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-
-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
-upcall interfaces that are used in order to provide the NFS client with
-some of the information that it requires in order to fully comply with
-the NFS spec.
-
-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
- http://tools.ietf.org/html/rfc3530#section-6
-and
- http://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
-=====================================
-
-#!/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}
-