Networking bafflement

Volume 3, Issue 14; 27 Apr 2019

When I said I was having a weird networking issue, I didn’t know the half of it.

A couple of weeks ago, more or less, I mentioned a weird networking issue: if my laptop was plugged into the network, everything was fine. If I disconnected the ethernet cable, everything stopped. And I don’t just mean for my laptop, which has a perfectly functional wireless connection to the router, I mean every device on the network is unable to reach the internet. If I logged into the router, I couldn’t even ping or (but I could, from the router, if my laptop was wired).

Here’s what my network looks like:

 Network configuration

Random chats with several folks lead me to the conclusion that my router had become flaky. It was a several-years-old Netgear R7000 running DD-WRT. Aside from the fact that in my experience residential networking gear has a half life of about five years, I had no real evidence to support my conclusion. But I didn’t have any other guesses, either.

Everything I know about networking I learned via web searching and experimentation. I am not well equipped to determine what’s really going on here. And I’m busy, this is not a fun challenge right now.

I fretted over the frustratingly opaque and jargon riddled pages at the DD-WRT and OpenWrt sites for a couple of days and ordered myself a Netgear R7800. Onto which I installed OpenWrt (because James ☺).

And…it made no difference.

Except I can ping numeric IP addresses now. (I haven’t tried to, and don’t have the time or energy to try to, reproduce the configuration where I couldn’t even ping IP addresses.) However, I can’t ping my DNS server, really cannot recommend running Pi-hole strongly enough. Whenever I have it turned off, I see ads in all kinds of places I don’t expect. The beauty of using Pi-hole is that it blocks ads for every device on the network. I had no idea, for example, that the Roku channels page has advertisements! No DNS certainly explains why the problem effects every device on my network!

I run Pi-hole on a RaspberryPi and all my DNS goes through that. I turned that off and sure enough, the problem went away. I’m not running my network without Pi-hole so I did a little dance where I unplugged my laptop, waited for it to fail, plugged my laptop back in and looked at dmesg on the Pi. And there it was:

The CPU reports a problem with the eth0 interface:

WARNING: CPU: 1 PID: 0 at net/sched/sch_generic.c:461 dev_watchdog+0x294/0x298
NETDEV WATCHDOG: eth0 (smsc95xx): transmit queue 0 timed out
Modules linked in: sha256_generic cfg80211 rfkill snd_bcm2835(C) snd_pcm …
CPU: 1 PID: 0 Comm: swapper/1 Tainted: G         C        4.19.36-v7+ #1213
Hardware name: BCM2835
[<80111eac>] (unwind_backtrace) from [<8010d430>] (show_stack+0x20/0x24)
[<8010d430>] (show_stack) from [<8080e560>] (dump_stack+0xd4/0x118)

I have absolutely no explanation for why eth0 on the Pi should crap out if and only if my laptop is disconnected. And it is my laptop; I put another laptop on the wired network and it still failed when I disconnected mine. I can’t concoct any explanation, no matter how far fetched, to explain this behavior.

I’ve been running this Pi-hole configuration for at least a couple of years. It has been flawless. And “I haven’t changed anything.” A bit of web searching lead me to the suggestion that upgrading the Pi firmware might help. It didn’t. Maybe, I concluded, the Pi has just gone flaky. I’ve got another one hooked up to the TV, so I swapped them.

And…exactly the same thing happened.

I was briefly under the misapprehension that the problem went away if I plugged the Pi directly into the router, instead of into the switch. It didn’t.

I put the original Pi back and simply connected it wirelessly. Relying on a wirless connection for DNS seems…suboptimal, but I’ve already burned more than all of the time and energy I have for this. Presumably this will fix the problem; time will tell.

It has occurred to me since that I may have run the ‘apt-get’ upgrade dance on the first Pi, and the process of installing Pi-hole on the second one may also have occasioned the upgrade dance. Maybe this is just a bug in the current Pi distribution. I dunno. I’m giving up for now (assuming the switch to a wireless connection fixes the problem!)

In theory, I can run Pi-hole on the NAS. I’ve put that on my todo list. Near the top of page 3,431 so not something I’m likely to get to in the near term.


It sounds to me like your laptop occasionally sends an ethernet packet that the RPi cannot handle properly. I see reports of jumbo packets causing RPI3 problems (e.g., so I would look at the MTU of the ethernet interface on your laptop. If it's not that simple, then you could run Wireshark on the laptop and look at the packets sent just before things stop working.

I think the easiest path to a more nearly optimal system is to use a USB ethernet adapter on the Pi in place of the built-in ethernet.

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