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NFS(5) BSD File Formats Manual NFS(5)
NAME
NFS -- Network File System
SYNOPSIS
NFS
DESCRIPTION
NFS is the standard UNIX file sharing protocol suite. MacOS supports
version two, NFSv2 (RFC 1094), version three, NFSv3 (RFC 1813), and in
addition for the client, version four, NFSv4 (RFC 3530). NFSv2 and NFSv3
also rely on adjunct protocols for mounting, Mountd (RFC 1094, RFC 1813)
and for locking, NLM (OpenGroup XNFS). NFSv4 subsumes both mount and
locking operations in to it's protocol. MacOS systems support TCP over
IPv4 and IPv6. In addition for versions two and three of the protocol,
UDP is supported over IPv4 and IPv6, however its use is discouraged. Ver-
sion two of the protocol is considered legacy and should only be used if
higher versions of the protocol are unavailable.
MacOS Extended Attribute Considerations
MacOS makes heavy use of extended attributes, resource forks, and possi-
bly ACLs. NFSv2 and NFSv3 do not support these operations over the wire.
The getting and setting of ACLs are not supported but extended attributes
and resource forks for MacOS clients are supported by storing extended
attributes and resource forks in an "Apple Double" (also known as a dot
under bar file) files on the server. On the other hand NFSv4 can option-
ally support ACLs. And can optionally support the storing of resource
forks and extended attributes as NFSv4 named attributes. Previous ver-
sions of NFSv4 on MacOS would use NFSv4 named attributes if supported by
the server for storing extended attributes and resource forks, however
this version of NFSv4 defaults to using "Apple Double" files. If a server
exports files systems with both NFSv3 and NFSv4 (the usual default case)
and supports named attributes as well, then there is a potential for data
loss, since changing an extended attribute or resource fork from an NFSv3
client is not visible to an NFSv4 client and visa versa. If the server
is known to only have NFSv4 clients or is only exporting file systems
with NFSv4 then MacOS clients should use the namedattr option. See
mount_nfs(8). If converting from NFSv3 to NFSv4 only with named
attribute support, the following procedure can be use to convert the
"Apple Double" files to NFSv4 named attributes.
Be sure that the file system on the server has no other MacOS NFS mounts.
Then allow one MacOS NFSv4 client that has been mounted with the
namedattr mount option (mount -o vers=4,namedattr), to mount the
export(s) on the server. Then at the mount point on the client run the
dot_clean -m command. This will read the "Apple Double" files and con-
vert them to native extended attributes on the server and then remove the
"Apple Double" file.
MacOS ACL support.
ACLs (access control list) are enforced at the server not the client. As
mentioned above NFSv3/NFSv2 do not support ACLs. So it is possible to get
access or permission errors even though the BSD permission bits indicate
that a file system operation should succeed. MacOS NFSv4 clients can
optionally support ACLs if the acl or aclonly options are given at mount
time. The reason that ACLs are not enabled by default is that there
interpretation can be different from one server to another. Particularly
in the way BSD mode bits are interpreted. A simplified explanation for
MacOS, is that it interprets ACLs by running over the list of ACEs
(access control entries) in order and if all requests for access have
been granted and no request for access has been denied then the access is
granted. If no request has been denied but some request have not been
granted then fall back to the BSD access permissions. A complete descrip-
tion of ACLs can be found in "Appendix B, File System Details" in the
File System Programming Guide. Also see acl, noacl, and aclonly in
mount_nfs(8) for more details. Future version of MacOS may enable ACL
support by default.
NFSv4 Name mapping.
Unlike NFSv3/NFSv2 identities for owners and groups are represented by
strings instead of 32 bit numbers. Thus NFSv4 clients and servers have to
convert these over the wire string identities to the local identities and
local identities to strings. These strings are principally used in
GETATTR, SETATTR, and ACEs for security identities. Note that the RPC
credential is used for permission and access checking for whether an
operation will be allowed. Those credentials are found in the RPC header,
such as a kerberos identity that is associated with the RPCSEC_GSS con-
text. These strings are of the form "identity@NFSv4Domain", where the
"NFSv4Domain" follows DNS domain naming conventions.
MacOS NFSv4 name mapping rules.
When processing identities by default we first check and see if the iden-
ity string from the server is a string of digits if so we use that as the
uid/gid to ask Open Directory to map that to a local idenitity. If that
fails or the idenity string is not a string of digits we next check for
well known names that NFSv4 supports. These names are of the form
"identity@" with no domain part. If we match any of the following
OWNER@ GROUP@ EVERYONE@
INTERACTIVE@ NETWORK@ DIALUP@
BATCH@ ANONYMOUS@ AUTHENTICATED@
SERVICE@
These identities are mapped to Open Directory internal identities. Along
with any other identity that ends in an "@", which are mapped to nobody.
These well known identities are used as generic security idenitifiers in
ACEs. These identities are mapped back to the above strings when going
back over the wire to the server. If the identites do not match the
strings ending in "@" then we attempt to map the identities as follows.
MacOS clients support a zero configuration option by not specifying a
default NFSv4 domain. String identies coming from the server are handed
as is to Open Directory to translate the string identity to the local
identity. This works if the NFSv4 domain is the same as an Open Directory
node name. Local identies are similarly translated to the fully scoped
Open Directory names on the way out. If Open Directory returns an error
in trying to map the identities we then by default try to use the fol-
lowing fall back idenities:
root@any_domain wheel@any_domain
nobody@any_domain nfsnobody@any_domain
Root and wheel identities are translated by Open Directory with uid/gid
of zero and nobody and nfsnobody are traslated to uid/gid of -2 (MacOS
internal representation for the nobody uid/gid).
