HFS

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HFS is a generic term used by TSK to refer to the HFS, HFS+, and HFSX file systems. They are commonly found on Apple systems and are supported by TSK (as of 3.1.0). As of TSK 4.0.0, HFS+ extended attributes, resource forks, hard links, symbolic links, and compressed files are also supported. HFS file systems (not HFS+ or HFSX) are not supported by TSK.

Background

HFS+ is the native file system for all versions Mac OS X and was introduced in 1998 to replace HFS. On Macs, HFS+ is often referred to as "Mac OS Extended." HFS (without the +) is rarely seen any more, except as a compatibility wrapper around early HFS+ file systems (from before OS X 10.4).

HFSX is a version of HFS+ that optionally supports case-sensitive path names. It is commonly used on iOS devices (iPhones, etc.).

For more information, refer to:

For reference, the source code to Apple's own implementation of HFS+ for Mac OS X is available at opensource.apple.com under xnu/bsd/hfs/ (this link is for OS X 10.7.4).

HFS in TSK

The SleuthKit supports HFS+ and HFSX. It also supports HFS, but only as a wrapper around an HFS+ file system.

Resource Forks

Files in HFS+ can have two sets of data, called forks: a data fork and a resource fork. The data fork of most files contains what is conventionally considered to be the file's content. With the exception of compressed files, resource forks are not often used in modern versions of Mac OS X. As of TSK 4.0.0, a file's resource fork is visible in its istat output and can be retrieved via icat. In TSK, a file's resource fork is made available as a file attribute called RSRC, number 4353-1, that can be passed to icat for examination. (The data fork is attribute 4352-0, DATA, and is normally the default one used by icat.)

istat also parses the resource fork's contents (if present) and prints a list of the individual resource entries. For each resource, it shows the resource type (four ASCII characters), the numeric ID, the offset (in bytes) within the file's resource fork, the size (in bytes), and the name of the resource (which is optional).

To access an individual resource within the resource fork, use icat on inum-4353-1 and examine the data at the offset and size given by istat.

HFS+ Attributes

HFS+ supports arbitrary named attributes, called extended attributes, on files and directories. Access Control Lists (ACLs) are the most common use for attributes in HFS+. Extended attributes are also used to mark compressed files.

As of TSK 4.0.0, istat shows all of a file's extended attributes. Each extended attribute is loaded as a TSK attribute, with type ExATTR (numerically, 4354-*) and the name of the extended attribute as its TSK attribute name. There is one exception: an attribute that marks a file as compressed, as explained in the next section, will have type CMPF (numerically, 4355).

HFS+ File Compression

In Mac OS X 10.6, Apple introduced file compression in HFS+. Compression is most often used for files installed as part of Mac OS X; user files are typically not compressed (but certainly can be!). Reading and writing compressed files is transparent as far as Apple's file system APIs.

Compressed files have an empty data fork. This means that forensic tools not aware of HFS+ file compression (including TSK before 4.0.0) will not see any data associated with a compressed file!

All compressed files have an extended attribute named com.apple.decmpfs which contains a compression header of 16 bytes. The actual data for compressed files is stored in one of three ways, depending on the size and compressibility of the file:

  1. The data is stored in the resource fork and compressed with zlib. (The resource fork will contain exactly one resource, of type cmpf. Apple's HFS+ implementation prevents compressed HFS+ files from having other resource fork data.) This compression strategy is used for large files.
  2. The data is stored in the com.apple.decmpfs extended attribute, compressed with zlib, immediately after the compression header. This compression strategy is used for mid-sized files (those that compress down to ~3800 bytes or less).
  3. The data is stored, uncompressed, in the com.apple.decmpfs extended attribute immediately after the compression header. This compression strategy is used for very small (or empty) files, effectively storing their data directly in the Attributes tree rather than reserving separate blocks on disk for it.

(The on-disk format allows for other compression strategies to be defined and used, but Mac OS X as of 10.7.4 only uses these three.)

As of TSK 4.0.0, istat will show these details about compressed HFS+ files. In addition, icat will automatically decompress the file data by default.

In cases 2 and 3 (above), TSK will load the uncompressed data of the file into resident DATA attribute 4352-0. In case 1, TSK will make the compressed data in the resource fork available as non-resident RSRC attribute 4353-1. The uncompressed data will be available as a virtual DATA attribute, 4352-0 (appearing as non-resident).

Thus, for any compressed file, icat of the default DATA attribute (4352-0) will show the uncompressed content of the file. To read the raw, compressed data, point icat at the resource fork attribute (4353-1) or at the com.apple.decmpfs attribute as appropriate.

HFS+ Hard Links

In HFS+, all hard linked files are really pointers to "actual" files in a special directory:

/^^^^HFS+ Private Data

Those four leading carets represent null characters (ASCII 0). We call this the metadata directory. The files all have names like

iNode<nnnn>

where <nnnn> is a link number. In practice, the link number is equal to the inode number (or CNID). However, this is not required in the specification, and TSK does not assume that this is so.

In TSK (in the standard build) those null characters, and all other nulls appearing in file names, are mapped to the caret character. Thus, in printed form, you will see carets, and you may enter carets when specifying such a path name.

The HFS+ hard link is a file in the file system Catalog which is marked as a "regular" file, but has some special characteristics that indicate that it is a hard link. One of its metadata fields is a "link number" which can be used to assemble the path name to the actual file which we refer to as the target of the link. The HFS+ file system is supposed to transparently direct all references to the hard link to the target file instead. Such target files are, themselves, never hard links.

