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SM-NOTIFY(8) System Manager's Manual SM-NOTIFY(8)

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 re‐
booted. After an NFS client reboots, an NFS server must release all file locks held by ap‐
plications 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 mon‐
itored. 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 man‐
agers on both peers to contact their local NSM service to store information about the oppo‐
site peer. On Linux, the local lock manager contacts rpc.statd.

rpc.statd records information about each monitored NFS peer on persistent storage. This in‐
formation 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 man‐
ager 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 no‐
tify 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 moni‐
tored 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 re‐
quest, 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-no‐‐
tify 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 unre‐
sponsive hosts. If this option is not specified, sm-notify attempts to send notifica‐
tions 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 re‐
mote's mon_name from being resolved to an address.

Hosts are not removed from the notification list until a valid reply has been re‐
ceived. 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 notifica‐
tions. 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 the subdirectory sm of this directory. After changing the effective ids, sm-
notify only needs to access files in sm and sm.bak within the state-directory-path.

-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 spec‐
ified, 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 no‐
tification from a specific network address.

CONFIGURATION FILE
Many of the options that can be set on the command line can also be controlled through values
set in the [sm-notify] or, in one case, the [statd] section of the /etc/nfs.conf configura‐
tion file.

Values recognized in the [sm-notify] section include: retry-time, outgoing-port, and outgo‐‐
ing-addr. These have the same effect as the command line options m, p, and v respectively.

An additional value recognized in the [sm-notify] section is lift-grace. By default, sm-no‐‐
tify will lift lockd's grace period early if it has no hosts to notify. Some high availabil‐
ity configurations will run one sm-notify per floating IP address. In these configurations,
lifting the grace period early may prevent clients from reclaiming locks. Setting lift-grace
to n will prevent sm-notify from ending the grace period early. lift-grace has no corre‐
sponding command line option.

The value recognized in the [statd] section is state-directory-path.

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 direc‐
tory. 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 unnec‐
essary 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 con‐
tact 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 re‐
moves 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 sys‐
tems 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 AT suse.de>
Chuck Lever <chuck.lever AT oracle.com>

1 November 2009 SM-NOTIFY(8)