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Network Working Group                              Marius Aamodt Eriksen
Internet Draft                                            February 2004
Document: draft-ietf-nfsv4-acl-mapping-01.txt



               Mapping Between NFSv4 and Posix Draft ACLs



SSttaattuuss ooff tthhiiss MMeemmoo


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
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   "Copyright (C) The Internet Society (2002-2004).  All Rights
   Reserved."

AAbbssttrraacctt


   The NFS (Network File System) version 4[rfc3010bis] (NFSv4) specifies
   a flavor of Access Control Lists (ACLs) that resembles that of Win-
   dows NT's.  ACLs are used to specify fine grained control of access
   to file system objects.  An ACL consists of a number of Access Con-
   trol Entries (ACEs), each specify some level of access for an entity;
   an entity can be a a user, group or a special entity.  The access
   level is described using an access mask, which is a bitmask where
   each bit describes a level of access, for example read, write and
   execute permissions on the file system object.




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   11..  IInnttrroodduuccttiioonn . . . . . . . . . . 3
   22..  NNFFSSvv44 AACCLLss . . . . . . . . . . . . . . 3
   33..  PPOOSSIIXX AACCLLss . . . . . . . . . . . . . . 4
   44..  MMaappppiinngg PPoossiixx AACCLLss
           ttoo NNFFSSvv44 AACCLLss  . . . . . . . . . . 5
   55..  SSeeccuurriittyy
           CCoonnssiiddeerraattiioonnss . . . . . . . 7
   66..  BBiibblliiooggrraapphhyy . . . . . . . . . . 8
   77..  AAcckknnoowwlleeddggmmeennttss  . . . . . 9
   88..  AAuutthhoorr''ss AAddddrreessss . . . .  10
   99..  CCooppyyrriigghhtt  . . . . . . . . . . . . .  10





































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   The NFS (Network File System) version 4 [rfc3010bis] (NFSv4) speci-
   fies a flavor of Access Control Lists (ACLs) that resembles that of
   Windows NT's.  ACLs are used to specify fine grained control of
   access to file system objects.  An ACL consists of a number of Access
   Control Entries (ACEs), each specifying some level of access for an
   entity; an entity can be a a user, group or a special entity.  The
   access level is described using an access mask, which is a bitmask
   where each bit describes a level of access, for example read, write
   and execute permissions on the file system object.


22..  NNFFSSvv44 AACCLLss


   NFSv4 ACLs are rich in nature and expand upon the traditional idea of
   ACLs.  An NFSv4 ACE can be of type ALLOW, DENY, LOG or ALARM; each
   specifies a different action to take should the ACE match a current
   request.  NFSv4 ACLs also have a rich set of access types that com-
   plements the traditional types.  These include appending data to the
   file object, deleting children of the file object, deleting the file
   object, etc [rfc3010bis].

   NFSv4 ACLs are interpreted in a straightforward manner.

   1) Walk through the list of ACEs from the ACL in order

   2) If the "who" (entity) field in the ACE does not match that of the
      requester, the particular ACE is not processed.

   3) Process all ACEs until all the bits in the requested access mask
      have been ALLOWed; once a particular bit has been ALLOWed by an
      ACE, it is no longer considered in further processing.

   4) If a particular access is DENYed (while that bit is still under
      consideration), the request is denied.

   5) If all bits have been ALLOWed, the access is granted, or else
      behavior is undefined.


   NFSv4 ACLs also specify a number of special entities such as OWNER,
   GROUP, and EVERYONE.  These refer to the traditional UNIX mode bits.
   Others include DIALUP, BATCH, and AUTHENTICATED, which have special-
   ized uses.




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   Additionally the NFSv4 ACLs specify a number of flags that can be
   applied to an ACL.  These include a specification on how an ACL on a
   directory may be propagated to newly created files or directories
   inside of said directory.