HFS+ file systems can also contain hard links to directories, although such links cannot be created by users of Apple software in any ordinary way (they are primarily used for Time Machine backups). They are implemented very similarly to file hard links, except that the targets are differently named, and occur in a different metadata directory. The metadata directory is:

/HFS+ Private Directory Data^

where that last caret is a carriage return character (ASCII 0x0D). In TSK, the fls and istat programs display this character as a caret, but in all other parts of TSK, the character is left as-is. Each hard linked directory has a name like:

dir_<nnn>

where <nnn> is the link number. As with hard linked files, the link number and the inode number (or CNID) are the same in practice, although this is not required by the specification, and TSK does not assume this.

With ordinary use of TSK on an HFS+ file system, you will never have "in hand" the inode number (or CNID) of a hard link. All of the utility programs and libraries that return inode numbers will only return the inode numbers of the link targets. Thus, if you do an istat, icat, or fls of such an inode number, you will see the results for the hard link target. So, the file name will be iNode<nnn> or dir_<nnn> for the appropriate link number. The istat program will tell you that this is a hard link to a file or directory. If you run fls on a directory that contains a hard link (file, or directory), the listing will show the name of the link, but will show the file type and inode number of the target. Here is an example listing showing an fls of a directory followed by an istat of a hard link that it contains.

$ ./fls -o 409640 -f hfs \\\\.\\PhysicalDrive1 39668
r/r 39669:      .com.apple.timemachine.supported
r/r 270:        .DS_Store
d/d 39671:      private
l/l 39681:      User Guides And Information
d/d 39682:      Users
d/d 32974:      usr
l/l 39873:      var


$ ./istat -o 409640 -f hfs \\\\.\\PhysicalDrive1 270
File Path: /^^^^HFS+ Private Data/iNode270
Catalog Record: 270
Allocated
Type:   File
Mode:   rrw-rw-r--
Size:   12292
uid / gid: 501 / 80
Link count:     24

File Name: iNode270
This is a hard link to a file
Admin flags: 0
Owner flags: 0
Has extended attributes
Has security data (ACLs)
File type:      0000
File creator:   0000
Is invisible
Text encoding:  0 = MacRoman
Resource fork size:     0

Times:
Created:        2009-02-26 11:54:19 (Eastern Standard Time)
Content Modified:       2009-03-27 21:53:00 (Eastern Daylight Time)
Attributes Modified:    2009-04-03 14:16:01 (Eastern Daylight Time)
Accessed:       2012-03-02 18:45:20 (Eastern Standard Time)
Backed Up:      0000-00-00 00:00:00 (UTC)

Data Fork Blocks:
202135-202138

Attributes:
Type: ExATTR (4354-2)   Name: com.apple.metadata:_kTimeMachineNewestSnapshot   Resident   size: 50
Type: ExATTR (4354-3)   Name: com.apple.metadata:_kTimeMachineOldestSnapshot   Resident   size: 50
Type: ExATTR (4354-4)   Name: com.apple.system.Security   Resident   size: 68
Type: DATA (4352-0)   Name: DATA   Non-Resident   size: 12292  init_size: 12292

This listing shows that entry .DS_Store occurs in the listed directory and is a hard link. The name of the target is iNode270, and it is a regular file.

If you istat a hard linked directory, such as "usr" (3274) above, you will get a similar result:

$ ./istat -o 409640 -f hfs \\\\.\\PhysicalDrive1 32974
File Path: /.HFS+ Private Directory Data^/dir_32974
Catalog Record: 32974
Allocated
Type:   Folder
Mode:   drwxr-xr-x
Size:   0
uid / gid: 0 / 0
Link count:     24

File Name: dir_32974
This is a hard link to a folder.
Admin flags: 0
Owner flags: 0
Has extended attributes
Has security data (ACLs)
Is invisible
Text encoding:  0 = MacRoman

Times:
Created:        2008-05-31 08:21:00 (Eastern Daylight Time)
Content Modified:       2009-02-26 13:38:14 (Eastern Standard Time)
Attributes Modified:    2009-04-03 14:16:06 (Eastern Daylight Time)
Accessed:       2012-03-02 18:47:40 (Eastern Standard Time)
Backed Up:      0000-00-00 00:00:00 (UTC)

Attributes:
Type: ExATTR (4354-2)   Name: com.apple.metadata:_kTimeMachineNewestSnapshot   Resident   size: 50
Type: ExATTR (4354-3)   Name: com.apple.metadata:_kTimeMachineOldestSnapshot   Resident   size: 50
Type: ExATTR (4354-4)   Name: com.apple.system.Security   Resident   size: 68
Type: ExATTR (4354-5)   Name: com.apple.system.hfs.firstlink   Resident   size:6

The istat of a file that occurs below such a hard linked directory in the file system hierarchy will show a path that begins with the link target, like this:

File Path: /.HFS+ Private Directory Data^/dir_32974/sbin

If you happen to istat the inode number of an actual link (file or directory), then istat will show you the path to the link. However, it will show all other information about the link target. This includes, several lines down, the name of the link target file or directory.

HFS+ Symbolic Links

Symbolic links are regular files that are specially marked, and contain the path of a "target" file as their data. When using fls, symbolic links will show up as type l/l. The listing above contains two examples of symbolic links. You can find the target of the symbolic link by using icat on it.

istat recognizes symbolic links, and, at the end of the listing, will print the target path. Note, that the target of a symbolic link does not need to exist in the file system. In contrast, the target of a hard link must exist in a well-formed HFS+ file system.