After missing several in-stock notifications I finally upgraded from a 1U CyberPower UPS to a 2U Ubiquiti UPS. The new UPS seemed like a great replacement (excluding a shuffle for space) as it supported Network Ups Tools (NUT) for arbitrary devices.

My Synology NAS runs NUT inside to allow orderly shutdown, but it only allows the NUT SNMP driver. NUT-to-NUT connectivity is always excluded by the UI and startup script. Many people have run into this problem and have solved it by editing the configuration. I don't trust such changes will continue to work so I decided I needed to adapt from NUT to SNMP.

Before attemting to build my own I found go-nut-snmpagent which uses the Agent Extensibility Protocol (AgentX). It said it was inspired by nut-snmpagent. go-nut-snmpagent seems like it was never finished as all the SNMP values appear to be hard-coded. nut-snmpagent hasn't been updated in over a decade, uses the older pass_persist integration with net-snmp and is a combination of awk, bash, ruby, and sed. Updating very old untouched code and keeping it running is too daunting.

I ended up writing snmp-nut in Rust and included OTEL export for metrics and logs.

For NUT communication, NUT published RFC 9271 describing their protocol for getting status information from NUT which is a plain-text protocol that is easy to parse. Section 2.11 links to all variables.

The agentx crate made implemeting the AgentX protocol straightforward. The NUT SNMP driver supports the RFC 1628 UPS-MIB (browsable) so the Synology NAS can consume it.

snmp-nut connects to net-snmp over the AgentX socket with an update to snmpd.conf:

com2sec notConfigUser   default public
group   notConfigGroup  v1      notConfigUser
group   notConfigGroup  v2c     notConfigUser
access  notConfigGroup  ""      any     oauth exact systemview none none

view    systemview      included        .1.3.6.1.2.1.33

master agentx
agentXPerms 770 770 snmpd snmpd
agentXSocket /var/agentx/master

I'm not sure if all the permissions at the top are required, but I didn't try without them. Once I restarted snmpd I was able to run snmp-nut under the snmpd user with a config file pointing to my UPS:

[nut]
address = "192.0.2.10:3493"
name = "UPS"

[snmp]
path = "/var/agentx/master"

Then test with snmpwalk:

$ snmpwalk -v 2c -c public localhost UPS-MIB::upsBattery
UPS-MIB::upsBatteryStatus.0 = INTEGER: batteryNormal(2)
UPS-MIB::upsSecondsOnBattery.0 = INTEGER: 0 seconds
UPS-MIB::upsEstimatedMinutesRemaining.0 = INTEGER: 3327 minutes
UPS-MIB::upsEstimatedChargeRemaining.0 = INTEGER: 100 percent
UPS-MIB::upsBatteryVoltage.0 = INTEGER: 272 0.1 Volt DC

Unplugging the UPS from the wall updates the status:

UPS-MIB::upsEstimatedChargeRemaining.0 = INTEGER: 96 percent
[…]
UPS-MIB::upsOutputSource.0 = INTEGER: battery(5)

Plugging it back in returns upsOutputSource:

UPS-MIB::upsEstimatedChargeRemaining.0 = INTEGER: 80 percent
[…]
UPS-MIB::upsOutputSource.0 = INTEGER: normal(3)

Also, the Synology NAS logs reported "Server is on battery."

I haven't yet had the opportunity to perform a full-rack orderly shutdown test as I need to configure the Synology NAS to turn off before the server running snmp-nut.