An inexpensive way to prevent unscheduled downtime or data loss due to power problems is with a UPS or Uninterruptible Power Supply. However, a UPS by itself is not enough for proper operation. Hardware, software, and configuration together make up a UPS system that will recover from unexpected power loss or power fluctuations that can damage systems and peripherals.
When considering data loss, system downtime and disaster recovery, backup methods are primarily discussed. There are many methods of preventing data loss, including clustering, backup, security and power conditioning. Proper power can prevent an initial disaster from ever occurring. Providing proper power can be in the form of an Uninterruptible Power Supply or UPS. A UPS has rechargeable batteries to supply emergency power in the event of immediate power loss. If the power loss is longer than the batteries can supply, then the UPS can signal the server to initiate a power down sequence to properly shutdown, preventing data loss. When power is returned the server can return to operation after having made a clean shutdown.
Other power related problems that occur can be minimized with the circuitry of a UPS. Voltage sags and spikes, brown outs and line noise (from other machinery like elevators, air conditioners or office equipment), can all be isolated by a UPS. These power related fluctuations can wreak havoc on systems and devices. For a relatively low cost a UPS can prevent downtime due to power anomalies.
The Network UPS Tools (NUT)  are a group of tools that are used to monitor and administer UPS hardware. NUT uses a layered scheme of equipment, drivers, server and clients. The equipment consists of the monitored UPS hardware. Drivers specific to the UPS hardware communicate or poll the UPS for status information in the form of variables. The driver programs talk directly to the UPS equipment and run on the same host as the server. The server
upsd serves data from the drivers to the network. Clients talk to the
upsd server and initiate tasks with the status data.
As indicated by the name, Network UPS Tools, NUT is a network based UPS system that works with multiple UPSs and systems. One of the many features of NUT allows multiple systems to monitor a single UPS, not requiring special UPS sharing hardware connections. The master/slave relationship synchronizes shut-downs so the slaves can initiate power-down sequences before the master switches off UPS power.
This article details the installation and configuration of a single system with a UPS connected to the serial port of the system. This is the natural first step of getting NUT installed and configured. If the UPS will supply more than one system, the second and subsequent systems can be configured as slaves.
The NUT developers also have a different take on when the systems should be powered down. NUT will wait until the UPS is “on battery” and “low battery” before it considers the UPS “critical”. This philosophy gets the most out of the UPS batteries and will wait until the critical moment to initiate a power down sequence, just in case the power comes back on line. There is an option to override this behavior if desired with
upssched, which can be found in the documentation. With the
upssched utility, commands can be invoked based on UPS events.
In typical GNU/Linux fashion, NUT is not the only tool available for monitoring a UPS. Apcupsd  is used for power management and control of APC model UPSs. There are also several graphical frontends for workstation class machines.
Prior to installing and using a UPS and its associated software, a few things must be in place. Since the system is going to be shutdown there must be a way to bring the system back up when power returns. The system BIOS needs to be configured correctly. Most modern BIOSes have an option to power-on when main power, now supplied by the UPS, is returned. Server system BIOSes will most likely support the “power on when power returns” option. If the BIOS does not support this option (more common with workstation class systems), a BIOS update may correct it. With servers configured headless, without a monitor or keyboard, there are also settings to ignore keyboard errors. Commonly these types of systems are administered via SSH or administration utilities like Webmin .
The UPS must also have the correct signal cable from the UPS to the system. With a USB type of UPS this is not a concern. A UPS that communicates via the serial port needs the correct signal cable that supports intelligent signaling between the UPS and system. See the UPS vendor for the correct cable or a custom cable can be built with information from the Network UPS Tools web site.
