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mrh53 avatar Ubuntu

built-in ethernet port cannot access particular lan device but USB ethernet port and wifi can

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mrh53
  1. Problem detected several months ago after system had be working normally for many months.
  2. Ubuntu 20.04
  3. Built-in ethernet port connected directly to wifi router
  4. Can ping any ethernet devices on LAN and have full access to WAN through router.
  5. cannot ping any wifi device associated with router. Destination is unreachable. Can ping other wifi devices connected to other access points (2 additional on LAN).
  6. Disable built-in ethernet port and enable either wifi or USB-ethernet port and all devices on lan are pingable.
  7. Other than the link device used I cannot see any difference in route tables for any of the links when each is individually enabled.
  8. Arp table is incomplete for unreachable devices when built-in ethernet link is used. Arp table shows those devices when either USB-ethernet or wifi links are used.

Since I've had workarounds I have not been pursuing very aggressively. However, it is now bugging me. What would cause arp to fail for the built-in specifically when trying to reach stations associated with the router but not the USB-ethernet link connected to the same router?

I will be looking into masquerading the MAC address to see if it is the router that is treating the links differently or truly within the network stack. Any advice on how to fix or further investigate is appreciated.

Update: I swapped MAC addresses between the built-in link and the USB-ethernet link and the problem stayed with the built-in link.

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heynnema avatar
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heynnema
This sounds like a router setting for "isolation" that separates wifi access from the LAN. Check your router settings. The naming might be different, depending on your vendor.
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mrh53 avatar
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mrh53
It turns out the unreachable wifi devices were connected to an AP and not the router as I originally stated. This does not explain why the USB-ethernet connection could reach the devices while the built-in when separately enabled could not. I have switched off the AP in question, which forced the devices to associate with the router and I no longer have the problem with the built-in link. I'm still puzzled why only that link would have problems.
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waltinator avatar
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waltinator
Be the Spark! Pretend to be a TCP/IP packet on one of the systems. How is the packet routed out of the system? `ip-route`. Which system does it go to? What rules is it subject to on that system? Where does it go next? and next?
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mrh53 avatar
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mrh53
That's my problem. ip route is the same for each of the three links, only one being active at a time. I use the USB-ethernet and built-in ethernet connected to the same port on the router when I test each, but get different results. I swapped MAC addresses between the two links and experienced no change in the results. Since this is a LAN I don't think hop counts come into play. That is for routing.
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