6. Configuration notes

This chapter describe most of the configuration and use aspects of NUT, including establishing communication with the device and configuring safe shutdowns when the UPS battery runs out of power.

There are many programs and features in this package. You should check out the NUT Overview and other accompanying documentation to see how it all works.


NUT does not currently provide proper graphical configuration tools. However, there is now support for Augeas, which will enable the easier creation of configuration tools. Moreover, nut-scanner(8) is available to discover supported devices (USB, SNMP, Eaton XML/HTTP and IPMI) and NUT servers (using Avahi or the classic connection method).

6.1. Details about the configuration files


All configuration files within this package are parsed with a common state machine, which means they all can use a number of extras described here.

First, most of the programs use an upper-case word to declare a configuration directive. This may be something like MONITOR, NOTIFYCMD, or ACCESS. The case does matter here. "monitor" won’t be recognized.

Next, the parser does not care about whitespace between words. If you like to indent things with tabs or spaces, feel free to do it here.

If you need to set a value to something containing spaces, it has to be contained within "quotes" to keep the parser from splitting up the line. That is, you want to use something like this:

SHUTDOWNCMD "/sbin/shutdown -h +0"

Without the quotes, it would only see the first word on the line.

OK, so let’s say you really need to embed that kind of quote within your configuration directive for some reason. You can do that too.

NOTIFYCMD "/bin/notifyme -foo -bar \"hi there\" -baz"

In other words, \ can be used to escape the ".

Finally, for the situation where you need to put the \ character into your string, you just escape it.

NOTIFYCMD "/bin/notifyme c:\\dos\\style\\path"

The \ can actually be used to escape any character, but you only really need it for \, ", and # as they have special meanings to the parser.

When using file names with space characters, you may end up having tricky things since you need to write them inside "" which must be escaped:

NOTIFYCMD "\"c:\\path with space\\notifyme\" \"c:\\path with space\\name\""

# is the comment character. Anything after an unescaped # is ignored.

Something like this…

identity = my#1ups

will actually turn into identity = my, since the # stops the parsing. If you really need to have a # in your configuration, then escape it.

identity = my\#1ups

Much better.

The = character should be used with care too. There should be only one "simple" = character in a line: between the parameter name and its value. All other = characters should be either escaped or within "quotes".

password = 123=123

is incorrect. You should use:

password = 123\=123


password = "123=123"

Line spanning

You can put a backslash at the end of the line to join it to the next one. This creates one virtual line that is composed of more than one physical line.

Also, if you leave the "" quote container open before a newline, it will keep scanning until it reaches another one. If you see bizarre behavior in your configuration files, check for an unintentional instance of quotes spanning multiple lines.

6.2. Basic configuration

This chapter describes the base configuration to establish communication with the device.

This will be sufficient for PDU. But for UPS and SCD, you will also need to configure automatic shutdowns for low battery events.


On operating systems with service management frameworks (such as Linux systemd and Solaris/illumos SMF), the life-cycle of driver, data server and monitoring client daemons is managed respectively by nut-driver (multi-instance service), nut-server and nut-monitor services. These are in turn wrapped by an "umbrella" service (or systemd "target") conveniently called nut which allows to easily start or stop all those of the bundled services, which are enabled on a particular deployment.

Driver configuration

Create one section per UPS in ups.conf


The default path for a source installation is /usr/local/ups/etc, while packaged installation will vary. For example, /etc/nut is used on Debian and derivatives, while /etc/ups or /etc/upsd is used on RedHat and derivatives.

To find out which driver to use, check the Hardware Compatibility List, or data/driver.list(.in) source file.

Once you have picked a driver, create a section for your UPS in ups.conf. You must supply values at least for "driver" and "port".

Some drivers may require other flags or settings. The "desc" value is optional, but is recommended to provide a better description of what useful load your UPS is feeding.

A typical device without any extra settings looks like this:

        driver = mydriver
        port = /dev/ttyS1
        desc = "Workstation"


USB drivers (such as usbhid-ups for non-SHUT mode, nutdrv_qx for non-serial mode, bcmxcp_usb, tripplite_usb, blazer_usb, riello_usb and richcomm_usb) are special cases and ignore the port value.

You must still set this value, but it does not matter what you set it to; a common and good practice is to set port to auto, but you can put whatever you like.

If you only own one USB UPS, the driver will find it automatically.

