Dear [Libvirt Developer Team], It has been quite some time since I initially sent my email, and I never expected to receive a response after so long. I sincerely appreciate your time and effort in addressing this case — thank you! Regarding your question: > To further clarify - when you say "vNIC" do you mean the tap device that > is connected to the bridge on the host side, and "target VM's MAC address" > is the MAC address of the network device inside the guest (VM)? Yes, that is precisely what I meant! Now, returning to the issue at hand: the distinctiveness of MAC addresses is a fundamental principle of network communication. This issue was originally raised by a friend of mine. While I was able to explain the theoretical importance of unique MAC addresses, I couldn't provide a clear explanation of how libvirt handles the "automatic modification" of the tap device's MAC address, as I had only observed changes in the device's state but not the modification itself in the logs. After carefully reviewing your response multiple times and conducting additional tests, I now fully understand the mechanism behind the "0xFE" modification of the first byte of the MAC address, which is handled in virNetDevTapCreateInBridgePort() at line 638 (starting at line 625). However, I wanted to confirm one specific point regarding tap device behavior. According to the official documentation <https://libvirt.org/formatdomain.html#generic-ethernet-connection>: > "If no target dev is specified, libvirt will create a new standard tap > device with a name of the pattern 'vnetN', where 'N' is replaced with a > number. If a target dev is specified and that device doesn't exist, then a > new standard tap device will be created with the exact dev name given." In contrast, you mentioned: > "Any time you stop/destroy a guest, any tap device(s) associated with that > guest will be destroyed, and the next time you start the guest, new tap > devices will be created."
