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Versions: 00 01 02 03 04 05                                             
Network Working Group                              Marius Aamodt Eriksen
Internet Draft                                            August 2003
Document: draft-ietf-nfsv4-acl-mapping-00.txt

               Mapping Between NFSv4 and Posix Draft ACLs

Status of this Memo

   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
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   "Copyright (C) The Internet Society (2003).  All Rights Reserved."


   The NFS (Network File System) version 4[rfc3530] (NFSv4) specifies a
   flavor of Access Control Lists (ACLs) that apply to file system
   objects.  ACLs specify an access level for a number of entities. The
   NFSv4 ACLs model resembles that of Windows NT.  A POSIX
   draft[posixacl] proposes another, more restrictive ACL model.  Many
   systems implement this proposed standard.  Differing in syntax,
   semantics and extensiveness, it is only feasible to create a correct
   representation for POSIX ACLs with NFSv4 ACLs.  This does not hold
   for an attempt to represent arbitrary NFSv4 ACLs with POSIX ACLs.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  NFSv4 ACLs . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  POSIX ACLs . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Mapping Posix ACLs to NFSv4 ACLs . . . . . . . . . . . . . . 5
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . 8
   6.  Bibliography . . . . . . . . . . . . . . . . . . . . . . . . 9
   7.  Author's Address . . . . . . . . . . . . . . . . . . . . .  10
   8.  Copyright  . . . . . . . . . . . . . . . . . . . . . . . .  10

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1.  Introduction

   The NFS (Network File System) version 4 [rfc3530] (NFSv4) specifies 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 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.  The POSIX Draft Standard
   17[posixacl] proposes a simpler, more limited ACL model.

   Due to the difference in syntax, semantics and extensiveness, it is
   only feasible to correctly represent POSIX ACLs using NFSv4 ACLs, and
   not the other way around.  Thus, we provide such a mapping for use in
   systems that implement NFSv4, already have POSIX Draft Standard ACL
   support and wish to continue to use this interface with NFSv4 and
   interoperate with other such systems.  A client may also use the
   mapping for storing, retrieving and interpreting ACLs on an NFSv4
   server that supports the storage, retrieval and interpretation of
   arbitrary NFSv4 ACLs.

2.  NFSv4 ACLs

   NFSv4 ACLs are rich in nature and expands 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
   complement the traditional types.  These include appending data to
   the file system object, deleting children of the file system object
   and deleting the file system object, etc [rfc3530].

   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; that is, the bits have entries in matching
        ALLOW ACEs that are not flagged ACE4_INHERIT_ONLY_ACE.  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

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        consideration), the request is denied.

   5)   If all bits have been ALLOWed, the access is granted, otherwise
        the access is denied

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

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

   The granularity of access control provided by NFSv4 ACLs is well
   beyond that provided by standard UNIX file system permissions.


   "POSIX ACLs" refer to POSIX 1003.1e/1003.2c Draft Standard 17
   [posixacl].  It was meant to specify a POSIX standard for ACLs, but
   unfortunately never materialized.  However, many systems still use
   it, in forms of its latest and earlier drafts.

   POSIX ACLs are simpler than its NFSv4 equivalent.  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, the request
        is granted if and only if the request access mask is allowed by
        the access mask of the ACE.

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

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   3)   Process the ACL_GROUP_OBJ ACE and all of the ACL_GROUP ACEs; the
        entity of each ACE specify a group on the system.  If none of
        the GIDs of the requester match the entity of 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.

   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 restriction; in addition to
   checking 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.  A default ACL
   governs the ACL a file system object will be assigned initially when
   it is created as a child of the particular directory.

4.  Mapping Posix ACLs to NFSv4 ACLs

   Given the difference in both extensiveness and interpretation of
   POSIX and NFSv4 ACLs, conversion of arbitrary NFSv4 ACLs to POSIX
   ACLs is infeasible.  However, POSIX ACLs are a subset of NFSv4 ACLs.
   Any POSIX ACL can be emulated with an NFSv4 ACL.

   The difference in the format of POSIX ACEs and NFSv4 ACEs can be
   compensated for by a direct mapping.

   The ACE entity is translated as follows.  The non-special entity in
   form of UIDs and GIDs is translated to equivalent strings (a system
   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 access mask is translated as follows.  The read bit of the POSIX

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   access mask is translated to the "ACE4_GENERIC_READ" NFSv4 access
   mask field.  The write bit of the POSIX access mask is translated to
   "ACE4_GENERIC_WRITE" NFSv4 access mask field. the execute bit of the
   POSIX access mask is translated into the "ACE4_GENERIC_EXECUTE" NFSv4
   access mask field.  Defined in [rfc3530], "ACE4_GENERIC_READ" is a
   logical OR of "ACE4_SYNCHRONIZE," "ACE4_READ_ACL,"
   "ACE4_GENERIC_WRITE" is a logical OR of "ACE4_SYNCHRONIZE,"
   logical OR of "ACE4_SYNCHRONIZE," "ACE4_READ_ACL,"
   "ACE4_READ_ATTRIBUTES" and "ACE4_EXECUTE."  These were chosen to
   represent the true meaning of the UNIX mode which are used by POSIX

   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_IDENTIFIER_GROUP" flag is set in the NFSv4 ACE.

   Completing the mapping reduces to being able to emulate an ACL_MASK
   and compensate for the difference in interpretation between to 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
   complementing DENY ACE which is also translated as specified by the
   above scheme, with the exception that the access mask is inverted.
   The ACL_USER_OBJ ACE is placed first in the list.

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

   Lastly, the POSIX ACL_OTHER ACE translates directly into one NFSv4
   ACE at the end of the group ACEs.  This is an allow ACE which is
   translated as specified by the above scheme.

   This translation strategy allows us to emulate POSIX ACL
   interpretation in an NFSv4 ACL.

   To handle the special POSIX entity ACL_MASK, we slightly modify the

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   above translation:

   With the exception of the "OWNER," "GROUP," and "EVERYONE" ACEs,
   another ACE is prepended to every 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.

   The fact that POSIX ACLs use separate ACLs for determining access to
   the file system object and determining inheritance of the ACL needs
   compensation in the translation scheme.  Whenever the server receives
   a request for an ACL, if the file system object in question is a
   directory, the server appends the default ACL to the access ACL.  It
   is then up to the client to separate the two ACLs and translate them
   individually.  Similarly, when the client wishes to set an ACL, it
   either sends the access and default ACLs individually in separate
   requests, or concatenates them.  Again the server should separate
   default and access ACLs, translating and setting them individually.

   The reverse mapping follows from the forward mapping described here.
   The forward mapping obeys a very strict template, and the implementer
   must ensure that when performing the reverse mapping, the ACL
   strictly adheres to this template.

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5.  Security Considerations

   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.

   It is therefore important that ACLs that do not match the above
   scheme are explicitly rejected.  Also, neither optimistic nor
   pessimistic translation between POSIX and NFSv4 ACLs should be
   carried out.  This can potentially lead to unintended granting or
   revoking of priveliges.

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6.  Bibliography

   Shepler, S. et. al., "NFS version 4 Protocol",
   http://www.ietf.org/rfc/rfc3530.txt, April 2003.

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


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7.  Author's Address

   Address comments related to this memorandum to:


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

   E-mail: marius@umich.edu

8.  Copyright

   Copyright (C) The Internet Society (2002, 2003).  All Rights

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published and
   distributed, in whole or in part, without restriction of any kind,
   provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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