списки контроля доступа NFSv4 (NFSv4 Access Control Lists)
Имя (Name)
nfs4_acl - NFSv4 Access Control Lists
Описание (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.
Предупреждение (Warning)
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.