   It is clear that the granularity of access control that NFSv4 ACLs
   specify is well beyond the standard UNIX capability of expressing
   file system object permissions.


33..  PPOOSSIIXX AACCLLss


   POSIX ACLs refer to POSIX 1003.1e/1003.2c Draft Standard 17 [posix-
   acl], which was meant to specify a POSIX standard for ACLs, but
   unfortunately never materialized.  However, many systems still use
   it, both in the form of it's latest draft as well as earlier drafts.

   POSIX ACLs are simpler than their NFSv4 equivalents.  Each ACE an has
   an entity and the traditional UNIX mode bits that are assigned to the
   particular entity.  The entity may be an arbitrary UID or GID or one
   of a few special entities, the most notable of which is the ACL_MASK
   entity.  POSIX ACLs are also interepreted differently than their
   NFSv4 equivalents.

   POSIX ACLs are interpreted as follows:


   1) Process the ACL_USER_OBJ (equivalent to UNIX file owner) ACE
      first; if the UID of the requester does not match that of the
      ACL_USER_OBJ, then the ACE is ignored.  Otherwise, if the
      requester's access mask is allowed by the access mask of the ACE,
      the request is granted, else the request is denied.

   2) Process all of the ACL_USER ACEs; the entity of these ACEs specify
      a user on the system.  If the UID of the requester does not match
      that of the ACE, then the ACE is ignored.  Otherwise, if the
      requester's access mask is allowed by the access mask of the ACE,
      the request is granted, else the request is denied.

   3) Process the ACL_GROUP_OBJ ACE and all of the ACL_GROUP ACEs; the
      entity of these ACEs specify a group on the system.  If none of
      the GIDs of the requester match the current ACE, the particular
      ACE is ignored.  For any matching ACE, if the the requester's
      access mask is allowed by the ACEs access mask, then access is
      permitted.  If there are matching ACEs, but none allow access,
      then access is denied.




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   4) If the requester's access mask is allowed by the ACL_OTHER ACE,
      then grant access.

   5) Deny access.


   Steps (2) and (3) have an additional criteria; in addition to check-
   ing whether the requested access mask is allowed by the access mask
   in the ACE, the requested bits also have to be in the access mask of
   the special ACE with the ACL_MASK entity.  This allows file owners to
   specify a maximum level of access allowed by any other user or group
   that has any access to the file system object.

   In addition to a regular POSIX ACL, a directory in the file system
   may also have associated with it a default ACL.  This default ACL
   governs the ACL a file system object will be assigned initially when
   it is created as a child of the particular directory.


44..  MMaappppiinngg PPoossiixx AACCLLss ttoo NNFFSSvv44 AACCLLss


   Given the difference in both extensiveness and interpretation of
   POSIX and NFSv4 ACLs, any conversion between the two is difficult.
   However, POSIX ACLs are a subset of NFSv4 ACLs.  Any POSIX ACL can be
   emulated with an NFSv4 ACL using the following mapping.

   The ACE entities are translated as follows.  The non-special entities
   in form of UIDs and GIDs is translated to equivalent strings (a sys-
   tem dependent process, typically done by lookups to /etc/passwd in
   UNIX).  The POSIX ACL_USER_OBJ entity is translated to the "OWNER"
   NFSv4 entity.  Similary, the POSIX ACL_GROUP_OBJ is translated to the
   "GROUP" NFSv4 entity.  The ACL_OTHER entity is translated to the
   "EVERYONE" NFSv4 entity.