If you own more than one, refer to the driver’s manual page for more information on matching a specific device.


On Windows systems, the second serial port (COM2), equivalent to "/dev/ttyS1" on Linux, would be "\\\\.\\COM2".

References: ups.conf(5), nutupsdrv(8), bcmxcp_usb(8), blazer_usb(8), nutdrv_qx(8), richcomm_usb(8), riello_usb(8), tripplite_usb(8), usbhid-ups(8)

Starting the driver(s)

Generally, you can just start the driver(s) for your hardware (all sections defined in ups.conf) using the following command:

upsdrvctl start

Make sure the driver doesn’t report any errors. It should show a few details about the hardware and then enter the background. You should get back to the command prompt a few seconds later. For reference, a successful start of the usbhid-ups driver looks like this:

# upsdrvctl start
Network UPS Tools - Generic HID driver 0.34 (2.4.1)
USB communication driver 0.31
Using subdriver: MGE HID 1.12
Detected EATON - Ellipse MAX 1100 [ADKK22008]

If the driver doesn’t start cleanly, make sure you have picked the right one for your hardware. You might need to try other drivers by changing the "driver=" value in ups.conf.

Be sure to check the driver’s man page to see if it needs any extra settings in ups.conf to detect your hardware.

If it says can't bind /var/state/ups/... or similar, then your state path probably isn’t writable by the driver. Check the permissions and mode on that directory vs. the user account your driver starts as.

After making changes, try the Ownership and permissions step again.

Driver(s) as a service

On operating systems with init-scripts managing life-cycle of the operating environment, the upsdrvctl program is also commonly used in those scripts. It has a few downsides, such as that if the device was not accessible during OS startup and the driver connection timed out, it would remain not-started until an administrator (or some other script) "kicks" the driver to retry startup. Also, startup of the upsd data server daemon and its clients like upsmon is delayed until all the NUT drivers complete their startup (or time out trying).

This can be a big issue on systems which monitor multiple devices, such as big servers with multiple power sources, or administrative workstations which monitor a datacenter full of UPSes.

For this reason, NUT starting with version 2.8.0 supports startup of its drivers as independent instances of a nut-driver service under the Linux systemd and Solaris/illumos SMF service-management frameworks (corresponding files and scripts may be not pre-installed in packaging for other systems).

Such service instances have their own and independent life-cycle, including parallel driver start and stop processing, and retries of startup in case of failure as implemented by the service framework in the OS. The Linux systemd solution also includes a nut-driver.target as a checkpoint that all defined drivers have indeed started up (as well as being a singular way to enable or disable startup of drivers).

In both cases, a service named nut-driver-enumerator is registered, and when it is (re-)started it scans the currently defined device sections in ups.conf and the currently defined instances of nut-driver service, and brings them in sync (adding or removing service instances), and if there were changes — it restarts the corresponding drivers (via service instances) as well as the data server which only reads the list of sections at its startup. This helper service should be triggered whenever your system (re-)starts the nut-server service, so that it runs against an up-to-date list of NUT driver processes.

Two service bundles are provided for this feature: a set of nut-driver-enumerator-daemon* units starts the script as a daemon to regularly inspect and apply the NUT configuration to OS service unit wrappings (mainly intended for monitoring systems with a dynamic set of monitored power devices, or for systems where filesystem events monitoring is not a clockwork-reliable mechanism to 100% rely on); while the other nut-driver-enumerator.* units run the script once per triggering of the service (usually during boot-up; configuration file changes can be detected and propagated by systemd most of the time, but not by SMF out of the box).

A service-oriented solution also allows to consider that different drivers have different dependencies — such as that networked drivers should begin startup after IP addresses have been assigned, while directly-connected devices might need nothing beside a mounted filesystem (or an activated USB stack service or device rule, in case of Linux). Likewise, systems administrators can define further local dependencies between services and their instances as needed on particular deployments.

This solution also adds the upsdrvsvcctl script to manage NUT drivers as system service instances, whose CLI mimics that of upsdrvctl program. One addition is the resync argument to trigger nut-driver-enumerator, another is a list argument to display current mappings of service instances to NUT driver sections. Also, original tool’s arguments such as the -u (user to run the driver as) or -D (debug of the driver) do not make sense in the service context — the accounts to use and other arguments to the driver process are part of service setup (and an administrator can manage it there).

