Ubuntu Manpage: sm-notify - send reboot notifications to NFS peers

Provided by: nfs-common_1.3.4-2.5ubuntu3.7_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 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
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

AUTHORS

       Olaf Kirch <okir@suse.de>
       Chuck Lever <chuck.lever@oracle.com>

                                         1 November 2009                             SM-NOTIFY(8)