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99 lines
3 KiB
Text
99 lines
3 KiB
Text
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The NFS client
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==============
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The NFS version 2 protocol was first documented in RFC1094 (March 1989).
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Since then two more major releases of NFS have been published, with NFSv3
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being documented in RFC1813 (June 1995), and NFSv4 in RFC3530 (April
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2003).
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The Linux NFS client currently supports all the above published versions,
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and work is in progress on adding support for minor version 1 of the NFSv4
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protocol.
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The purpose of this document is to provide information on some of the
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upcall interfaces that are used in order to provide the NFS client with
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some of the information that it requires in order to fully comply with
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the NFS spec.
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The DNS resolver
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================
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NFSv4 allows for one server to refer the NFS client to data that has been
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migrated onto another server by means of the special "fs_locations"
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attribute. See
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http://tools.ietf.org/html/rfc3530#section-6
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and
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http://tools.ietf.org/html/draft-ietf-nfsv4-referrals-00
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The fs_locations information can take the form of either an ip address and
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a path, or a DNS hostname and a path. The latter requires the NFS client to
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do a DNS lookup in order to mount the new volume, and hence the need for an
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upcall to allow userland to provide this service.
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Assuming that the user has the 'rpc_pipefs' filesystem mounted in the usual
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/var/lib/nfs/rpc_pipefs, the upcall consists of the following steps:
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(1) The process checks the dns_resolve cache to see if it contains a
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valid entry. If so, it returns that entry and exits.
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(2) If no valid entry exists, the helper script '/sbin/nfs_cache_getent'
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(may be changed using the 'nfs.cache_getent' kernel boot parameter)
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is run, with two arguments:
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- the cache name, "dns_resolve"
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- the hostname to resolve
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(3) After looking up the corresponding ip address, the helper script
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writes the result into the rpc_pipefs pseudo-file
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'/var/lib/nfs/rpc_pipefs/cache/dns_resolve/channel'
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in the following (text) format:
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"<ip address> <hostname> <ttl>\n"
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Where <ip address> is in the usual IPv4 (123.456.78.90) or IPv6
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(ffee:ddcc:bbaa:9988:7766:5544:3322:1100, ffee::1100, ...) format.
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<hostname> is identical to the second argument of the helper
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script, and <ttl> is the 'time to live' of this cache entry (in
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units of seconds).
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Note: If <ip address> is invalid, say the string "0", then a negative
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entry is created, which will cause the kernel to treat the hostname
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as having no valid DNS translation.
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A basic sample /sbin/nfs_cache_getent
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=====================================
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#!/bin/bash
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#
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ttl=600
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#
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cut=/usr/bin/cut
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getent=/usr/bin/getent
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rpc_pipefs=/var/lib/nfs/rpc_pipefs
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#
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die()
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{
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echo "Usage: $0 cache_name entry_name"
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exit 1
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}
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[ $# -lt 2 ] && die
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cachename="$1"
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cache_path=${rpc_pipefs}/cache/${cachename}/channel
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case "${cachename}" in
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dns_resolve)
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name="$2"
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result="$(${getent} hosts ${name} | ${cut} -f1 -d\ )"
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[ -z "${result}" ] && result="0"
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;;
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*)
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die
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;;
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esac
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echo "${result} ${name} ${ttl}" >${cache_path}
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