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NFS(5) File Formats Manual NFS(5)

NFS — Network File System

NFS

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. Version two of the protocol is considered legacy and should only be used if higher versions of the protocol are unavailable.

As part of macOS Ventura (macOS 13.0), NFS client code was moved out from kernel into kernel extension (kext). The new kext is now loaded by default on boot. Hence, from the user perspective, there is no difference from the previous releases. This behavior will change in a future release, i.e., NFS kext will not be loaded on boot but automatically when mount_nfs is called ( NetFSFramework and autofs use mount_nfs in order to mount) or manually using kextload tool. This means that mount("nfs", ...) syscall would fail with ENODEV and NFS client sysctl parameters will not be available until kext is up.

MacOS makes heavy use of extended attributes, resource forks, and possibly 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 optionally support ACLs. And can optionally support the storing of resource forks and extended attributes as NFSv4 named attributes. Previous versions 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 convert them to native extended attributes on the server and then remove the "Apple Double" file.

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 description of ACLs can be found in "Appendix B, File System Details" in the . Also see acl, noacl, and aclonly in mount_nfs(8) for more details. Future version of MacOS may enable ACL support by default.

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 context. These strings are of the form "identity@NFSv4Domain", where the "NFSv4Domain" follows DNS domain naming conventions.

While crossing file system boundries on the server side, nfs client will automatically create mount points on the client side. Such mount points will be marked as ephemeral in nfsstat -m output. Mount options are inherited from the parent mount point.

When processing identities by default we first check and see if the idenity 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 following 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 number 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.

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

Appendix B, File System Details, File System Programming Guide, Apple, https://developer.apple.com.

dot_clean(1), nfsstat(1), nfs.conf(5), nfs4mapid(8), mount_nfs(8), opendirectoryd(8), kextload(8)

[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.
May 25, 2017 macOS