Note that while this solution tries to register service instances with same names as NUT configuration sections for the devices, this can not always be possible due to constraints such as syntax supported by a particular service management framework. In this case, the enumerator falls back to MD5 hashes of such section names, and the upsdrvsvcctl script supports this to map the user-friendly NUT configuration section names to actual service names that it would manage.

References: man pages: nutupsdrv(8), upsdrvctl(8), upsdrvsvcctl(8)

Data server configuration (upsd)

Configure upsd, which serves data from the drivers to the clients.

First, edit upsd.conf to allow access to your client systems. By default, upsd will only listen to localhost port 3493/tcp. If you want to connect to it from other machines, you must specify each interface you want upsd to listen on for connections, optionally with a port number.

LISTEN ::1 3493

As a special case, LISTEN * <port> (with an asterisk) will try to listen on "ANY" IP address for both and IPv6 (::0) and IPv4 (, subject to upsd command-line arguments, or system configuration or support. Note that if the system supports IPv4-mapped IPv6 addressing per RFC-3493, and does not allow to disable this mode, then there may be one listening socket to handle both address families.


Refer to the NUT user manual security chapter for information on how to access and secure upsd clients connections.

Next, create upsd.users. For now, this can be an empty file. You can come back and add more to it later when it’s time to configure upsmon or run one of the management tools.

Do not make either file world-readable, since they both hold access control data and passwords. They just need to be readable by the user you created in the preparation process.

The suggested configuration is to chown it to root, chgrp it to the group you created, then make it readable by the group.

chown root:nut upsd.conf upsd.users
chmod 0640 upsd.conf upsd.users

References: man pages: upsd.conf(5), upsd.users(5), upsd(8)

Starting the data server

Start the network data server:


Make sure it is able to connect to the driver(s) on your system. A successful run looks like this:

# upsd
Network UPS Tools upsd 2.4.1
listening on port 3493
listening on ::1 port 3493
Connected to UPS [eaton]: usbhid-ups-eaton

upsd prints dots while it waits for the driver to respond. Your system may print more or less depending on how many drivers you have and how fast they are.


If upsd says that it can’t connect to a UPS or that the data is stale, then your ups.conf is not configured correctly, or you have a driver that isn’t working properly. You must fix this before going on to the next step.


Normally upsd requires that at least one driver section is defined in the ups.conf file, and refuses to start otherwise. If you intentionally do not have any driver sections defined (yet) but still want the data server to run, respond and report zero devices (e.g. on an automatically managed monitoring deployment), you can enable the ALLOW_NO_DEVICE true option in the upsd.conf file.


Normally upsd requires that at all LISTEN directives defined in the upsd.conf file are honoured (except for mishaps possible with many names of localhost), and refuses to start otherwise. If you want to allow start-up in cases where at least one but possibly not all of the LISTEN directives were honoured, you can enable the ALLOW_NOT_ALL_LISTENERS true option in the upsd.conf file. Note you would have to restart upsd to pick up the LISTEN`ed IP address if it appears later, so probably configuring `LISTEN * is a better choice in such cases.

On operating systems with service management frameworks, the data server life-cycle is managed by nut-server service.

Reference: man page: upsd(8)

Check the UPS data

Status data

Make sure that the UPS is providing good status data. You can use the upsc command-line client for this:

upsc myupsname@localhost ups.status

You should see just one line in response:


OL means your system is running on line power. If it says something else (like OB — on battery, or LB — low battery), your driver was probably misconfigured during the Driver configuration step. If you reconfigure the driver, use upsdrvctl stop to stop it, then start it again as shown in the Starting driver(s) step.

Reference: man page: upsc(8)

All data

Look at all of the status data which is being monitored.

upsc myupsname@localhost

What happens now depends on the kind of device and driver you have. In the list, you should see ups.status with the same value you got above. A sample run on an UPS (Eaton Ellipse MAX 1100) looks like this:

