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

Provided by: nfs-common_2.6.4-3ubuntu5.1_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.

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 configuration file.

       Values recognized in the [sm-notify]  section  include:  retry-time,  outgoing-port,  and  outgoing-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-notify will lift
       lockd's grace period early if it has no hosts to notify.  Some high availability 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 corresponding 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 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)