NFSV4(4) FreeBSD Kernel Interfaces Manual NFSV4(4)
NAME
NFSv4 - NFS Version 4 Protocol
DESCRIPTION
The NFS client and server provides support for the NFSv4 specification;
see Network File System (NFS) Version 4 Protocol RFC 7530, Network File
System (NFS) Version 4 Minor Version 1 Protocol RFC 5661, Network File
System (NFS) Version 4 Minor Version 2 Protocol RFC 7862, File System
Extended Attributes in NFSv4 RFC 8276 and Parallel NFS (pNFS) Flexible
File Layout RFC 8435. The protocol is somewhat similar to NFS Version 3,
but differs in significant ways. It uses a single compound RPC that
concatenates operations to-gether. Each of these operations are similar
to the RPCs of NFS Version 3. The operations in the compound are
performed in order, until one of them fails (returns an error) and then
the RPC terminates at that point.
It has integrated locking support, which implies that the server is no
longer stateless. As such, the NFSv4 server remains in recovery mode for
a grace period (always greater than the lease duration the server uses)
after a reboot. During this grace period, clients may recover state but
not perform other open/lock state changing operations. To provide for
correct recovery semantics, a small file described by stablerestart(5) is
used by the server during the recovery phase. If this file is missing or
empty, there is a backup copy maintained by nfsd(8) that will be used.
If either file is missing, they will be created by the nfsd(8). If both
the file and the backup copy are empty, it will result in the server
starting without providing a grace period for recovery. Note that
recovery only occurs when the server machine is rebooted, not when the
nfsd(8) are just restarted.
It provides several optional features not present in NFS Version 3:
- NFS Version 4 ACLs
- Referrals, which redirect subtrees to other servers
(not yet implemented)
- Delegations, which allow a client to operate on a file locally
- pNFS, where I/O operations are separated from Metadata operations
And for NFSv4.2 only
- User namespace extended attributes
- lseek(SEEK_DATA/SEEK_HOLE)
- File copying done locally on the server for copy_file_range(2)
- posix_fallocate(2)
- posix_fadvise(POSIX_FADV_WILLNEED/POSIX_FADV_DONTNEED)
The NFSv4 protocol does not use a separate mount protocol and assumes
that the server provides a single file system tree structure, rooted at
the point in the local file system tree specified by one or more
V4: <rootdir> [-sec=secflavors] [host(s) or net]
line(s) in the exports(5) file. (See exports(5) for details.) The
nfsd(8) allows a limited subset of operations to be performed on non-
exported subtrees of the local file system, so that traversal of the tree
to the exported subtrees is possible. As such, the ``<rootdir>'' can be
in a non-exported file system. The exception is ZFS, which checks
exports and, as such, all ZFS file systems below the ``<rootdir>'' must
be exported. However, the entire tree that is rooted at that point must
be in local file systems that are of types that can be NFS exported.
Since the NFSv4 file system is rooted at ``<rootdir>'', setting this to
anything other than ``/'' will result in clients being required to use
different mount paths for NFSv4 than for NFS Version 2 or 3. Unlike NFS
Version 2 and 3, Version 4 allows a client mount to span across multiple
server file systems, although not all clients are capable of doing this.
NFSv4 uses strings for users and groups instead of numbers. On the wire,
these strings can either have the numbers in the string or take the form:
<user>@<dns.domain>
where ``<dns.domain>'' is not the same as the DNS domain used for host
name lookups, but is usually set to the same string. Most systems set
this ``<dns.domain>'' to the domain name part of the machine's
hostname(1) by default. However, this can normally be overridden by a
command line option or configuration file for the daemon used to do the
name<->number mapping. Under FreeBSD, the mapping daemon is called
nfsuserd(8) and has a command line option that overrides the domain
component of the machine's hostname. For use of this form of string on
NFSv4, either client or server, this daemon must be running.
The form where the numbers are in the strings can only be used for
AUTH_SYS. To configure your systems this way, the nfsuserd(8) daemon
does not need to be running on the server, but the following sysctls need
to be set to 1 on the server.
vfs.nfs.enable_uidtostring
vfs.nfsd.enable_stringtouid
On the client, the sysctl
vfs.nfs.enable_uidtostring
must be set to 1 and the nfsuserd(8) daemon does not need to be running.
If these strings are not configured correctly, ``ls -l'' will typically
report a lot of ``nobody'' and ``nogroup'' ownerships.
Although uid/gid numbers are no longer used in the NFSv4 protocol except
optionally in the above strings, they will still be in the RPC
authentication fields when using AUTH_SYS (sec=sys), which is the
default. As such, in this case both the user/group name and number
spaces must be consistent between the client and server.
