NTFS

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NTFS (New Technology File System) is Windows NT's native file system. It is not only based on HPFS, but also supports security features such as access control. Since Windows NT is entirely unicode, NTFS is a unicode filesystem, with each character (e.g. in names) being 16-bits instead of 8-bits.

About

NTFS doesn't only add security features to HPFS. In NTFS, there is a lot more built-in redundancy. For example: in HPFS, wiping out a sector in the wrong place can render the entire volume inaccessible. Support for multiple hard-links to a file (before NTFS, the only easy access was through the POSIX subsystem, but Windows 2000 (NT 5) added this to Win32 as well) was also added.

NTFS supports an arbitrary number of file forks (much like Mac OS, except Mac OS always has exactly 2 forks for each file).

HPFS decrees that a cluster is always 512 bytes long and a cluster is always one sector. For the sake of performance and compatibility with some (especially Japanese) machines, NTFS allows sectors of different sizes. It also supports clusters of more than one sector, which can be beneficial on performance.

In short, NTFS' most significant changes:

• Better and more security.
• Multiple hard-links to one file.
• An arbitrary number of forks.
• Variable cluster and sectors sizes (usually resulting in better performance).

Implementation

NTFS is probably one of the most difficult file system to deal with, especially because of the lack of hacking experience and reliable documents. A read-only stable driver can be found in the Linux source code base since kernel 2.4, while an experimental read-write driver is coming with linux 2.6.

The NTFS-3G project apparently has a read/write implementation for Linux/FreeBSD/BeOS that is currently in beta development status.

Structure

The NTFS format is built around "file" tables that allow both pre-defined and custom attributes to be stored and read by the operating system.

The NTFS boot sector is similar to other file systems, like FAT.

Field	Type	Length
JMP	byte[3]	3
OEM System	char[8]	8
Bytes Per Sector	unsigned short	2
Sectors Per Cluster	byte	1
Reserved Sector Count	unsigned short	2
Table Count	byte	1
Root Entry Count	unsigned short	2
Sector Count	unsigned short	2
Media Type	byte	1
Sectors Per Table	unsigned short	2
Sectors Per Track	unsigned short	2
Hidden Sector Count	unsigned int	4
Sector Count (32-bit)	unsigned int	4
Reserved	unsigned int	4
Total Sectors (64-bit)	unsigned long	8

This is followed immediately by a NTFS specific header.

Field	Type	Length
Master File Table Cluster	unsigned long	8
Master File Table Mirror Cluster	unsigned long	8
Clusters Per Record	signed byte	1
Reserved	byte[3]	3
Clusters Per Index Buffer	signed byte	1
Reserved	byte[3]	3
Serial Number	unsigned int	4
Checksum	unsigned short	2

Using the "Master File Table Cluster" and "Sectors Per Cluster" values, you can find the Master File Table. This table contains entries for every object in the file system, including files, folders, and the tables themselves. The size of each record in the Master File Table can be calculated using the "Clusters Per Record" and "Sectors Per Cluster" fields from the boot sector.

Each record starts with the same header structure.

Field	Type	Length
Record Type	char[4]	4
Update Sequence Offset	unsigned short	2
Update Sequence Length	unsigned short	2
Log File Sequence Number	unsigned long	8
Record Sequence Number	unsigned short	2
Hard Link Count	unsigned short	2
Attributes Offset	unsigned short	2
Flags	unsigned short	2
Bytes In Use	unsigned int	4
Bytes Allocated	unsigned int	4
Parent Record Number	unsigned long	8
Next Attribute Index	unsigned short	2
Reserved	unsigned short	2
Record Number	unsigned int	4

The remainder of the file record contains additional tables and data for this record. The "Attributes Offset" field contains the byte offset (from the start of the record) of the beginning of the attribute list for this record.

Attributes have a variable length, but always start with the same sequence.

Field	Type	Length
Attribute Type	unsigned int	4

If the "Attribute Type" field contains the value 0xffffffff, this marks the end of the attribute list. Otherwise, the attribute sequence continues with the length of the attribute table.

Field	Type	Length
Attribute Length	unsigned int	4

This length value defines the total length of the attribute record, including the "Attribute Type" and "Attribute Length" fields.

Links

• The Linux NTFS project.
• NTFS-3G Read/Write Drivers For Linux/FreeBSD/BeOS
• GIT Linux
• Apple Open Source NTFS site