Provided by: nfs-common_1.2.8-6ubuntu1.2_amd64 
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 my_name string recorded when that remote was monitored. 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 my_name recorded when the remote was
monitored as the mon_name argument when sending SM_NOTIFY requests.
The ipaddr form can be expressed as either an IPv4 or an IPv6 presentation address. If the ipaddr
form is used, the sm-notify command converts this address to a hostname for use as the mon_name
argument when sending SM_NOTIFY requests.
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
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 SM-NOTIFY(8)