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A Bug Caused by MAC Address

When developing CoHPC, I use QEMU to launch virtual machine and setup VM with cloud-init.

Add NIC in QEMU boot command:

${QEMU} \
    -netdev vde,id=net0,sock=./vdesock \
    -device e1000,netdev=net0,mac=${MAC_ADDRESS} \
    -netdev user,id=user0,hostfwd=tcp::64306-:22 \
    -device e1000,netdev=user0 \
    ...

I randomly generates MAC address into ${MAC_ADDRESS}. In Network Config section in cloud-init, I adopted the same MAC address as in the QEMU command, and allocated a static IP address.

But QEMU log indicates that NIC might not be configured correctly randomly. It turns out that MAC address generation is wrong.

48-bit MAC address(from Wikipedia)

Since MAC address is randomly generated, so b0 bit in the first byte might be 1, meaning multicast and IP address could not be claimed by that NIC correctly. MAC should be unicast.

So the first 3 bytes of MAC address should not be randomly generated. Usually 52:00:00 is good. And the randomness could be left to latter 3 bytes.