Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 2 The NFS client 3 ============== 4 5 The NFS version 2 protocol was first documented in RFC1094 (March 1989). 6 Since then two more major releases of NFS have been published, with NFSv3 7 being documented in RFC1813 (June 1995), and NFSv4 in RFC3530 (April 8 2003). 9 10 The Linux NFS client currently supports all the above published versions, 11 and work is in progress on adding support for minor version 1 of the NFSv4 12 protocol. 13 14 The purpose of this document is to provide information on some of the 15 special features of the NFS client that can be configured by system 16 administrators. 17 18 19 The nfs4_unique_id parameter 20 ============================ 21 22 NFSv4 requires clients to identify themselves to servers with a unique 23 string. File open and lock state shared between one client and one server 24 is associated with this identity. To support robust NFSv4 state recovery 25 and transparent state migration, this identity string must not change 26 across client reboots. 27 28 Without any other intervention, the Linux client uses a string that contains 29 the local system's node name. System administrators, however, often do not 30 take care to ensure that node names are fully qualified and do not change 31 over the lifetime of a client system. Node names can have other 32 administrative requirements that require particular behavior that does not 33 work well as part of an nfs_client_id4 string. 34 35 The nfs.nfs4_unique_id boot parameter specifies a unique string that can be 36 used instead of a system's node name when an NFS client identifies itself to 37 a server. Thus, if the system's node name is not unique, or it changes, its 38 nfs.nfs4_unique_id stays the same, preventing collision with other clients 39 or loss of state during NFS reboot recovery or transparent state migration. 40 41 The nfs.nfs4_unique_id string is typically a UUID, though it can contain 42 anything that is believed to be unique across all NFS clients. An 43 nfs4_unique_id string should be chosen when a client system is installed, 44 just as a system's root file system gets a fresh UUID in its label at 45 install time. 46 47 The string should remain fixed for the lifetime of the client. It can be 48 changed safely if care is taken that the client shuts down cleanly and all 49 outstanding NFSv4 state has expired, to prevent loss of NFSv4 state. 50 51 This string can be stored in an NFS client's grub.conf, or it can be provided 52 via a net boot facility such as PXE. It may also be specified as an nfs.ko 53 module parameter. Specifying a uniquifier string is not support for NFS 54 clients running in containers. 55 56 57 The DNS resolver 58 ================ 59 60 NFSv4 allows for one server to refer the NFS client to data that has been 61 migrated onto another server by means of the special "fs_locations" 62 attribute. See 63 http://tools.ietf.org/html/rfc3530#section-6 64 and 65 http://tools.ietf.org/html/draft-ietf-nfsv4-referrals-00 66 67 The fs_locations information can take the form of either an ip address and 68 a path, or a DNS hostname and a path. The latter requires the NFS client to 69 do a DNS lookup in order to mount the new volume, and hence the need for an 70 upcall to allow userland to provide this service. 71 72 Assuming 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. 100 101 102 103 104 A basic sample /sbin/nfs_cache_getent 105 ===================================== 106 107 #!/bin/bash 108 # 109 ttl=600 110 # 111 cut=/usr/bin/cut 112 getent=/usr/bin/getent 113 rpc_pipefs=/var/lib/nfs/rpc_pipefs 114 # 115 die() 116 { 117 echo "Usage: $0 cache_name entry_name" 118 exit 1 119 } 120 121 [ $# -lt 2 ] && die 122 cachename="$1" 123 cache_path=${rpc_pipefs}/cache/${cachename}/channel 124 125 case "${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 ;; 134 esac 135 echo "${result} ${name} ${ttl}" >${cache_path}