The example system is running a basic install of Debian GNU/Linux V4.0  and utilizing an APC SmartUPS 700. Debian is an excellent, long term supported GNU/Linux, which is ideal for small enterprise deployments, as well as much larger environments. Different GNU/Linux distributions may install the software and configuration files in different directories. Since the server is configured without a GUI interface, all commands and configuration are done through the command line as the
root user. Using the APT package tool,
apt-get, the package
nut can easily be installed:
# apt-get install nut
The package tool installs the NUT software, documentation, man pages and example configuration files. Debian specific documentation is found in the
/usr/share/doc/nut/ directory. Extensive NUT documentation can be found in
/usr/share/doc/nut/docs/ and the example configuration files in
/usr/share/doc/nut/examples/. Some of the documentation is compressed with
gzip which can be uncompressed or viewed with the
# zcat /usr/share/doc/nut/README.Debian.gz | less
The configuration files exist in the
/etc/nut/ directory. The
ups.conf configuration file contains the UPS definitions. The UPS is defined with the
[labsvr] entry. The
port fields must be defined, the
desc field is optional and describes the UPS. Additional UPS definitions can be configured in this file; however, this example is a single UPS and server configuration.
[labsvr] driver = apcsmart port = /dev/ttyS0 desc = "Lab Server"
The definition, between the square brackets, is user definable, with the exception of word
default, since it is used by the Network UPS Tools. The correct driver name for your UPS can be found in the file
/usr/share/nut/driver.list. For proper permissions to use the serial port the
nut user must be added to the “dialout” group, and can be accomplished with the
addgroup command. To manually test the configuration and to verify the configuration is correct the
upsdrvctl (UPS driver controller) command is used. After verification the driver can be stopped.
# addgroup nut dialout Adding user `nut' to group `dialout' ... Done. # /sbin/upsdrvctl start labsvr Network UPS Tools - UPS driver controller 2.0.4 Network UPS Tools (version 2.0.4) - APC Smart protocol driver Driver version 1.99.8, command table version 2.0 Detected SMART-UPS 700 [QS0331213446] on /dev/ttyS0 # /sbin/upsdrvctl stop labsvr Network UPS Tools - UPS driver controller 2.0.4 Stopping UPS: labsvr #
Since this example is a single server, the access control list for server communication is minimal. Configuring the access control lists is accomplished in the configuration file
upsd.conf. The ACL (access control list)
all is defined with the netblock in CIDR format, the old style
address/network format can also be used. Additionally the ACL
localhost is defined. The
ACCEPT field allows communication to the server for
localhost and the
REJECT field blocks all other access. Similar to other access control lists, flow goes from the top down. The
ACCEPT is evaluated before the
REJECT, if the
REJECT line were before
localhost would meet the rule and not be allowed access.
ACL all 0.0.0.0/0 ACL localhost 127.0.0.1/32 ACCEPT localhost REJECT all
upsd.users configuration file is used to define users that have access to administrative commands. Here are defined the users and what access each user is allowed; each section begins with user names in brackets and continues to the next bracketed user or end of the file. The
password field defines the user’s password. The
allowfrom field grants access based on the user’s source IP address, the values are defined in the ACL lists configuration file
upsmon field is set to either
slave to allow the upsmon process to work.
[monmaster] password = p455w0rd allowfrom = localhost upsmon = master
The final configuration file
upsmon.conf defines which systems the
upsmon process will monitor, as well as how to shutdown systems when necessary. The
MONITOR line defines the UPS to monitor. The first field is the UPS to monitor, in this case
labsvr@localhost. The second field is the power value that defines the number of power supplies the UPS supplies. In simple configurations this is normally set to
1. The next two fields are the user name and password that were previously defined in
upsd.users. The last field will be either
slave process. A
master process is one in which the process is running on the system that is plugged in directly and communicates with the UPS. A
slave is a process that gets power from the UPS but doesn’t communicate directly to it.
MONITOR labsvr@localhost 1 monmaster p455w0rd master POWERDOWNFLAG /etc/killpower SHUTDOWNCMD "/sbin/shutdown -h +0"
POWERDOWNFLAG defines a file name to be created in master mode when the UPS needs to be powered off. This file is cleared when the system comes back up. Finally,
SHUTDOWNCMD is the actual shutdown command performed enclosed in quotes.