battery.charge: 100
battery.charge.low: 20
battery.runtime: 2525
battery.type: PbAc
device.mfr: EATON
device.model: Ellipse MAX 1100
device.serial: ADKK22008
device.type: ups
driver.name: usbhid-ups
driver.parameter.pollfreq: 30
driver.parameter.pollinterval: 2
driver.parameter.port: auto
driver.version: 2.4.1-1988:1990M
driver.version.data: MGE HID 1.12
driver.version.internal: 0.34
input.sensitivity: normal
input.transfer.boost.low: 185
input.transfer.high: 285
input.transfer.low: 165
input.transfer.trim.high: 265
input.voltage.extended: no
outlet.1.desc: PowerShare Outlet 1
outlet.1.id: 2
outlet.1.status: on
outlet.1.switchable: no
outlet.desc: Main Outlet
outlet.id: 1
outlet.switchable: no
output.frequency.nominal: 50
output.voltage: 230.0
output.voltage.nominal: 230
ups.beeper.status: enabled
ups.delay.shutdown: 20
ups.delay.start: 30
ups.firmware: 5102AH
ups.load: 0
ups.mfr: EATON
ups.model: Ellipse MAX 1100
ups.power.nominal: 1100
ups.productid: ffff
ups.serial: ADKK22008
ups.status: OL CHRG
ups.timer.shutdown: -1
ups.timer.start: -1
ups.vendorid: 0463

Reference: man page: upsc(8), NUT command and variable naming scheme

Startup scripts


This step is not necessary if you installed from packages.

Edit your startup scripts, and make sure upsdrvctl and upsd are run every time your system starts. In newer versions of NUT, you may have a nut.conf file which sets the MODE variable for bundled init-scripts, to facilitate enabling of certain features in the specific end-user deployments.

If you installed from source, check the scripts directory for reference init-scripts, as well as systemd or SMF service methods and manifests.

6.3. Configuring automatic shutdowns for low battery events

The whole point of UPS software is to bring down the OS cleanly when you run out of battery power. Everything else is roughly eye candy.

To make sure your system shuts down properly, you will need to perform some additional configuration and run upsmon. Here are the basics.

Shutdown design

When your UPS batteries get low, the operating system needs to be brought down cleanly. Also, the UPS load should be turned off so that all devices that are attached to it are forcibly rebooted, and subsequently start in the predictable order and state suitable for your data center.

Here are the steps that occur when a critical power event happens, for the simpler case of one UPS device feeding one or several systems:

  1. The UPS goes on battery
  2. The UPS reaches low battery (a "critical" UPS), that is to say, upsc displays:

    ups.status: OB LB

    The exact behavior depends on the specific device, and is related to such settings and readings as:

    • battery.charge and battery.charge.low
    • battery.runtime and battery.runtime.low
  3. The upsmon primary notices the "critical UPS" situation and sets "FSD" — the "forced shutdown" flag to tell all secondary systems that it will soon power down the load.


    By design, since we require power-cycling the load and don’t want some systems to be powered off while others remain running if the "wall power" returns at the wrong moment as usual, the "FSD" flag can not be removed from the data server unless its daemon is restarted. If we do take the first step in critical mode, then we intend to go all the way — shut down all the servers gracefully, and power down the UPS.

    Keep in mind that some UPS devices and corresponding drivers would latch the "FSD" again even if "wall power" is available, but the remaining battery charge is below a threshold configured as "safe" in the device (usually if you manually power on the UPS after a long power outage). This is by design of respective UPS vendors, since in such situation they can not guarantee that if a new power outage happens, their UPS would safely shut down your systems again. So it is deemed better and safer to stay dark until batteries become sufficiently charged.

    (If you have no secondary systems, skip to step 6)

  4. upsmon secondary systems see "FSD" and:

    • generate a NOTIFY_SHUTDOWN event
    • wait FINALDELAY seconds — typically 5
    • call their SHUTDOWNCMD
    • disconnect from upsd
  5. The upsmon primary system waits up to HOSTSYNC seconds (typically 15) for the secondary systems to disconnect from upsd. If any are still connected after this time, upsmon primary stops waiting and proceeds with the shutdown process.
  6. The upsmon primary:

    • generates a NOTIFY_SHUTDOWN event
    • waits FINALDELAY seconds — typically 5
    • creates the POWERDOWNFLAG file in its local filesystem —  usually /etc/killpower
    • calls the SHUTDOWNCMD
  7. On most systems, init takes over, kills your processes, syncs and unmounts some filesystems, and remounts some read-only.
  8. init then runs your shutdown script. This checks for the POWERDOWNFLAG, finds it, and tells the UPS driver(s) to power off the load by sending commands to the connected UPS device(s) they manage.
  9. All the systems lose power.
  10. Time passes. The power returns, and the UPS switches back on.
  11. All systems reboot and go back to work.

