mirror of https://gitee.com/openkylin/linux.git
99 lines
3.0 KiB
Plaintext
99 lines
3.0 KiB
Plaintext
|
|
||
|
|
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}
|
||
|
|
|