After the configuration of the UPS and Network UPS Tools is complete, a couple of housekeeping tasks need to be accomplished. Since several of the configuration files contain user names and passwords, they will have permissions set so only the
root user and
nut group can read them with the following commands:
# chown root:nut /etc/nut/* # chmod 640 /etc/nut/*
With Debian GNU/Linux, two items must also be changed in the file
START_UPSMON are changed from “no” to “yes”.
nut init.d script can be run to start the UPS monitor tools and
/var/log/syslog is checked to verify everything is running correctly.
# /etc/init.d/nut start Starting Network UPS Tools: upsdrvctl upsd upsmon. # tail /var/log/syslog Sep 01 13:36:48 labserver apcsmart: Startup successful Sep 01 13:36:48 labserver upsd: Connected to UPS [labsvr]: apcsmart-ttyS0 Sep 01 13:36:50 labserver upsd: Startup successful Sep 01 13:36:50 labserver upsmon: Startup successful Sep 01 13:36:50 labserver upsd: Connection from 127.0.0.1 Sep 01 13:36:50 labserver upsd: Client email@example.com logged into UPS [labsvr]
To quickly poll the status of a UPS server, the
upsc UPS client utility is used. The first example displays the value of the
ups.status variable, which is
OL (or “on line”), meaning the UPS
labsvr@localhost is on line power. If the value of
OB (“on battery”), then the UPS would be supplying battery power to the server. The second invocation displays all available variables and the values for
# upsc labsvr@localhost ups.status OL # upsc labsvr@localhost battery.alarm.threshold: 0 battery.charge: 100.0 battery.charge.restart: 00 battery.date: 08/02/03 battery.packs: 000 battery.runtime: 7860 battery.runtime.low: 120 battery.voltage: 27.60 battery.voltage.nominal: 024 driver.name: apcsmart driver.parameter.port: /dev/ttyS0 driver.version: 2.0.4 driver.version.internal: 1.99.8 input.frequency: 60.00 input.quality: FF input.sensitivity: H input.transfer.high: 132 input.transfer.low: 103 input.transfer.reason: S input.voltage: 120.2 input.voltage.maximum: 121.5 input.voltage.minimum: 119.6 output.voltage: 120.2 output.voltage.target.battery: 115 ups.delay.shutdown: 180 ups.delay.start: 000 ups.firmware: 50.14.D ups.id: UPS_IDEN ups.load: 008.3 ups.mfr: APC ups.mfr.date: 08/02/03 ups.model: SMART-UPS 700 ups.serial: QS0331213446 ups.status: OL ups.temperature: 037.8 ups.test.interval: 1209600 ups.test.result: NO #
Testing power loss is accomplished with the
upsdrvctl utility. The value of
ups.delay.shutdown is the amount of time in seconds the UPS will wait before shutting down. In the above listing the value is 180 seconds. This value can be changed with the
upsrw utility, though a valid user in
upsd.users with proper permissions must be defined to change variable values. Refer to the
upsrw man pages for more information. The 180 second delay is enough time to allow for a proper shut down of this system.
# upsdrvctl shutdown labsvr; shutdown -h +0
After this command is run,
upsdrvctl tells the UPS to issue its shutdown sequence. The second command tells the system to shutdown immediately. The server shuts down and after the 180 second delay the UPS shuts down and powers back up. If the BIOS is set correctly in the server, when the UPS supplies line power again the server system comes back on line.
In addition to the extensive documentation installed in
/usr/share/doc/nut/ and on the NUT web site, the man pages contain excellent detailed information on the utilities, configuration files and drivers.
Power interruptions are a common problem in many areas and can cause eventual failure of components and systems. Having an uninterruptible power supply to prevent damage and initiate proper power down sequences can save many headaches as well as avoid disaster. Just implementing a UPS system is not enough and the proper UPS, server cabling and motherboard BIOS are all part of a reliable system.