SFP+ DDM Monitor

A small C daemon that polls ethtool -m for standard SFP+ DDM (Digital Diagnostics Monitoring) data. It writes hwmon-format sensor files for CoolerControl to read.

The daemon works with any SFP+ transceiver that supports DDM: copper RJ45 (RealHD, MikroTik S+RJ10, and others), SR/LR optical, DAC, and AOC. It parses only the standard Module temperature and Module voltage fields. It ignores all other DDM data (laser bias, optical power, and rx power). As a result, copper transceivers that spoof optical identity work correctly.

What it monitors

• Module temperature → temp1_input (millidegrees C)
• Supply voltage → in1_input (millivolts)
• Alarm and warning thresholds (written one time, on the first successful read)

Only the temperature is useful in CoolerControl. I am still not sure about the other two.

Quick install

# Copy this folder to the target machine
scp -r sfp-monitor/ user@hostname:~/

# Connect with SSH, then install
ssh user@hostname
cd ~/sfp-monitor
sudo ./install.sh

The install script automatically detects ixgbe (Intel 82599) interfaces. If the script finds more than one interface, it shows a numbered menu. If it finds none, it prompts you with tab-autocomplete over all network interfaces. You can also give the interface name directly: sudo ./install.sh enp3s0f0.

The script compiles the C source and creates a systemd service for the interface you chose. Then it enables the service, starts it, and checks the sensor output.

The default poll interval is 5 seconds. To change it, edit the third argument of the ExecStart line in /etc/systemd/system/sfp-monitor.service. Then run:

sudo systemctl daemon-reload && sudo systemctl restart sfp-monitor

Files installed

File	Location
Binary	/usr/local/bin/sfp_monitor
Service (generated)	/etc/systemd/system/sfp-monitor.service
Sensor data	/run/sfp-monitor/<interface>/ (tmpfs, recreated on boot)

Check that your transceiver supports DDM

Before you install the daemon, check that ethtool -m <interface> returns a Module temperature line:

sudo ethtool -m <interface> | grep "Module temperature"

If it does not, then the transceiver does not support DDM, or the driver needs allow_unsupported_sfp=1. The second cause is common with ixgbe.

Uninstall

sudo ./uninstall.sh

Acknowledgments

I developed this project with help from Claude (Anthropic).