 1 Flags
4 4 Last modification time
Contains a POSIX timestamp.
8 1 Extra flags
9 1 Operating system
Value that indicates on which operating system the gzip file was created.

Compression method

Value Identifier Description
0 - 7 Reserved
8 "deflate" zlib compressed data


Value Identifier Description
0x01 FTEXT
0x02 FHCRC
0x08 FNAME The file contains an original file name string.
0x20 Reserved
0x40 Reserved
0x80 Reserved

Extra flags

If compression method is 8 the following extra flags can be defined:

Value Identifier Description
0x02 compressor used maximum compression, slowest algorithm
0x04 compressor used fastest algorithm

Operating System

Value Identifier Description
0 FAT filesystem (MS-DOS, OS/2, NT/Win32)
1 Amiga
2 VMS (or OpenVMS)
3 Unix
5 Atari TOS
6 HPFS filesystem (OS/2, NT)
7 Macintosh
8 Z-System
9 CP/M
10 TOPS-20
11 NTFS filesystem (NT)
13 Acorn RISCOS
255 unknown

External Links