nfs4_acl(5) — Linux manual page

NAME | DESCRIPTION | ACL FORMAT | INHERITANCE FLAGS COMMENTARY | A WARNING ABOUT DENY ACES | AUTHORS | CONTACT | SEE ALSO | COLOPHON

NFS4_ACL(5)            NFSv4 Access Control Lists            NFS4_ACL(5)

NAME         top

       nfs4_acl - NFSv4 Access Control Lists

DESCRIPTION         top

       An ACL is a list of permissions associated with a file or
       directory and consists of one or more Access Control Entries
       (ACEs).  NFSv4 ACLs provide finer granularity than typical POSIX
       read/write/execute permissions and are similar to CIFS ACLs.

       A sample NFSv4 file ACL might look like the following (see the
       ACL FORMAT section for detailed information):

              A::OWNER@:rwatTnNcCy
              A::alice@nfsdomain.org:rxtncy
              A::bob@nfsdomain.org:rwadtTnNcCy
              A:g:GROUP@:rtncy
              D:g:GROUP@:waxTC
              A::EVERYONE@:rtncy
              D::EVERYONE@:waxTC

       Some observations:

       -  In the example output above, the user `alice@nfsdomain.org'
          has the equivalent of "read" and "execute" permissions,
          `bob@nfsdomain.org' has "read" and "write", and both `GROUP@'
          and `EVERYONE@' have "read".

       -  NFSv4 ACLs are "default-deny"; that is, if a permission is not
          explicitly granted by an Allow ACE, it is denied.  Because of
          this, the two Deny ACEs above are superfluous and could be
          excluded by the server.  See the A WARNING ABOUT DENY ACES
          section for more information.

       -  NFSv4 servers may return an ACL slightly different than one
          you set.  For example, a server that always allows reading the
          attributes of a file may silently turn on the read-attributes
          permission, and a server that does not support separate write-
          data and append-data permissions, e.g., may choose to turn off
          both if you set only one.  In extreme cases the server may
          also reorder or combine ACEs.  As a general rule, however,
          servers will attempt to ensure that the ACLs they return are
          no more permissive than the ones you set.

ACL FORMAT         top

       An NFSv4 ACL is written as an acl_spec, which is a comma- or tab-
       delimited string consisting of one or more ace_specs.  A single
       NFSv4 ACE is written as an ace_spec, which is a colon-delimited,
       4-field string in the following format:

              type:flags:principal:permissions

   ACE TYPES:
       There are four types of ACEs, each represented by a single
       character.  An ACE must have exactly one type.

       A      Allow - allow principal to perform actions requiring
              permissions.

       D      Deny - prevent principal from performing actions requiring
              permissions.

       U      Audit - log any attempted access by principal which
              requires permissions.  Requires one or both of the
              successful-access and failed-access flags.  System-
              dependent; not supported by all servers.

       L      Alarm - generate a system alarm at any attempted access by
              principal which requires permissions.  Requires one or
              both of the successful-access and failed-access flags.
              System-dependent; not supported by all servers.

   ACE FLAGS:
       There are three kinds of ACE flags: group, inheritance, and
       administrative.  An Allow or Deny ACE may contain zero or more
       flags, while an Audit or Alarm ACE must contain at least one of
       the successful-access and failed-access flags.

       Note that ACEs are inherited from the parent directory's ACL at
       the time a file or subdirectory is created.  Accordingly,
       inheritance flags can be used only in ACEs in a directory's ACL
       (and are therefore stripped from inherited ACEs in a new file's
       ACL).  Please see the INHERITANCE FLAGS COMMENTARY section for
       more information.

       GROUP FLAG - can be used in any ACE

       g      group - indicates that principal represents a group
              instead of a user.

       INHERITANCE FLAGS - can be used in any directory ACE

       d      directory-inherit - newly-created subdirectories will
              inherit the ACE.

       f      file-inherit - newly-created files will inherit the ACE,
              minus its inheritance flags.  Newly-created subdirectories
              will inherit the ACE; if directory-inherit is not also
              specified in the parent ACE, inherit-only will be added to
              the inherited ACE.

       n      no-propagate-inherit - newly-created subdirectories will
              inherit the ACE, minus its inheritance flags.

       i      inherit-only - the ACE is not considered in permissions
              checks, but it is heritable; however, the inherit-only
              flag is stripped from inherited ACEs.

       ADMINISTRATIVE FLAGS - can be used in Audit and Alarm ACEs

       S      successful-access - trigger an alarm/audit when principal
              is allowed to perform an action covered by permissions.

       F      failed-access - trigger an alarm/audit when principal is
              prevented from performing an action covered by
              permissions.

   ACE PRINCIPALS:
       A principal is either a named user (e.g., `myuser@nfsdomain.org')
       or group (provided the group flag is also set), or one of three
       special principals: `OWNER@', `GROUP@', and `EVERYONE@':

       OWNER@ matches the file's owner

       GROUP@ matches the file's group

       EVERYONE@
              always matches.  (Note this is different from the "other"
              used in mode bits or POSIX ACLs, which does not match the
              file's owner or group.)

