NAME
sm-notify - send reboot notifications to NFS peers
SYNOPSIS
/usr/sbin/sm-notify [-dfn] [-m minutes] [-v name] [-p notify-port] [-P
path]
DESCRIPTION
File locks are not part of persistent file system state. Lock state is
thus lost when a host reboots.
Network file systems must also detect when lock state is lost because a
remote host has rebooted. After an NFS client reboots, an NFS server
must release all file locks held by applications that were running on
that client. After a server reboots, a client must remind the server
of file locks held by applications running on that client.
For NFS version 2 and version 3, the Network Status Monitor protocol
(or NSM for short) is used to notify NFS peers of reboots. On Linux,
two separate user-space components constitute the NSM service:
sm-notify
A helper program that notifies NFS peers after the local system
reboots
rpc.statd
A daemon that listens for reboot notifications from other hosts,
and manages the list of hosts to be notified when the local
system reboots
The local NFS lock manager alerts its local rpc.statd of each remote
peer that should be monitored. When the local system reboots, the sm-
notify command notifies the NSM service on monitored peers of the
reboot. When a remote reboots, that peer notifies the local rpc.statd,
which in turn passes the reboot notification back to the local NFS lock
manager.
NSM OPERATION IN DETAIL
The first file locking interaction between an NFS client and server
causes the NFS lock managers on both peers to contact their local NSM
service to store information about the opposite peer. On Linux, the
local lock manager contacts rpc.statd.
rpc.statd records information about each monitored NFS peer on
persistent storage. This information describes how to contact a remote
peer in case the local system reboots, how to recognize which monitored
peer is reporting a reboot, and how to notify the local lock manager
when a monitored peer indicates it has rebooted.
An NFS client sends a hostname, known as the client’s caller_name, in
each file lock request. An NFS server can use this hostname to send
asynchronous GRANT calls to a client, or to notify the client it has
rebooted.
The Linux NFS server can provide the client’s caller_name or the
client’s network address to rpc.statd. For the purposes of the NSM
protocol, this name or address is known as the monitored peer’s
mon_name. In addition, the local lock manager tells rpc.statd what it
thinks its own hostname is. For the purposes of the NSM protocol, this
hostname is known as my_name.
There is no equivalent interaction between an NFS server and a client
to inform the client of the server’s caller_name. Therefore NFS
clients do not actually know what mon_name an NFS server might use in
an SM_NOTIFY request. The Linux NFS client records the server’s
hostname used on the mount command to identify rebooting NFS servers.
Reboot notification
When the local system reboots, the sm-notify command reads the list of
monitored peers from persistent storage and sends an SM_NOTIFY request
to the NSM service on each listed remote peer. It uses the mon_name
string as the destination. To identify which host has rebooted, the
sm-notify command normally sends the results of gethostname(3) as the
my_name string. The remote rpc.statd matches incoming SM_NOTIFY
requests using this string, or the caller’s network address, to one or
more peers on its own monitor list.
If rpc.statd does not find a peer on its monitor list that matches an
incoming SM_NOTIFY request, the notification is not forwarded to the
local lock manager. In addition, each peer has its own NSM state
number, a 32-bit integer that is bumped after each reboot by the sm-
notify command. rpc.statd uses this number to distinguish between
actual reboots and replayed notifications.
Part of NFS lock recovery is rediscovering which peers need to be
monitored again. The sm-notify command clears the monitor list on
persistent storage after each reboot.
OPTIONS
-d Keeps sm-notify attached to its controlling terminal and running
in the foreground so that notification progress may be monitored
directly.
-f Send notifications even if sm-notify has already run since the
last system reboot.
-m retry-time
Specifies the length of time, in minutes, to continue retrying
notifications to unresponsive hosts. If this option is not
specified, sm-notify attempts to send notifications for 15
minutes. Specifying a value of 0 causes sm-notify to continue
sending notifications to unresponsive peers until it is manually
killed.
