MAC addresses belong to the NIC and not the machine, and while the OS can override them, it won't do so without express user intervention. I'm especially skeptical of the Ethernet MAC being a duplicate of the Wi-Fi MAC, as this would cause obvious issues (especially considering both Wi-Fi stays up even in the presence of Ethernet, it's just that the routing table is configured to prefer the Ethernet over it) - if this was indeed the case, he would never have had network access on that machine.
However, it is known that some Realtek USB Ethernet controllers have unexpected behavior when powered but no longer enumerated on the USB bus, and they send some low-level Ethernet frame that effectively causes all traffic to stop on that L2 network segment. I'm not sure who is at fault (whether the controller or the switch it's connected to mistakenly rebroadcasting that frame), but here are more details: http://jeffq.com/blog/the-ethernet-pause-frame/ and https://lucumr.pocoo.org/2020/7/6/usb-c-network-hubs/.
It's the people who thought Ethernet Pause was ever a good idea.
I've only seen it lead to tears. In all of the situations I've run into, it would be better to drop packets that are arriving too fast to handle than to try to fix it up by trying to do flow control. Ethernet is unreliable, and packet loss is the overload signal; PAUSE changes overload from drop to buffer, and then you end up with problems with overlarge buffers. In addition to things like where a multi-queue nic has one queue overloaded and sends PAUSE, which results in starvation for the other queues; or PAUSE getting broadcast over a switch when it shouldn't have been, etc.
I would go on a walk and every time the missus would call me to tell that the network is down again...
The switches one is a bear to debug. I just tossed out two old switches, because I wasn't sure which one was the one that polluted the network. I could have spent several hours, setting up little sandboxes, but it was easier just to toss them both, and get new ones.
Never found out the reason. In the end I divorced the wife and she got that notebook.
Wifi is weird.
It's one of the reasons given for why people say Macs "feel better" e.g. you sit down in a room with a friend who has a Dell. Your Mac gets WiFi almost immediately while they have to wait a few seconds even though both of you have been on this network before.
One of the benefits of owning the stack all the way from bare metal to OS to applications.
Our switches didn't handle this correctly and would forward the PAUSE frame as it if it were a broadcast. When this happened, the entire network would cease to function until the offending dock was found and disconnected.
Simply opening WireShark for a few minutes might have helped the poster identify their problem faster. Maybe you would observe the same frame transmitting again and again, maybe you would see corrupted frames, or maybe nothing at all.
First time I run a dhcp server by accident and suddenly went sideways but the blast radius was small,
Second time and more interesting one, My campus had a mac address allow list; when I got a new computer and didn't want to handle the process of updating my access access permission, I just run a script to change my mac to the old known address.
Later, I also sold the old computer to another colleague which didn't bother to register as well since everything was working. Long story short, we keep disconnecting each other.
One day I was, "c'mon, I'll fix this". Opened wireshark and started to capture network traffic. I've got a list of mac addresses from the pcap dump and every time I got disconnected, I ran the script spoofing my address to the next one in my list.
That worked fine until the day I spoofed the mac address of a central managed switch that shit itself out of the network.
:)
Sadly, the immense joy that I felt upon resolution is mysteriously lacking from this person's post.
Edit: DHCP not DNS
And when I found out: "Mother f
> The dock apparently holds the mac address of the MacBook and retains the IP even when the MacBook isn’t docked. Thus when the MacBook is removed from the dock and connects to the Network via Wifi with the same Mac address it causes a conflict and worse, it locks the whole network.
Other annoying problem: if I start my computer with the jack plugged in, unplugging it doesn't switch to speakers.