How you set it up

NUT user creation

Create a upsd user for upsmon to use while monitoring this UPS.

Edit upsd.users and create a new section. The upsmon will connect to upsd and use these user name (in brackets) and password to authenticate (as specified in its configuration via MONITOR line).

This example is for defining a user called "monuser":

        password = mypass
        upsmon primary
        # or upsmon secondary

References: upsd(8), upsd.users(5)

Reloading the data server

Reload upsd. Depending on your configuration, you may be able to do this without stopping the upsd daemon process (if it had saved a PID file earlier):

upsd -c reload

If that doesn’t work (check the syslog), just restart it:

upsd -c stop

For systems with integrated service management (Linux systemd, illumos/Solaris SMF) their corresponding reload or refresh service actions should handle this as well. Note that such integration generally forgoes saving of PID files, so upsd -c <cmd> would not work. If your workflow requires to manage these daemons beside the OS provided framework, you can customize it to start upsd -FF and save the PID file.

NUT releases after 2.8.0 define aliases for these units, so if your Linux distribution uses NUT-provided unit definitions, systemctl reload upsd may also work.


If you want to make reloading work later, see the entry in the FAQ about starting upsd as a different user.

Power Off flag file

Set the POWERDOWNFLAG location for upsmon.

In upsmon.conf, add a POWERDOWNFLAG directive with a filename. The upsmon will create this file when the UPS needs to be powered off during a power failure when low battery is reached.

We will test for the presence of this file in a later step.

POWERDOWNFLAG /etc/killpower

References: man pages: upsmon(8), upsmon.conf(5)

Securing upsmon.conf

The recommended setting is to have it owned by root:nut, then make it readable by the group and not by the world. This file contains passwords that could be used by an attacker to start a shutdown, so keep it secure.

chown root:nut upsmon.conf
chmod 0640 upsmon.conf

This step has been placed early in the process so you secure this file before adding sensitive data in the next step.

Create a MONITOR directive for upsmon

Edit upsmon.conf and create a MONITOR line with the UPS definition (<upsname>@<hostname>), username and password from the NUT user creation step, and the "primary" or "secondary" setting.

If this system is the UPS manager (i.e. it’s connected to this UPS directly and can manage it using a suitable NUT driver), its upsmon is the primary:

MONITOR myupsname@mybox 1 monuser mypass primary

If it’s just monitoring this UPS over the network, and some other system is the primary, then this one is a secondary:

MONITOR myupsname@mybox 1 monuser mypass secondary

The number 1 here is the "power value". This should always be set to 1, unless you have a very special (read: expensive) system with redundant power supplies. In such cases, refer to the User Manual:

Note that the "power value" may also be 0 for a monitoring (administrative) system which only observes the remote UPS status but is not impacted by its power events, and so does not shut down when the UPS does.

References: upsmon(8), upsmon.conf(5)

Define a SHUTDOWNCMD for upsmon

Still in upsmon.conf, add a directive that tells upsmon how to shut down your system. This example seems to work on most systems:

SHUTDOWNCMD "/sbin/shutdown -h +0"

Notice the presence of "quotes" here to keep it together.

If your system has special needs (e.g. system-provided shutdown handler is ungracefully time constrained), you may want to set this to a script which does customized local shutdown tasks before calling init or shutdown programs to handle the system side of this operation.

Start upsmon

If it complains about something, then check your configuration.

On operating systems with service management frameworks, the monitoring client life-cycle is managed by nut-monitor service.

Checking upsmon

Look for messages in the syslog to indicate success. It should look something like this:

May 29 01:11:27 mybox upsmon[102]: Startup successful
May 29 01:11:28 mybox upsd[100]: Client monuser@
logged into UPS [myupsname]

Any errors seen here are probably due to an error in the config files of either upsmon or upsd. You should fix them before continuing.

Startup scripts


This step is not need if you installed from packages.

Edit your startup scripts, and add a call to upsmon.

Make sure upsmon starts when your system comes up. On systems with upsmon primary (also running the data server), do it after upsdrvctl and upsd, or it will complain about not being able to contact the server.

You may delete the POWERDOWNFLAG in the startup scripts, but it is not necessary. upsmon will clear that file for you when it starts.


Init script examples are provide in the scripts directory of the NUT source tree, and in the various packages that exist.

