1
2The NFS client
3==============
4
5The NFS version 2 protocol was first documented in RFC1094 (March 1989).
6Since then two more major releases of NFS have been published, with NFSv3
7being documented in RFC1813 (June 1995), and NFSv4 in RFC3530 (April
82003).
9
10The Linux NFS client currently supports all the above published versions,
11and work is in progress on adding support for minor version 1 of the NFSv4
12protocol.
13
14The purpose of this document is to provide information on some of the
15special features of the NFS client that can be configured by system
16administrators.
17
18
19The nfs4_unique_id parameter
20============================
21
22NFSv4 requires clients to identify themselves to servers with a unique
23string.  File open and lock state shared between one client and one server
24is associated with this identity.  To support robust NFSv4 state recovery
25and transparent state migration, this identity string must not change
26across client reboots.
27
28Without any other intervention, the Linux client uses a string that contains
29the local system's node name.  System administrators, however, often do not
30take care to ensure that node names are fully qualified and do not change
31over the lifetime of a client system.  Node names can have other
32administrative requirements that require particular behavior that does not
33work well as part of an nfs_client_id4 string.
34
35The nfs.nfs4_unique_id boot parameter specifies a unique string that can be
36used instead of a system's node name when an NFS client identifies itself to
37a server.  Thus, if the system's node name is not unique, or it changes, its
38nfs.nfs4_unique_id stays the same, preventing collision with other clients
39or loss of state during NFS reboot recovery or transparent state migration.
40
41The nfs.nfs4_unique_id string is typically a UUID, though it can contain
42anything that is believed to be unique across all NFS clients.  An
43nfs4_unique_id string should be chosen when a client system is installed,
44just as a system's root file system gets a fresh UUID in its label at
45install time.
46
47The string should remain fixed for the lifetime of the client.  It can be
48changed safely if care is taken that the client shuts down cleanly and all
49outstanding NFSv4 state has expired, to prevent loss of NFSv4 state.
50
51This string can be stored in an NFS client's grub.conf, or it can be provided
52via a net boot facility such as PXE.  It may also be specified as an nfs.ko
53module parameter.  Specifying a uniquifier string is not support for NFS
54clients running in containers.
55
56
57The DNS resolver
58================
59
60NFSv4 allows for one server to refer the NFS client to data that has been
61migrated onto another server by means of the special "fs_locations"
62attribute. See
63	http://tools.ietf.org/html/rfc3530#section-6
64and
65	http://tools.ietf.org/html/draft-ietf-nfsv4-referrals-00
66
67The fs_locations information can take the form of either an ip address and
68a path, or a DNS hostname and a path. The latter requires the NFS client to
69do a DNS lookup in order to mount the new volume, and hence the need for an
70upcall to allow userland to provide this service.
71
72Assuming that the user has the 'rpc_pipefs' filesystem mounted in the usual
73/var/lib/nfs/rpc_pipefs, the upcall consists of the following steps:
74
75   (1) The process checks the dns_resolve cache to see if it contains a
76       valid entry. If so, it returns that entry and exits.
77
78   (2) If no valid entry exists, the helper script '/sbin/nfs_cache_getent'
79       (may be changed using the 'nfs.cache_getent' kernel boot parameter)
80       is run, with two arguments:
81		- the cache name, "dns_resolve"
82		- the hostname to resolve
83
84   (3) After looking up the corresponding ip address, the helper script
85       writes the result into the rpc_pipefs pseudo-file
86       '/var/lib/nfs/rpc_pipefs/cache/dns_resolve/channel'
87       in the following (text) format:
88
89		"<ip address> <hostname> <ttl>\n"
90
91       Where <ip address> is in the usual IPv4 (123.456.78.90) or IPv6
92       (ffee:ddcc:bbaa:9988:7766:5544:3322:1100, ffee::1100, ...) format.
93       <hostname> is identical to the second argument of the helper
94       script, and <ttl> is the 'time to live' of this cache entry (in
95       units of seconds).
96
97       Note: If <ip address> is invalid, say the string "0", then a negative
98       entry is created, which will cause the kernel to treat the hostname
99       as having no valid DNS translation.
100
101
102
103
104A basic sample /sbin/nfs_cache_getent
105=====================================
106
107#!/bin/bash
108#
109ttl=600
110#
111cut=/usr/bin/cut
112getent=/usr/bin/getent
113rpc_pipefs=/var/lib/nfs/rpc_pipefs
114#
115die()
116{
117	echo "Usage: $0 cache_name entry_name"
118	exit 1
119}
120
121[ $# -lt 2 ] && die
122cachename="$1"
123cache_path=${rpc_pipefs}/cache/${cachename}/channel
124
125case "${cachename}" in
126	dns_resolve)
127		name="$2"
128		result="$(${getent} hosts ${name} | ${cut} -f1 -d\ )"
129		[ -z "${result}" ] && result="0"
130		;;
131	*)
132		die
133		;;
134esac
135echo "${result} ${name} ${ttl}" >${cache_path}
136
137