SM-NOTIFY(8)

💖 SM-NOTIFY(8) — NAME 💖

sm-notify — send reboot notifications to NFS peers

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✨ SYNOPSIS ✨

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

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🌸 DESCRIPTION 🌸

Okay so bestie, file locks? They are not part of persistent file system state. Which means when a host reboots, that lock state is just... gone. Poof. 💀

Network file systems also have to clock when lock state gets lost because a remote host rebooted. After an NFS client reboots, an NFS server has to release all file locks held by apps that were running on that client. And after a server reboots, a client has to remind the server about file locks held by apps running on that client. It's giving mutual accountability. 🫶

For NFS version 2 and version 3, the Network Status Monitor protocol — NSM for short — is what notifies NFS peers about reboots. On Linux, two separate user-space components make up 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. It's a whole relay race. 🏃‍♀️💨

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🔥 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 info 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 info describes how to contact a remote peer if 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 says 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 NSM protocol purposes, this name or address is known as the monitored peer's mon_name. Also, the local lock manager tells rpc.statd what it thinks its own hostname is. For NSM protocol purposes, 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. So 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 the 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. Also, 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. Clean slate, periodt. ✨

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💅 OPTIONS 💅

• -d — Keeps sm-notify attached to its controlling terminal and running in the foreground so you can monitor notification progress directly. It's giving front-row seat. 👀
• -f — Send notifications even if sm-notify has already run since the last system reboot. Like, ignore the "already did that" energy.
• -m retry-time — Specifies how long, in minutes, to keep retrying notifications to unresponsive hosts. If you don't specify this option, 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. So it's lowkey a mystery. 🕵️‍♀️

• -n — Prevents sm-notify from updating the local system's NSM state number. No touchy. 🙅‍♀️
• -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. Firewall? We're getting through. 🔥

• -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. Main character IP energy. 👑

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🧚 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.

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🔒 SECURITY 🔒

The sm-notify command is, generally, 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. Safety first, bestie. 🛡️

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.

The sm-notify command can also be started by the user ID that owns the state directory. That is useful for the situation when it is called from an rpc-stat ha-callout program that is already running as the non-privileged user. The non-root user will not have the capabilities to drop the root privileges as described above.

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📝 ADDITIONAL NOTES 📝

Lock recovery after a reboot is critical to maintaining data integrity and preventing unnecessary application hangs. No cap.

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 📁

okay so here's the little file-system bestie tour, no cap these paths matter:

• /var/lib/nfs/sm — the directory that holds the monitor list ✨
• /var/lib/nfs/sm.bak — the directory that holds the notify list 💖
• /var/lib/nfs/state — the NSM state number for this host, like its lil ID badge 🎀
• /proc/sys/fs/nfs/nsm_local_state — the kernel's own copy of the NSM state number 👑

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👀 SEE ALSO 👀

go read your friends, they're all iconic:

• rpc.statd(8)
• nfs(5)
• uname(2)
• hostname(7)

and the certified academic baddies, aka the required reading 📚:

• RFC 1094 — "NFS: Network File System Protocol Specification"
• RFC 1813 — "NFS Version 3 Protocol Specification"
• OpenGroup Protocols for Interworking: XNFS, Version 3W — Chapter 11

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💅 AUTHORS 💅

the masterminds, the main characters, the ones who literally wrote this:

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

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and that's the whole page bestie, we ate and left no crumbs 🍓✨ shoutout to sm-notify(8) from 1 November 2009 for being the moment, the icon, the legend 🫶 you're all caught up, go notify somebody 💜