Shutdown scripts


This step is not need if you installed from packages.

Edit your shutdown scripts, and add upsdrvctl shutdown.

You should configure your system to power down the UPS after the filesystems are remounted read-only. Have it look for the presence of the POWERDOWNFLAG (from upsmon.conf(5)), using this as an example:

        if (/sbin/upsmon -K)
                echo "Killing the power, bye!"
                /sbin/upsdrvctl shutdown

                sleep 120

                # uh oh... the UPS power-off failed
                # you probably want to reboot here so you don't get stuck!
                # *** see also the section on power races in the FAQ! ***


  • Be careful that upsdrvctl shutdown command will probably power off your machine and others fed by the UPS(es) which it manages. Don’t use it unless your system is ready to be halted by force. If you run RAID, read the RAID warning below!
  • Make sure the filesystem(s) containing upsdrvctl, upsmon, the POWERDOWNFLAG file, ups.conf and your UPS driver(s) are mounted (possibly in read-only mode) when the system gets to this point. Otherwise it won’t be able to figure out what to do.
  • If for some reason you can not ensure upsmon program is executable at this point, your script can (test -f /etc/killpower) in a somewhat non-portable manner, instead of asking upsmon -K for the verdict according to its current configuration.
Testing shutdowns

UPS equipment varies from manufacturer to manufacturer and even within model lines. You should test the shutdown sequence on your systems before leaving them unattended. A successful sequence is one where the OS halts before the battery runs out, and the system restarts when power returns.

The first step is to see how upsdrvctl will behave without actually turning off the power. To do so, use the -t argument:

upsdrvctl -t shutdown

It will display the sequence without actually calling the drivers.

You can finally test a forced shutdown sequence (FSD) using:

upsmon -c fsd

This will execute a full shutdown sequence, as presented in Shutdown design, starting from the 3rd step.

If everything works correctly, the computer will be forcibly powered off, may remain off for a few seconds to a few minutes (depending on the driver and UPS type), then will power on again.

If your UPS just sits there and never resets the load, you are vulnerable to a power race and should add the "reboot after timeout" hack at the very least.

Also refer to the section on power races in the FAQ.

Using suspend to disk

Support for suspend to RAM and suspend to disk has been available in the Linux kernel for a while now. For obvious reasons, suspending to RAM isn’t particularly useful when the UPS battery is getting low, but suspend to disk may be an interesting concept.

This approach minimizes the amount of disruption which would be caused by an extended outage. The UPS goes on battery, then reaches low battery, and the system takes a snapshot of itself and halts. Then it is turned off and waits for the power to return.

Once the power is back, the system reboots, pulls the snapshot back in, and keeps going from there. If the user happened to be away when it happened, they may return and have no idea that their system actually shut down completely in the middle (although network connections will drop).

In order for this to work, you need to shutdown NUT (UPS driver, upsd server and upsmon client) in the suspend script and start them again in the resume script. Don’t try to keep them running. The upsd server will latch the FSD state (so it won’t be usable after resuming) and so will the upsmon client. Some drivers may work after resuming, but many don’t and some UPS devices will require re-initialization, so it’s best not to keep them running either.

After stopping NUT driver, server and client you’ll have to send the UPS the command to shutdown only if the POWERDOWNFLAG is present. Note that most likely you’ll have to allow for a grace period after calling upsdrvctl shutdown since the system will still have to take a snapshot of itself after that. Not all drivers and devices support this, so before going down this road, make sure that the one you’re using does.

  • see if you can query or configure settings named like load.off.delay, ups.delay.shutdown, offdelay and/or shutdown_delay

RAID warning

If you run any sort of RAID equipment, make sure your arrays are either halted (if possible) or switched to "read-only" mode. Otherwise you may suffer a long resync once the system comes back up.

The kernel may not ever run its final shutdown procedure, so you must take care of all array shutdowns in userspace before upsdrvctl shutdown runs.

If you use software RAID (md) on Linux, get mdadm and try using mdadm --readonly to put your arrays in a safe state. This has to happen after your shutdown scripts have remounted the filesystems.

On hardware RAID or other kernels, you have to do some detective work. It may be necessary to contact the vendor or the author of your driver to find out how to put the array in a state where a power loss won’t leave it "dirty".

Our understanding is that most if not all RAID devices on Linux will be fine unless there are pending writes. Make sure your filesystems are remounted read-only and you should be covered.

