A denial-of-service attacker in my Ethernet: Philips screen 329P9H

I use a Philips P-line 329P9H screen. A strong reason for why I chose this particular model is its 1 Gbit/s Ethernet (RJ-45) port: the 329P9H has the rare property that I can attach it to my network switch with a common Ethernet cable.

When I then attach my notebook to the screen through a single USB-C cable that connection does not only transmit video data and periphery (keyboard and mouse, …) data but also Ethernet. Quite neat, right?

I thought: that way, I could easily continue enjoying the stability of a cabled Ethernet connection at my desk just like I did for most of my professional life, whereas my current notebook does not even have an RJ-45 port anymore.

I also listen to music quite a lot with my music streaming system which is connected to the Internet through the same cabled Ethernet, attached to the same Ethernet switch that my screen is attached to.

Since I have had this new screen the music stream stopped more often than before. And I couldn’t quite believe it: when the music stops and when I then unplug the screen’s Ethernet cable from the switch the music starts again, within one or two seconds.

The first time I tried this was when I ran out of options debugging the absence of Internet connectivity at my music stream device. I looked at the switch and the activity LED for the screen’s Ethernet port (on the switch) was flashing at its highest frequency. At that very moment, I did not have the notebook attached to the screen. That was fishy: where would the Ethernet activity come from if not from the (idle, sleeping) screen itself? I powered off the screen and the Ethernet (and therefore Internet) started to work again, quasi-instantaneously, for my music streaming device, and for all other devices attached to the switch.

I observed this about five times so far. It’s no coincidence, seems to be an actual fault in the screen’s Ethernet “controller”. This only seems to happen when no notebook is attached to the screen. It happens when the screen is in ‘sleep’ mode. Power-cycling the screen is sufficient to make the problem go away (until it comes back after a couple of days or weeks).

The Ethernet switch I use is consumer-grade but it is a decent 5-port 1 Gbit/s switch which has served me well over the years. Several different networking devices just worked fine with that switch. This new screen is the first device that trips up the switch.

This happened again today. I think now I leave the screen unplugged from the switch using just the wireless network from my notebook. :-(.