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I have a set of 1G Ethernet USB adapters. To keep them organized, I wanted to label them with MAC address so I could easily identify the correct interface when more than one was connected.

I plugged three of them into a Linux machine, and got three similar addresses:

20:23:51:92:5b:64 20:23:51:92:62:15 20:23:51:92:6c:ce

One was already plugged into a Windows system, so I did IPCONFIG /ALL and looked at the hardware (Physical) address:

E4-B9-7A-C9-7A-6D

I looked up 20:23:51 at https://macaddress.io, and it was correctly identify as TP-Link.

I looked up E4:B9:7A and the OUI is registered to Dell. The Windows system is Dell, but the USB adapter is the same TP-Link as the others above.

Out of curiosity, I moved the adapter to the Linux system and ip a told me the link/ether address for the adapter is 20:23:51:92:5b:66 which is consistent with the other adapters.

I thought maybe Windows was configured for Random MAC, but E4 is not a local address. There was no an override MAC address in the adapter settings either.

Why would Windows 11 present the physical address differently?

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    my first guess is that the mac address you are seeing in ipconfig /all is not the address of the USB adapter, but the one for the built in ethernet port. even if an adapter is not in use, it will still have a MAC address, and it may be that ipconfig is confused by the USB connection. just a guess. Commented Oct 15, 2024 at 20:06
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    I looked at the adapter's advanced settings in Windows and discovered "MAC Passthrough" was enabled. I was unfamiliar with that term, so a quick search brought me to dell.com/support/kbdoc/en-us/000143263/…. Commented Oct 15, 2024 at 21:26
  • there's another reason this could happen - on wireless there's an option for randomised mac addresses. Its clearly not the case here, but if its not a fixed connection, its worth considering Commented Oct 17, 2024 at 14:47
  • @tim11g before "MAC Passthrough" it was common for software licensing systems to fail when doctoring stations had a network connection as well as the ethernet port in the laptop. Commented Oct 17, 2024 at 22:12

1 Answer 1

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It is caused by MAC Passthrough in the adapter settings. Apparently enabled by default by Dell for USB Ethernet.

For more details, see What is MAC Address Pass-through on Dell Support.

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    the reason Dell enables this by default is to allow corporate network to whitelist PCs but not have to individually whitelist docking stations (which is counter-productive, since then any PC can use the docking station as if it's whitelisted). I'm actually surprised they eventually did this, a few years ago this was our main gripe with them. Commented Oct 16, 2024 at 9:56
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    @Themoonisacheese Business-class laptops from most (all?) major vendors have supported MAC passthrough for ages, it just wasn't always on by default. But then in a corporate environment, firmware configuration is typically centrally managed, so does it really matter much if this is the default or just one of the many things you need to set when provisioning a new laptop? Commented Oct 16, 2024 at 13:14
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    @TooTea it's been years since i left that place but i think our main issue was that we wanted to provision laptops over the network, but they only had usb-c ports and no rj-45, so we had "blessed" usb-c to rj45 adapters that were the only ones whitelisted and it didn't feel very good. Commented Oct 16, 2024 at 14:56
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    @Themoonisacheese: The DELL explanation screams "We're doing security wrong!" The problem is using an easily-spoofed identifier for security purposes. The correct solution would be to use a client certificate preloaded in UEFI... then the same certificate is presented regardless of network adapter, and the network adapters themselves still have distinct MAC addresses and don't collide if multiple are plugged into the same network segment. Commented Oct 16, 2024 at 20:41
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    @Peter-ReinstateMonica: from what i can tell the EUFI program is responsible for initializing the USB device at boot, and configures itself to use MAC passthrough, but the OS loaded after it may reinit or reconfigure usb devices as it sees fit. I think linux pretty much always reinits USB devices on boot. Commented Oct 17, 2024 at 8:31

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