[vbox-dev] Setting the hardware MAC address on a virtual Ethernet interface is not working correctly

David Garrod dgarrod at extremenetworks.com
Thu Aug 25 14:06:04 UTC 2016


Many thanks for your answer. But I still don't fully understand what is happening. Specifically I stated:

> System A sends a Broadcast ARP to resolve the MAC address for B. This packet is received by system B and of course a UNICAST reply is sent back with the resolution for MAC B.

> System B then sends a unicast ARP packet (source MACB dest MACA) asking for the resolution of systemA's MAC.

> The pcap trace file I've put on System B's NIC (foobar2.pcap) shows all of these packets. But the pcap trace file on system A (foobar.pcap) only shows the original ARP broadcast packet from system A to system B. It does NOT show either of the Unicast replies. So of course they don't make it into the System A Libux system and hence the ping doesn't work.

> This is the behavior I'd expect had I not set promiscuous mode on the virtual NICs inside VirtualBox. But I have set promiscuous mode and I cannot understand why it is not working.

> Various web searches led me to one post that said that PROMISC mode in VirtualBox has no effect unless the guest has set the interface into promiscuous mode. Indeed I found this to be true. Once I set promiscuous mode on the guest the Unicast packets got through.

So given I've set the interfaces into PROMISCUOUS mode from the virtualbox console why are the Unicast packets not being delivered to "system A's NIC"? From what you say it sounds like the MAC I've programmed on system A's NIC isn't being properly communicated to the internal switch as a MAC when packets with that MAC as the source MAC appear (i.e. that MAC is not learned) But even if this is not the case how come PROMISCUOUS mode in virtualbox doesn't deliver all packets to system A's NIC regardless of destination MAC. As I stated if the guest sets promiscuous mode on the NIC then the packets are delivered. So does this mean that this promiscuous mode setting is being communicated? Why is the setting of promiscuous mode in virtualbox contingent on promiscuous mode being set inside the guest?

Could you give me a pointer to the code you are referring to inside Virtualbox at https://www.virtualbox.org/browser/vbox/trunk As I said I did look here but I got lost.



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