Note: This article is part of a series. See the Index for more information.

Self-promotion: I’ve recorded this series as a screencast for Pluralsight:
(http://www.pluralsight.com/courses/raspberry-pi-home-server)
If you have a Pluralsight subscription, please consider watching it. Thanks!

Updates: I haven’t had any reports of issues with this post under the Jessie release of Raspbian, so I assume it all still works. I don’t have a spare UPS to test with, though, so unless someone reports an issue under Jessie, I think everything still works.

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Adding more Pis to the same UPS (optional)

With Raspberry Pis being so cheap, you may very well have more than one. Perhaps you’ve split CrashPlan or MiniDLNA off onto a Pi all by itself. Maybe you went a little crazy and built a Pi-powered super-computer cluster. If, for any of these reasons, you have multiple Pis, and you’d like them to share the same UPS, how will they all know when the power’s gone out? After all, you can only plug the USB cable into one of them, and buying a separate UPS for each Pi would be overkill, right?

The great thing about NUT being split up into layers is that you can run multiple client instances against one server. Normally, this is the sort of thing you’d only see in a large server rack, where you’ve assigned one computer to watch the battery and tell all the others when the power goes out. It’s not the sort of thing regular home users usually care about, but Raspberry Pi owners are not regular home users.

If you do happen to have multiple Pis, getting them all to share a single UPS and shut down gracefully is actually really simple.

Prerequisites

For this article, I’m assuming you’ve already completed the standalone version in the previous article. If you came straight here from a link or a search, go look at that article first, and get everything working before continuing.

Reconfigure NUT on the server

In the previous article, NUT was configured in “standalone” mode, which means that the server and client were on the same machine. It also means that the server isn’t expecting any other clients to be talking to it, so it simply isn’t listening for them.

Log in to the Raspberry Pi that is connected to the UPS. With a little bit of reconfiguration, it will become a NUT server. Edit the NUT configuration file.

sudo nano /etc/nut/nut.conf

Scroll down to the bottom, and change the mode from “standalone” to “netserver” (not “nutsever”, mind you).

Close and save the file (ctrl-x, y, enter)

Next, you’ll need to configure credentials for the remote machines to use when connecting to the server.

sudo nano /etc/nut/upsd.users

Add a new user at the bottom. You can call it anything you like, but I’ve chosen “upsmon-remote”. Give it a password, and specify that it is a upsmon slave.

Next, you need to tell the NUT server to listen for client connections. Edit the UPS daemon configuration file.

sudo nano /etc/nut/upsd.conf

Find the “listen” section, and add a listen directive using the IP address of your server. If you don’t configure anything, the NUT server will listen only to the computer that it’s on, which is localhost, or IP address 127.0.0.1. Since you now want NUT to listen for clients on other systems as well, you’ll need to specify a real IP address.

Adding the server’s own IP address might seem a little redundant, but remember that NUT is made to work on all kinds of equipment, and it’s very common for larger-scale servers to have multiple network interface cards, and you’d only want it to listen for NUT traffic on the “inward” facing one.

Because you will still want to run a ups monitoring client locally on the server itself, and you’ve now overridden the defaults by specifying one IP address, you’ll need to explicitly add an entry for localhost (127.0.0.1) back in as well, otherwise the local ups monitor will stop working. The result should look something like this:

Note: You’ll need to substitute your own Pi’s address in place of 192.168.1.11. That’s just the address where my own Pi server lives. The default port that NUT listens to is 3493. If you wanted to change that, for some reason, you would do that here. I’m going to leave it at the default port, though.

Reboot the server to get things reconfigured and restarted.

sudo reboot

Double check that you can still communicate with the NUT locally:

sudo upsc rphs

You should get the same kind of listing of UPS settings and status as you did in the previous article:

If you get some kind of error instead, make sure that you added an entry for localhost (127.0.0.1) in addition to the Pi’s regular IP address.

You can actually run NUT configured this way, but without any clients connected, and it will behave in pretty much the same way as it did in standalone mode, but it will still be listening for additional clients over the network.

Configure the client machines

Install NUT on each additional Pi that you intend to run from the UPS.

sudo apt-get install nut

For these additional client-only installations, you can skip configuring a UPS driver and proceed directly to configuring the client.

sudo nano /etc/nut/nut.conf

Go to the bottom of the file, and set MODE to “netclient”.

Next, edit the upsmon configuration file.

sudo nano /etc/nut/upsmon.conf

This time, configure the client to talk to the remote server for information, and configure it as a slave. This time, the “system name” will not be “RPHS@localhost”, but “RPHS@192.168.1.11” (again, substitute your server’s IP address).

If you remember from last time, the first part of this “system name” is the name you gave your UPS, and the second part is the address of the server it’s attached to. The name and password should match the “upsmon-remote” name and password from above, and the final parameter will be “slave” instead of “master”. The result should look like this:

The difference between “master” and “slave” has to do with when each one will shut down. When the UPS says the power has dipped below the safe threshold (20% for my UPS), the server (the master), will tell all of the clients (the slaves) to shut down first before shutting down itself.

Like on the server, you’ll need to set up the permissions so that the NUT configuration files are readable by the nut user. Unlike the server, though, you don’t need to touch the upsd.users file because users are defined on the server, not the client.

sudo chown nut:nut /etc/nut/*

sudo chmod 640 /etc/nut/upsmon.conf

Check connectivity

From the slave system, you can issue the same kinds of commands you could run from the master system. Use this to verify that everything is communicating properly.

sudo upsc RPHS@192.168.1.11

I’ve used the IP address because name resolution didn’t work for me. It’s one of those things I’ll probably learn to set up someday, just not today.

Test the system

Once again, unplug the UPS, and you should see warning messages appear on both the server and client(s) right away. You may want to mute the beeper if your hardware supports it, and plug something in to help the battery drain faster.

Here is what it looks like from the server’s perspective:

And this is from the client’s perspective:

Apart from the name of the server (RPHS vs. RPHS2), and the fact that the client shut down first, you wouldn’t be able to tell the difference between the two, even though only the server is actually attached to the UPS.

Lather, Rinse, Repeat

Repeat as necessary for each additional Raspberry Pi you are plugging in to the same UPS. That’s all there is to it. When the power goes out, all your Pis can now shut themselves down safely.