Synology NAS UPS Setup: USB, Network UPS Server and NUT Clients

Three ways DSM can talk to a UPS

In recent DSM versions, the UPS tab offers three UPS types. Which one you pick depends on where the UPS data comes from.

Setting up a USB UPS

1. Connect the data cable. Use the USB cable that came with the UPS, from the UPS's USB port to a USB port on the NAS. Power cords alone carry no data.
2. Enable UPS support. Control Panel, Hardware & Power, UPS tab. Tick Enable UPS support and select USB UPS.
3. Apply and check Device Information. DSM should show the manufacturer, model and status (for example "OL" or a plain-language equivalent). If the fields are empty, DSM is not reading the UPS.
4. Choose the Safe Mode timing. Either a fixed time on battery, or wait until the UPS reports low battery.
5. Apply. DSM writes its NUT configuration and starts monitoring.

Fixed time or low battery?

The time option tells DSM to enter Safe Mode after a set number of minutes on battery. The low battery option waits until the UPS itself signals low battery, which uses the most runtime but depends on the UPS's own low battery threshold.

What Safe Mode actually does

Safe Mode is not a full shutdown. DSM stops packages and services, flushes write caches and unmounts volumes so the file system is consistent. Depending on model and DSM version, the NAS then powers off or stays in a low-activity state. Settings available in some versions let DSM also send a command to turn the UPS off once Safe Mode is reached, so the NAS genuinely loses power and can restart cleanly when mains returns.

Because of that, set the automatic restart option: Control Panel, Hardware & Power, General, Restart automatically when power supply issue is fixed. Without it, a NAS that fully powered down will stay off until someone presses the button.

Sharing the UPS: the network UPS server

Tick Enable network UPS server on the NAS that owns the USB cable, then click Permitted DiskStation Devices (the label varies slightly between versions) and add the IP address of every client. Under the hood, DSM runs the Network UPS Tools server (upsd) on TCP port 3493, so any NUT client can connect, not just other Synology units.

The details that matter for non-Synology clients have been consistent across DSM versions documented by users, though Synology does not prominently publish them, so verify on your own system:

• UPS name: ups
• Username: monuser
• Password: secret
• Role: secondary (called "slave" in NUT versions before 2.8)

First test from the client machine with the NUT client tools installed:

# Replace 192.168.1.20 with the NAS address
upsc ups@192.168.1.20

# Expected: a list of variables such as
# battery.charge: 100
# ups.status: OL
# ...

If that works, add a MONITOR line to the client's /etc/nut/upsmon.conf. On NUT 2.8 and later:

MONITOR ups@192.168.1.20 1 monuser secret secondary

On older NUT versions, use slave instead of secondary. Also set MODE=netclient in /etc/nut/nut.conf. Full client setup, including SHUTDOWNCMD and notifications, is covered in the NUT guide, and the Proxmox-specific ordering in Proxmox UPS shutdown.

Synology as a client of a non-Synology NUT server

The reverse case is common in homelabs: the UPS USB cable goes to a Linux server, Raspberry Pi or TrueNAS box running NUT, and the Synology should shut down when that server says so. DSM's "Synology UPS server" option only asks for an IP address. It does not let you enter a UPS name, user or password, so it assumes the same defaults it uses as a server.

Configure the NUT server so those defaults work:

# /etc/nut/ups.conf on the NUT server: the section name must be "ups"
[ups]
    driver = usbhid-ups
    port = auto
    desc = "Rack UPS"

# /etc/nut/upsd.conf: listen on the LAN, not only localhost
LISTEN 0.0.0.0 3493

# /etc/nut/upsd.users: add the account DSM will use
[monuser]
    password = secret
    upsmon secondary

Restart the NUT server services, then on the Synology choose Synology UPS server and enter the NUT server's IP. If the server already uses a different UPS name such as [rackups], you can rename the section to ups and update other clients' MONITOR lines accordingly. Listening on 0.0.0.0 exposes port 3493 on every interface, so restrict it with a host firewall or listen only on the LAN address.

Planning a whole-network setup around a Synology

In many homes and small offices the Synology is the most reliable always-on device, which makes it a natural UPS owner. A typical plan, with every device on battery-backed outlets:

The point of this layout is that every device knows the UPS state, and anything that depends on the NAS leaves before the NAS does. A plain secondary client only reacts when the server declares the UPS critical, so devices that must leave early need their own timer. Proxmox UPS shutdown shows how to set one with upssched.

Network gear has no shutdown to perform, but keeping it powered matters: clients lose contact with the Synology the moment the switch goes dark, and a NUT client that loses its server cannot receive the shutdown signal. Most NUT clients treat a lost server on a UPS last seen on battery as critical and shut down, but that is a fallback, not a plan.

Troubleshooting

After any change, run a real test: pull the UPS plug from the wall and watch DSM's notification center and each client's logs. For runtime planning across several devices, see choosing a UPS for a NAS.