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 ==========
 NFS Client
 ==========
 
 The NFS client
 ==============
 
 The NFS version 2 protocol was first documented in RFC1094 (March 1989).
 Since then two more major releases of NFS have been published, with NFSv3
 being documented in RFC1813 (June 1995), and NFSv4 in RFC3530 (April
 2003).
 
 The Linux NFS client currently supports all the above published versions,
 and work is in progress on adding support for minor version 1 of the NFSv4
 protocol.
 
 The purpose of this document is to provide information on some of the
 special features of the NFS client that can be configured by system
 administrators.
 
 
 The nfs4_unique_id parameter
 ============================
 
 NFSv4 requires clients to identify themselves to servers with a unique
 string.  File open and lock state shared between one client and one server
 is associated with this identity.  To support robust NFSv4 state recovery
 and transparent state migration, this identity string must not change
 across client reboots.
 
 Without any other intervention, the Linux client uses a string that contains
 the local system's node name.  System administrators, however, often do not
 take care to ensure that node names are fully qualified and do not change
 over the lifetime of a client system.  Node names can have other
 administrative requirements that require particular behavior that does not
 work well as part of an nfs_client_id4 string.
 
 The nfs.nfs4_unique_id boot parameter specifies a unique string that can be
 used together with  a system's node name when an NFS client identifies itself to
 a server.  Thus, if the system's node name is not unique, its
 nfs.nfs4_unique_id can help prevent collisions with other clients.
 
 The nfs.nfs4_unique_id string is typically a UUID, though it can contain
 anything that is believed to be unique across all NFS clients.  An
 nfs4_unique_id string should be chosen when a client system is installed,
 just as a system's root file system gets a fresh UUID in its label at
 install time.
 
 The string should remain fixed for the lifetime of the client.  It can be
 changed safely if care is taken that the client shuts down cleanly and all
 outstanding NFSv4 state has expired, to prevent loss of NFSv4 state.
 
 This string can be stored in an NFS client's grub.conf, or it can be provided
 via a net boot facility such as PXE.  It may also be specified as an nfs.ko
 module parameter.
 
 This uniquifier string will be the same for all NFS clients running in
 containers unless it is overridden by a value written to
 /sys/fs/nfs/net/nfs_client/identifier which will be local to the network
 namespace of the process which writes.
 
 
 The DNS resolver
 ================
 
 NFSv4 allows for one server to refer the NFS client to data that has been
 migrated onto another server by means of the special "fs_locations"
 attribute. See `RFC3530 Section 6: Filesystem Migration and Replication`_ and
 `Implementation Guide for Referrals in NFSv4`_.
 
 .. _RFC3530 Section 6\: Filesystem Migration and Replication: https://tools.ietf.org/html/rfc3530#section-6
 .. _Implementation Guide for Referrals in NFSv4: https://tools.ietf.org/html/draft-ietf-nfsv4-referrals-00
 
 The fs_locations information can take the form of either an ip address and
 a path, or a DNS hostname and a path. The latter requires the NFS client to
 do a DNS lookup in order to mount the new volume, and hence the need for an
 upcall to allow userland to provide this service.
 
 Assuming that the user has the 'rpc_pipefs' filesystem mounted in the usual
 /var/lib/nfs/rpc_pipefs, the upcall consists of the following steps:
 
    (1) The process checks the dns_resolve cache to see if it contains a
        valid entry. If so, it returns that entry and exits.
 
    (2) If no valid entry exists, the helper script '/sbin/nfs_cache_getent'
        (may be changed using the 'nfs.cache_getent' kernel boot parameter)
        is run, with two arguments:
        - the cache name, "dns_resolve"
        - the hostname to resolve
 
    (3) After looking up the corresponding ip address, the helper script
        writes the result into the rpc_pipefs pseudo-file
        '/var/lib/nfs/rpc_pipefs/cache/dns_resolve/channel'
        in the following (text) format:
 
                 "<ip address> <hostname> <ttl>\n"
 
        Where <ip address> is in the usual IPv4 (123.456.78.90) or IPv6
        (ffee:ddcc:bbaa:9988:7766:5544:3322:1100, ffee::1100, ...) format.
        <hostname> is identical to the second argument of the helper
        script, and <ttl> is the 'time to live' of this cache entry (in
        units of seconds).
 
        .. note::
             If <ip address> is invalid, say the string "0", then a negative
             entry is created, which will cause the kernel to treat the hostname
             as having no valid DNS translation.
 
 
 
 
 A basic sample /sbin/nfs_cache_getent
 =====================================
 .. code-block:: sh
 
     #!/bin/bash
     #
     ttl=600
     #
     cut=/usr/bin/cut
     getent=/usr/bin/getent
     rpc_pipefs=/var/lib/nfs/rpc_pipefs
     #
     die()
     {
         echo "Usage: $0 cache_name entry_name"
         exit 1
     }
 
     [ $# -lt 2 ] && die
     cachename="$1"
     cache_path=${rpc_pipefs}/cache/${cachename}/channel
 
     case "${cachename}" in
         dns_resolve)
             name="$2"
             result="$(${getent} hosts ${name} | ${cut} -f1 -d\ )"
             [ -z "${result}" ] && result="0"
             ;;
         *)
             die
             ;;
     esac
     echo "${result} ${name} ${ttl}" >${cache_path}

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