6.4. Typical setups for enterprise networks and data rooms

The split nature of this UPS monitoring software allows a wide variety of power connections. This chapter will help you identify how things should be configured using some general descriptions.

There are two main elements:

  1. There’s a UPS attached to a communication (serial, USB or network) port on this system.
  2. This system depends on a UPS for power.

You can play "mix and match" with those two to arrive at these descriptions for individual hosts:

  • A: 1 but not 2
  • B: 2 but not 1
  • C: 1 and 2

A small to medium sized data room usually has one C and a bunch of Bs. This means that there’s a system (type C) hooked to the UPS which depends on it for power. There are also some other systems in there (type B) which depend on that same UPS for power, but aren’t directly connected to it communications-wise.

Larger data rooms or those with multiple UPSes may have several "clusters" of the "single C, many Bs" depending on how it’s all wired.

Finally, there’s a special case. Type A systems are connected to an UPS’s communication port, but don’t depend on it for power. This usually happens when an UPS is physically close to a box and can reach the serial port, but the power wiring is such that it doesn’t actually feed that box.

Once you identify a system’s type, use this list to decide which of the programs need to be run for monitoring:

  • A: driver and upsd
  • B: upsmon (in secondary mode)
  • C: driver, upsd, and upsmon (in primary mode, as the UPS manager)


To further complicate things, you can have a system that is hooked to multiple UPSes, but only depends on one for power. This particular situation makes it an A relative to one UPS, and a C relative to the other. The software can handle this — you just have to tell it what to do.


NUT can also serve as a data proxy to increase the number of clients, or share the communication load between several upsd instances.

If you are running large server-class systems that have more than one power feed, see the next section for information on how to handle it properly.

6.5. Typical setups for big servers with UPS redundancy

By using multiple MONITOR statements in upsmon.conf, you can configure an environment where a large machine with redundant power monitors multiple separate UPSes.


Example configuration

For the examples in this section, we will use a server with four power supplies installed and locally running the full NUT stack, including upsmon in primary mode — as the UPS manager.

Two UPSes, Alpha and Beta, are each driving two of the power supplies (by adding up, we know about the four power supplies of the current system). This means that either Alpha or Beta can totally shut down and the server will be able to keep running.

The upsmon.conf configuration which reflects this is the following:

MONITOR ups-alpha@myhost 2 monuser mypass primary
MONITOR ups-beta@myhost  2 monuser mypass primary

With such configuration, upsmon on this system will only shut down when both UPS devices reach a critical (on battery + low battery) condition, since Alpha and Beta each provide the same power value.

As an added bonus, this means you can move a running server from one UPS to another (for maintenance purpose for example) without bringing it down since the minimum sufficient power will be provided at all times.

The MINSUPPLIES line tells upsmon that we need at least 2 power supplies to be receiving power from a good UPS (on line or on battery, just not on battery and low battery).


We could have used a Power Value of 1 for both UPS, and have MINSUPPLIES set to 1 too. These values are purely arbitrary, so you are free to use your own rules. Here, we have linked these values to the number of power supplies that each UPS is feeding (2) since this maps better to physical topology and allows to throw a third or fourth UPS into the mix without much configuration headache.

Multiple UPS shutdowns ordering

If you have multiple UPSes connected to your system, chances are that you need to shut them down in a specific order. The goal is to shut down everything but the one keeping upsmon alive at first, then you do that one last.

To set the order in which your UPSes receive the shutdown commands, define the sdorder value in your ups.conf device sections.

        driver = usbhid-ups
        port = auto
        sdorder = 2
        driver = mge-shut
        port = /dev/ttyS0
        sdorder = 1
        driver = blazer_ser
        port = /dev/ttyS1
        sdorder = 0

The order runs from 0 to the highest number available. So, for this configuration, the order of shutdowns would be misc, littleguy, and then bigone.


If you have a UPS that shouldn’t be powered off when running upsdrvctl shutdown, set its sdorder to -1.

Other redundancy configurations

There are a lot of ways to handle redundancy and they all come down to how many power supplies, power cords and independent UPS connections you have. A system with a 1:1 cord:supply ratio has more wires stuffed behind it, but it’s much easier to move things around since any given UPS drives a smaller percentage of the overall power.

More information can be found in the NUT user manual, and the various user manual pages.