Notifications are retried if sending fails, the remote does not
respond, the remote’s NSM service is not registered, or if there
is a DNS failure which prevents the remote’s mon_name from being
resolved to an address.
Hosts are not removed from the notification list until a valid
reply has been received. However, the SM_NOTIFY procedure has a
void result. There is no way for sm-notify to tell if the
remote recognized the sender and has started appropriate lock
recovery.
-n Prevents sm-notify from updating the local system’s NSM state
number.
-p port
Specifies the source port number sm-notify should use when
sending reboot notifications. If this option is not specified,
a randomly chosen ephemeral port is used.
This option can be used to traverse a firewall between client
and server.
-P, --state-directory-path pathname
Specifies the pathname of the parent directory where NSM state
information resides. If this option is not specified, sm-notify
uses /var/lib/nfs by default.
After starting, sm-notify attempts to set its effective UID and
GID to the owner and group of this directory.
-v ipaddr | hostname
Specifies the network address from which to send reboot
notifications, and the mon_name argument to use when sending
SM_NOTIFY requests. If this option is not specified, sm-notify
uses a wildcard address as the transport bind address, and uses
the results of gethostname(3) as the mon_name argument.
The ipaddr form can be expressed as either an IPv4 or an IPv6
presentation address.
This option can be useful in multi-homed configurations where
the remote requires notification from a specific network
address.
SECURITY
The sm-notify command must be started as root to acquire privileges
needed to access the state information database. It drops root
privileges as soon as it starts up to reduce the risk of a privilege
escalation attack.
During normal operation, the effective user ID it chooses is the owner
of the state directory. This allows it to continue to access files in
that directory after it has dropped its root privileges. To control
which user ID rpc.statd chooses, simply use chown(1) to set the owner
of the state directory.
ADDITIONAL NOTES
Lock recovery after a reboot is critical to maintaining data integrity
and preventing unnecessary application hangs.
To help rpc.statd match SM_NOTIFY requests to NLM requests, a number of
best practices should be observed, including:
The UTS nodename of your systems should match the DNS names that
NFS peers use to contact them
The UTS nodenames of your systems should always be fully
qualified domain names
The forward and reverse DNS mapping of the UTS nodenames should
be consistent
The hostname the client uses to mount the server should match
the server’s mon_name in SM_NOTIFY requests it sends
The use of network addresses as a mon_name or a my_name string
should be avoided when interoperating with non-Linux NFS
implementations.
Unmounting an NFS file system does not necessarily stop either the NFS
client or server from monitoring each other. Both may continue
monitoring each other for a time in case subsequent NFS traffic between
the two results in fresh mounts and additional file locking.
On Linux, if the lockd kernel module is unloaded during normal
operation, all remote NFS peers are unmonitored. This can happen on an
NFS client, for example, if an automounter removes all NFS mount points
due to inactivity.
IPv6 and TI-RPC support
TI-RPC is a pre-requisite for supporting NFS on IPv6. If TI-RPC
support is built into the sm-notify command ,it will choose an
appropriate IPv4 or IPv6 transport based on the network address
returned by DNS for each remote peer. It should be fully compatible
with remote systems that do not support TI-RPC or IPv6.
Currently, the sm-notify command supports sending notification only via
datagram transport protocols.
FILES
/var/lib/nfs/sm directory containing monitor list
/var/lib/nfs/sm.bak directory containing notify list
/var/lib/nfs/state NSM state number for this host
/proc/sys/fs/nfs/nsm_local_state
kernel’s copy of the NSM state number
SEE ALSO
rpc.statd(8), nfs(5), uname(2), hostname(7)
RFC 1094 - "NFS: Network File System Protocol Specification"
RFC 1813 - "NFS Version 3 Protocol Specification"
OpenGroup Protocols for Interworking: XNFS, Version 3W - Chapter 11
AUTHORS
Olaf Kirch <okir@suse.de>
Chuck Lever <chuck.lever@oracle.com>
1 November 2009