This is useful in situations where the client and server do not share a
common naming database. It is recommended where sites have a large num-
ber of MacOS clients, set their NFSv4 domain to be the LDAP node that is
being used to bind Open Directory so that there is no other configuration
needed for the Mac clients.
For environments which have a different NFSv4 domain name from the bound
LDAP node name, the NFSv4 domain name needs to be set by editing
/etc/nfs.conf and adding the line
nfs.client.default_nfs4domain = <my_nfsv4_domain>
See nfs.conf(5). The rules for mapping are then as follows. If a string
identity comes in over the wire whose domain portion matches the client's
NFSv4 domain, then its stripped off and that unscoped name is this given
to Open Directory to map to a local identity. On the way back to the
server any Open Directory node name is stripped off and the NFSv4 domain
name is appended. If the identity coming from the server does not match
the NFSv4 domain name, then its passed to Open Directory as is and the
rules described above for having no NFSv4 domain name set are followed.
Examples
The server has set its NFSv4 domain that is not the same as any MacOS
client Open Directory node, so that the identity mapping is not set up
correctly. Users will see output like the following example:
3-$ ls -l /tmp/mp
total 16851
-rw-r--r-- 1 nobody nobody 29 Oct 12 2015 Foo.txt
drwxr-xr-x 3 nobody nobody 4 Jan 31 16:27 Q102/
drwxrwxrwx 3 nobody nobody 7 Jan 24 16:59 acl/
-rw-r--r-- 1 nobody wheel 0 Feb 8 11:54 file1
-rw-r--r-- 1 root wheel 0 Feb 8 12:00 file2
-rw-r--r-- 1 nobody nobody 0 Feb 9 11:06 fooby
drwx------ 2 nobody nobody 5 Sep 22 2015 keyring-GbeUpi/
drwx------ 2 65432 nobody 5 Sep 8 2015 keyring-OX5G6P/
Most of the mappings comeback as "nobody/nobody". Note "file1" comes
back with group wheel. This is an example of fall back identity mapping.
Similarly for "file2" for both the user and group return root and wheel
respectfully. The directory "keyring-OX5G6P" has ownership of 65432 this
is because the server could not map that id locally and so sent it over
the wire as a string of digits. After correcting the NFSv4 domain on the
server we have:
4-$ ls -l /tmp/mp
total 16851
-rw-r--r-- 1 lbricker staff 29 Oct 12 2015 Foo.txt
drwxr-xr-x 3 lbricker staff 4 Jan 31 16:27 Q102/
drwxrwxrwx 3 lbricker staff 7 Jan 24 16:59 acl/
-rw-r--r-- 1 lbricker staff 0 Feb 8 11:54 file1
-rw-r--r-- 1 root nobody 0 Feb 8 12:00 file2
-rw-r--r-- 1 lbricker nobody 0 Feb 9 11:06 fooby
drwx------ 2 lbricker staff 5 Sep 22 2015 keyring-GbeUpi/
drwx------ 2 65432 staff 5 Sep 8 2015 keyring-OX5G6P/
What is surprising is that file1 and file2's group is now nobody. The
reason is that the server is sending those group ids as
"root@<open_directroy_node>". Open Directory will not find that mapping
so it will map it to nobody (had "wheel@<open_directory_node" had been
used, wheel would have been returned). In the previous example the
server sent "root@bogus.nfsv4.com". Open Directory will now return an
error since it can not find a valid Open Directory node "bogus.nfsv4.com"
and thus use the fall back to a gid of 0.
Debugging NFSv4 name mapping can be done with the nfs4mapid command. See
nfs4mapid(8). This allows testing of name/identity translations by using
a system call into the kernel that calls the same routines as the MacOS
nfs client uses. For example we determine the group translations above.
83-$ sudo nfs4mapid -G root@nod.apple.com
group root@nod.apple.com maps to id -2
mapping done through guid ABCDEFAB-CDEF-ABCD-EFAB-CDEFFFFFFFFE
84-$ sudo nfs4mapid -G wheel@nod.apple.com
group wheel@nod.apple.com maps to id 0
mapping done through guid ABCDEFAB-CDEF-ABCD-EFAB-CDEF00000000
85-$ sudo nfs4mapid -G wheel@foobar.com
group wheel@foobar.com maps to id 0
mapping done through guid ABCDEFAB-CDEF-ABCD-EFAB-CDEF00000000
86-$ sudo nfs4mapid -G root@foobar.com group
root@foobar.com maps to id 0
mapping done through guid ABCDEFAB-CDEF-ABCD-EFAB-CDEF00000000
See Also
"Appendix B, File System Details", File System Programming Guide, Apple,
https://developer.apple.com.
dot_clean(1), nfs.conf(5), nfs4mapid(8), mount_nfs(8), opendirectoryd(8),
Standards
[RFC1094] B. Nowicki, NFS: Network File System Protocol specification,
RFC1094, March 1989, http://www.rfc-editor.org/info/rfc1094.
[RFC1813] B. Callaghan, B. Pawlowski, and P. Staubach, NFS Version 3
Protocol Specification, RFC1813, June 1995, http://www.rfc-
editor.org/info/rfc1813.
[RFC3530] S. Shepler, B. Callaghan, D. Robinson, R. Thurlow, C. Beame,
M. Eisler, and D. Noveck, Network File System (NFS) version 4
Protocol, RFC3530, April 2003, http://www.rfc-
editor.org/info/rfc3530.
[XNFS] Protocols for Interworking: XNFS, Version 3W, Open Group
Technical Standard, February, 1998, ISBN: 1-85912-184-5.
BSD May 25, 2017 BSD