Based on kernel version 2.6.32. Page generated on 2009-12-11 16:22 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 upcall interfaces that are used in order to provide the NFS client with 16 some of the information that it requires in order to fully comply with 17 the NFS spec. 18 19 The DNS resolver 20 ================ 21 22 NFSv4 allows for one server to refer the NFS client to data that has been 23 migrated onto another server by means of the special "fs_locations" 24 attribute. See 25 http://tools.ietf.org/html/rfc3530#section-6 26 and 27 http://tools.ietf.org/html/draft-ietf-nfsv4-referrals-00 28 29 The fs_locations information can take the form of either an ip address and 30 a path, or a DNS hostname and a path. The latter requires the NFS client to 31 do a DNS lookup in order to mount the new volume, and hence the need for an 32 upcall to allow userland to provide this service. 33 34 Assuming that the user has the 'rpc_pipefs' filesystem mounted in the usual 35 /var/lib/nfs/rpc_pipefs, the upcall consists of the following steps: 36 37 (1) The process checks the dns_resolve cache to see if it contains a 38 valid entry. If so, it returns that entry and exits. 39 40 (2) If no valid entry exists, the helper script '/sbin/nfs_cache_getent' 41 (may be changed using the 'nfs.cache_getent' kernel boot parameter) 42 is run, with two arguments: 43 - the cache name, "dns_resolve" 44 - the hostname to resolve 45 46 (3) After looking up the corresponding ip address, the helper script 47 writes the result into the rpc_pipefs pseudo-file 48 '/var/lib/nfs/rpc_pipefs/cache/dns_resolve/channel' 49 in the following (text) format: 50 51 "<ip address> <hostname> <ttl>\n" 52 53 Where <ip address> is in the usual IPv4 (123.456.78.90) or IPv6 54 (ffee:ddcc:bbaa:9988:7766:5544:3322:1100, ffee::1100, ...) format. 55 <hostname> is identical to the second argument of the helper 56 script, and <ttl> is the 'time to live' of this cache entry (in 57 units of seconds). 58 59 Note: If <ip address> is invalid, say the string "0", then a negative 60 entry is created, which will cause the kernel to treat the hostname 61 as having no valid DNS translation. 62 63 64 65 66 A basic sample /sbin/nfs_cache_getent 67 ===================================== 68 69 #!/bin/bash 70 # 71 ttl=600 72 # 73 cut=/usr/bin/cut 74 getent=/usr/bin/getent 75 rpc_pipefs=/var/lib/nfs/rpc_pipefs 76 # 77 die() 78 { 79 echo "Usage: $0 cache_name entry_name" 80 exit 1 81 } 82 83 [ $# -lt 2 ] && die 84 cachename="$1" 85 cache_path=${rpc_pipefs}/cache/${cachename}/channel 86 87 case "${cachename}" in 88 dns_resolve) 89 name="$2" 90 result="$(${getent} hosts ${name} | ${cut} -f1 -d\ )" 91 [ -z "${result}" ] && result="0" 92 ;; 93 *) 94 die 95 ;; 96 esac 97 echo "${result} ${name} ${ttl}" >${cache_path}