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

       sm-notify - send reboot notifications to NFS peers

       /usr/sbin/sm-notify  [-dfn] [-m minutes] [-v name] [-p notify-port] [-P

       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:

              A  helper program that notifies NFS peers after the local system

              A daemon that listens for reboot notifications from other hosts,
              and manages the list of hosts to be notified when the local sys-
              tem 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  re-
       boot.   When  a remote reboots, that peer notifies the local rpc.statd,
       which in turn passes the reboot notification back to the local NFS lock

       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  persis-
       tent  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

       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 host-
       name 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  re-
       mote  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 num-
       ber, a 32-bit integer that is bumped after each reboot by the sm-notify
       command.   rpc.statd uses this number to distinguish between actual re-
       boots and replayed notifications.

       Part of NFS lock recovery is rediscovering which peers need to be moni-
       tored  again.  The sm-notify command clears the monitor list on persis-
       tent storage after each reboot.

       -d     Keeps sm-notify attached to its controlling terminal and running
              in the foreground so that notification progress may be monitored

       -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 min-
              utes.  Specifying a value of  0  causes  sm-notify  to  continue
              sending notifications to unresponsive peers until it is manually

              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 re-
              mote recognized the sender and has started appropriate lock  re-

       -n     Prevents  sm-notify  from  updating the local system's NSM state

       -p port
              Specifies the source port number sm-notify should use when send-
              ing  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 direc-
              tory.  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  notifi-
              cations, 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 ad-

       The sm-notify command must be started as  root  to  acquire  privileges
       needed  to access the state information database.  It drops root privi-
       leges as soon as it starts up to reduce the risk of a privilege escala-
       tion 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.

       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  quali-
              fied 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  moni-
       toring 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  opera-
       tion,  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 sup-
       port 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.

       /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

                                kernel's copy of the NSM state number

       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

       Olaf Kirch <>
       Chuck Lever <>

                                1 November 2009                   SM-NOTIFY(8)
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