Ubuntu Manpage: nfs4_acl - NFSv4 Access Control Lists

Provided by: nfs4-acl-tools_0.3.7-1_amd64

NAME

       nfs4_acl - NFSv4 Access Control Lists

DESCRIPTION

       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

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

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

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

       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

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