   The ACE access mask is translated as follows.  The read bit of the
   POSIX access mask is translated to the logical OR of the
   ACE4_READ_DATA and ACE4_READ_NAMED_ATTRS NFSv4 access mask fields.
   The write bit of the POSIX access mask is translated to the logical
   OR of the ACE4_WRITE_DATA, ACE4_WRITE_NAMED_ATTRIBUTES and
   ACE4_APPEND_DATA NFSv4 access mask fields.  The execute bit of the
   POSIX access mask is translated into the ACE4_EXECUTE  and
   ACE4_READ_DATA NFSv4 acess mask fields.  Note that NFSv4 defines
   ACE4_READ_DATA, ACE4_WRITE_DATA, and ACE4_APPEND_DATA to be equal to
   ACE4_LIST_DIRECTORY, ACE4_ADD_FILE, and ACE4_ADD_SUBDIRECTORY,
   respectively, so this translation makes sense for directories as
   well.  However, on directories the ACE4_DELETE_CHILD field must be
   included in the translation of the POSIX write bit.



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   In addition to the above, the OWNER entity must always be given
   ACE4_WRITE_ACL and ACE4_WRITE_ATTRIBUTES, and all entities must be
   given ACE4_READ_ACL and ACE4_READ_ATTRIBUTES.

   The ACE flag field also has a simple translation.  If the file system
   object is a directory, and the POSIX ACE belongs to a default ACL,
   the "ACE4_INHERIT_ONLY_ACE" flag is set in the NFSv4 ACE.  If the
   entity in the POSIX ACE refers to a group, the "ACE4_IDENTI-
   FIER_GROUP" flag is set in the NFSv4 ACE.

   The POSIX ACL_USER_OBJ ACE is also always given the permission bits
   "ACE4_READ_ACL" and "ACE4_WRITE_ACL."

   Completing the mapping reduces to being able to emulate an ACL_MASK
   and compensate for the difference in interpretation between two ACL
   implementations.

   The difference in interpretation of the two ACL types call for a
   translation scheme.  The scheme follows:

   Every user ACE in the POSIX ACL maps into 2 NFSv4 ACEs; one ALLOW ACE
   which is translated as specified by the above scheme, then a comple-
   menting DENY ACE which is also translated as specified by the above
   scheme, with the exception that the access mask is inverted.  Note
   that the ACL_USER_OBJ ACE is placed first in this list.

   Every group ACE in the POSIX ACL produces a similar pair, but instead
   of being in sequence, all of the ALLOW ACEs are all in sequence, fol-
   lowed by all the DENY ACEs.  The ACL_GROUP_OBJ ACE is placed first in
   both lists.

   Lastly, the POSIX ACL_OTHER ACE is translated into a pair of ACEs as
   in the user ACE case.

   This translation strategy allows us to emulate POSIX ACL interpreta-
   tion in an NFSv4 ACL.

   To handle the special POSIX entity ACL_MASK, we slightly modify the
   above translation:

   With the exception of the "OWNER" and "EVERYONE" ACEs, another ACE is
   prepended to the ACE.  The prepended ACE is a DENY ACE with the same
   entity as the following ALLOW ACE, but with a permission mask set to
   the complement of the POSIX ACL_MASK.

   This method allows us to preserve the real permission bits of each
   ACE should the ACL_MASK be changed.




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   Since this draft deals with the mapping of Access Control Lists, it
   is deeply involved with security.  The body of this document needs to
   address the issue of mapping ACLs in a way as to not disobey the
   intent of or mislead the user.












































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   [rfc3010bis]
   Shepler, S. et. al., "NFS version 4 Protocol", draft-ietf-
   nfsv4-rfc3010bis-05.txt, April 2003.

   http://www.ietf.org/internet-drafts/draft-ietf-
   nfsv4-rfc3010bis-05.txt


   [posixacl]
   IEEE, "IEEE Draft P1003.1e", October 1997 (last draft).

   http://wt.xpilot.org/publications/posix.1e/download.html



































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   The author would like to thank and acknowledge Bruce Fields for his
   careful scrutiny and excellent comments and suggestions.














































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   Address comments related to this memorandum to:

      marius@umich.edu

   Marius Aamodt Eriksen
   University of Michigan / CITI
   535 West William
   Ann Arbor, Michigan

   E-mail: marius@umich.edu

99..  CCooppyyrriigghhtt


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