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NAME | DESCRIPTION | ACL FORMAT | INHERITANCE FLAGS COMMENTARY | A WARNING ABOUT DENY ACES | AUTHORS | CONTACT | SEE ALSO | COLOPHON |
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NFS4_ACL(5) NFSv4 Access Control Lists NFS4_ACL(5)
nfs4_acl - NFSv4 Access Control Lists
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.
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@', which are,
respectively, analogous to the POSIX user/group/other distinctions
used in, e.g., chmod(1).
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 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.
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.
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.
Please send bug reports, feature requests, and comments to
<nfsv4@linux-nfs.org>.
nfs4_getfacl(1), nfs4_setfacl(1), RFC3530 (NFSv4.0), NFSv4.1 Minor
Version Draft.
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
2018-02-02. (At that time, the date of the most recent commit that
was found in the repository was 2017-11-29.) 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 0.3.3, August 2008 NFS4_ACL(5)
Pages that refer to this page: nfs4_getfacl(1), nfs4_setfacl(1)
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