However, if you run NFSv4 with RPCSEC_GSS (sec=krb5, krb5i, krb5p), only
names and KerberosV tickets will go on the wire.
SERVER SETUP
To set up the NFS server that supports NFSv4, you will need to set the
variables in rc.conf(5) as follows:
nfs_server_enable="YES"
nfsv4_server_enable="YES"
plus
nfsuserd_enable="YES"
if the server is using the ``<user>@<domain>'' form of user/group strings
or is using the ``-manage-gids'' option for nfsuserd(8).
You will also need to add at least one ``V4:'' line to the exports(5)
file for NFSv4 to work.
If the file systems you are exporting are only being accessed via NFSv4
there are a couple of sysctl(8) variables that you can change, which
might improve performance.
vfs.nfsd.issue_delegations
when set non-zero, allows the server to issue Open Delegations to
clients. These delegations permit the client to manipulate the
file locally on the client. Unfortunately, at this time, client
use of delegations is limited, so performance gains may not be
observed. This can only be enabled when the file systems being
exported to NFSv4 clients are not being accessed locally on the
server and, if being accessed via NFS Version 2 or 3 clients,
these clients cannot be using the NLM.
vfs.nfsd.enable_locallocks
can be set to 0 to disable acquisition of local byte range locks.
Disabling local locking can only be done if neither local
accesses to the exported file systems nor the NLM is operating on
them.
Note that Samba server access would be considered ``local access'' for
the above discussion.
To build a kernel with the NFS server that supports NFSv4 linked into it,
the
options NFSD
must be specified in the kernel's config(5) file.
CLIENT MOUNTS
To do an NFSv4 mount, specify the ``nfsv4'' option on the mount_nfs(8)
command line. This will force use of the client that supports NFSv4 plus
set ``tcp'' and NFSv4.
The nfsuserd(8) must be running if name<->uid/gid mapping is being used,
as above. Also, since an NFSv4 mount uses the host uuid to identify the
client uniquely to the server, you cannot safely do an NFSv4 mount when
hostid_enable="NO"
is set in rc.conf(5).
If the NFSv4 server that is being mounted on supports delegations, you
can start the nfscbd(8) daemon to handle client side callbacks. This
will occur if
nfsuserd_enable="YES" <-- If name<->uid/gid mapping is being used.
nfscbd_enable="YES"
are set in rc.conf(5).
Without a functioning callback path, a server will never issue
Delegations to a client.
For NFSv4.0, by default, the callback address will be set to the IP
address acquired via rtalloc() in the kernel and port# 7745. To override
the default port#, a command line option for nfscbd(8) can be used.
To get callbacks to work when behind a NAT gateway, a port for the
callback service will need to be set up on the NAT gateway and then the
address of the NAT gateway (host IP plus port#) will need to be set by
assigning the sysctl(8) variable vfs.nfs.callback_addr to a string of the
form:
N.N.N.N.N.N
where the first 4 Ns are the host IP address and the last two are the
port# in network byte order (all decimal #s in the range 0-255).
For NFSv4.1 and NFSv4.2, the callback path (called a backchannel) uses
the same TCP connection as the mount, so none of the above applies and
should work through gateways without any issues.
To build a kernel with the client that supports NFSv4 linked into it, the
option
options NFSCL
must be specified in the kernel's config(5) file.
Options can be specified for the nfsuserd(8) and nfscbd(8) daemons at
boot time via the ``nfsuserd_flags'' and ``nfscbd_flags'' rc.conf(5)
variables.
NFSv4 mount(s) against exported volume(s) on the same host are not
recommended, since this can result in a hung NFS server. It occurs when
an nfsd thread tries to do an NFSv4 VOP_RECLAIM() / Close RPC as part of
acquiring a new vnode. If all other nfsd threads are blocked waiting for
lock(s) held by this nfsd thread, then there isn't an nfsd thread to
service the Close RPC.
FILES
/var/db/nfs-stablerestart NFS V4 stable restart file
/var/db/nfs-stablerestart.bak backup copy of the file
SEE ALSO
stablerestart(5), mountd(8), nfscbd(8), nfsd(8), nfsdumpstate(8),
nfsrevoke(8), nfsuserd(8)
BUGS
At this time, there is no recall of delegations for local file system
operations. As such, delegations should only be enabled for file systems
that are being used solely as NFS export volumes and are not being
accessed via local system calls nor services such as Samba.
FreeBSD 13.1-RELEASE-p6 December 20, 2019 FreeBSD 13.1-RELEASE-p6
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