Documentation/filesystems/nfs.txt

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}