NFS: Add a dns resolver for use with NFSv4 referrals and migration
The NFSv4 and NFSv4.1 protocols both allow for the redirection of a client
from one server to another in order to support filesystem migration and
replication. For full protocol support, we need to add the ability to
convert a DNS host name into an IP address that we can feed to the RPC
client.
We'll reuse the sunrpc cache, now that it has been converted to work with
rpc_pipefs.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/Documentation/filesystems/nfs.txt b/Documentation/filesystems/nfs.txt
new file mode 100644
index 0000000..f50f26c
--- /dev/null
+++ b/Documentation/filesystems/nfs.txt
@@ -0,0 +1,98 @@
+
+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}
+