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RedHat 9 (Linux i386) - man page for attr (redhat section 5)

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ATTR(5) 										  ATTR(5)

       attr - Extended attributes

       Extended  attributes  are  name:value pairs associated permanently with files and directo-
       ries, similar to the environment strings associated with a process.  An attribute  may  be
       defined or undefined.  If it is defined, its value may be empty or non-empty.

       Extended  attributes are extensions to the normal attributes which are associated with all
       inodes in the system (i.e. the stat(2) data).  They are often used to  provide  additional
       functionality  to  a filesystem - for example, additional security features such as Access
       Control Lists (ACLs) may be implemented using extended attributes.

       Users with search access to a file or directory may retrieve a  list  of  attribute  names
       defined for that file or directory.

       Extended  attributes are accessed as atomic objects.  Reading retrieves the whole value of
       an attribute and stores it in a buffer.	Writing replaces any previous value with the  new

       Space  consumed	for  extended  attributes  is counted towards the disk quotas of the file
       owner and file group.

       Currently, support for extended attributes is implemented on Linux by the ext2,	ext3  and
       XFS   filesystem  patches,  which  can  be  downloaded  from  http://acl.bestbits.at/  and
       http://oss.sgi.com/projects/xfs/ respectively.

       Attribute names are zero-terminated strings.  The attribute name is  always  specified  in
       the fully qualified namespace.attribute form, eg.  user.mime_type, trusted.md5sum, or sys-

       The namespace mechanism is used to define different classes of extended attributes.  These
       different  classes  exist  for  several	reasons,  e.g.	the  permissions and capabilities
       required for manipulating extended attributes of one namespace may differ to another.

       Currently the user,  trusted,  and  system  extended  attribute	classes  are  defined  as
       described below. Additional classes may be added in the future.

   Extended user attributes
       Extended  user  attributes  may be assigned to files and directories for storing arbitrary
       additional information such as the mime type, character set or encoding	of  a  file.  The
       access permissions for user attributes are defined by the file permission bits.

       The file permission bits of regular files and directories are interpreted differently from
       the file permission bits of special files and symbolic links. For regular files and direc-
       tories  the  file  permission bits define access to the file's contents, while for special
       files they define access to the device described by the special file.   The  file  permis-
       sions of symbolic links are not used in access checks. These differences would allow users
       to consume filesystem resources in a way not controllable by  disk  quotas  for	group  or
       world writable special files and directories.

       For  this  reason,  extended user attributes are disallowed for symbolic links and special
       files, and access to extended user attributes is restricted to the owner and to users with
       appropriate  capabilities for directories with the sticky bit set (see the chmod(1) manual
       page for an explanation of Sticky Directories).

   Trusted extended attributes
       Trusted extended attributes are visible and accessible only to  processes  that	have  the
       CAP_SYS_ADMIN capability (the super user usually has this capability).  Attributes in this
       class are used to implement mechanisms in user space (i.e., outside the kernel) which keep
       information in extended attributes to which ordinary processes should not have access.

   Extended system attributes
       Extended  system  attributes are used by the kernel to store system objects such as Access
       Control Lists and Capabilities.	Read and write access permissions  to  system  attributes
       depend on the policy implemented for each system attribute implemented in the kernel.

       The  kernel and the filesystem may place limits on the maximum number and size of extended
       attributes that can be associated with a file.

       In the current ext2 and ext3 filesystem implementations, all extended attributes must  fit
       on a single filesystem block (1024, 2048 or 4096 bytes, depending on the block size speci-
       fied when the filesystem was created). This limit may be removed in a future version.

       In the XFS filesystem implementation, there  is	no  practical  limit  on  the  number  of
       extended  attributes  associated  with  a  file, and the algorithms used to store extended
       attribute information on disk are scalable (stored either  inline  in  the  inode,  as  an
       extent, or in a B+ tree).

       Since the filesystems on which extended attributes are stored might also be used on archi-
       tectures with a different byte order and machine word size, care should be taken to  store
       attribute values in an architecture independent format.

       Andreas	Gruenbacher,  <a.gruenbacher@computer.org>  and  the  SGI  XFS	development team,

       getfattr(1), setfattr(1).

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