   ACE PERMISSIONS:
       There are a variety of different ACE permissions (13 for files,
       14 for directories), each represented by a single character.  An
       ACE should have one or more of the following permissions
       specified:

       r      read-data (files) / list-directory (directories)

       w      write-data (files) / create-file (directories)

       a      append-data (files) / create-subdirectory (directories)

       x      execute (files) / change-directory (directories)

       d      delete - delete the file/directory.  Some servers will
              allow a delete to occur if either this permission is set
              in the file/directory or if the delete-child permission is
              set in its parent directory.

       D      delete-child - remove a file or subdirectory from within
              the given directory (directories only)

       t      read-attributes - read the attributes of the
              file/directory.

       T      write-attributes - write the attributes of the
              file/directory.

       n      read-named-attributes - read the named attributes of the
              file/directory.

       N      write-named-attributes - write the named attributes of the
              file/directory.

       c      read-ACL - read the file/directory NFSv4 ACL.

       C      write-ACL - write the file/directory NFSv4 ACL.

       o      write-owner - change ownership of the file/directory.

       y      synchronize - allow clients to use synchronous I/O with
              the server.

INHERITANCE FLAGS COMMENTARY         top

       Inheritance flags can be divided into two categories: "primary"
       (file-inherit and directory-inherit); and "secondary" (no-
       propagate-inherit and inherit-only), which are significant only
       insofar as they affect the two "primary" flags.

       The no-propagate-inherit and inherit-only flags can be tricky to
       remember: the former determines whether or not a new child
       directory's inherited ACE is itself heritable by a grandchild
       subdirectory; the latter determines whether or not a heritable
       ACE affects the parent directory itself (in addition to being
       heritable).  They can be used in-tandem.

       When a subdirectory inherits an ACE from its parent directory's
       ACL, this can happen in one of two different ways, depending on
       the server implementation:

       -  In the simple case, that exact same ACE is set in the
          subdirectory's ACL.

       -  In the other case, two different ACEs will instead be set in
          the subdirectory's ACL: one with all inheritance flags
          removed, and one with the inherit-only flag added.  The former
          is the "effective" inherited ACE (used in the subdirectory's
          own permissions checks); the latter is the "heritable"
          inherited ACE (when the subdirectory has directories created
          within it, they inherit it).  This approach makes it easier to
          modify access rights to the subdirectory itself without
          modifying its heritable ACEs. The Linux kernel server uses
          this approach.

A WARNING ABOUT DENY ACES         top

       Deny ACEs should be avoided whenever possible.  Although they are
       a valid part of NFSv4 ACLs, Deny ACEs can be confusing and
       complicated.  This stems primarily from the fact that, unlike
       POSIX ACLs and CIFS ACLs, the ordering of ACEs within NFSv4 ACLs
       affects how they are evaluated.

       First, it is important to note that (despite some unfortunate
       ambiguity in RFC3530) NFSv4 ACLs are "default-deny" in practice.
       That is, if a permission is not explicitly granted, it is denied.

       In general, when a principal is attempting to perform an action
       over NFSv4 which requires one or more permissions, an access
       check is performed.  The NFSv4 ACL (assuming one is present) is
       evaluated ACE-by-ACE until every one of those permissions has
       been addressed, or until the end of the ACL is reached.  If every
       requisite permission was granted by Allow ACEs and was not
       forbidden by Deny ACEs (see next paragraph), the action is
       allowed to proceed.  Otherwise, the action is forbidden.

       Note that each requisite permission is only addressed once --
       that is, after a permission has been explicitly Allowed or Denied
       once during an access check, any subsequent ACEs in the ACL which
       affect that permission are no longer considered.  This often
       introduces problematic ordering issues when Deny ACEs are
       present.

       Additionally, in some cases Group-Deny ACEs can be difficult (if
       not impossible) to enforce, since a server might not know about
       all of a given principal's memberships in remote groups, e.g.

       Because NFSv4 ACLs are "default-deny", the use of Deny ACEs can
       (and should) be avoided entirely in most cases.

AUTHORS         top

       Tools for viewing and manipulating NFSv4 ACLs, nfs4_getfacl and
       nfs4_setfacl, were written by people at CITI, the Center for
       Information Technology Integration (http://www.citi.umich.edu ).
       This manpage was written by David Richter and J. Bruce Fields.

CONTACT         top

       Please send bug reports, feature requests, and comments to
       <nfsv4@linux-nfs.org>.

SEE ALSO         top

       nfs4_getfacl(1), nfs4_setfacl(1), RFC3530 (NFSv4.0), NFSv4.1
       Minor Version Draft.

COLOPHON         top

       This page is part of the nfs4-acl-tools (NFSv4 ACL tools)
       project.  Information about the project can be found at 
       ⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩.  If you have a
       bug report for this manual page, see
       ⟨http://linux-nfs.org/wiki/index.php/Main_Page⟩.  This page was
       obtained from the project's upstream Git repository
       ⟨git://git.linux-nfs.org/projects/bfields/nfs4-acl-tools.git⟩ on
       2021-08-27.  (At that time, the date of the most recent commit
       that was found in the repository was 2020-06-04.)  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to man-pages@man7.org

Linux                  version 0.3.4, August 2018            NFS4_ACL(5)

Pages that refer to this page: nfs4_getfacl(1)nfs4_setfacl(1)