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     NFSv4 — NFS Version 4 Protocol


     The NFS client and server provides support for the NFSv4 specification; see Network File
     System (NFS) Version 4 Protocol RFC 7530 and Network File System (NFS) Version 4 Minor
     Version 1 Protocol RFC 5661.  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

     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:


     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.


     On the client, the sysctl


     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.


     To set up the NFS server that supports NFSv4, you will need to set the variables in
     rc.conf(5) as follows:




     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

     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.

             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.

             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.


     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


     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.

     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:


     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, 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.


     /var/db/nfs-stablerestart      NFS V4 stable restart file
     /var/db/nfs-stablerestart.bak  backup copy of the file


     stablerestart(5), mountd(8), nfscbd(8), nfsd(8), nfsdumpstate(8), nfsrevoke(8), nfsuserd(8)


     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.