IPv6 Neighbor Solicitation incorrectly retransmitted by PFSense?


  • I'm seeing some strange behaviour on a private IPv6 network with PFSense configured as an IPv6 RA.

    During SLAAC configuration / DAD, a client is sending a Neighbor Solicitation packet to PFSense's Ethernet MAC with source, destination, and target address IPs all set to its global address (the prefix from the RA and it's interface ID).

    When PFSense receives this packet, it transmits an NS for the target address on the same interface it was received on. This NS has a source of PFSense's local Ethernet MAC and IP, and a multicast destination (Ethernet MAC of IPv6mcast, IP of the "All Nodes" multicast for the target address). This causes the client to report a duplicate IP address, and prevents it from using the SLAAC-discovered address.

    Why would PFSense retransmit this packet? Aren't NS requests supposed to be entirely local and point-to-point?

    Even if the PFSense behaviour is correct, is there a way I can configure PFSense to drop this specific packet without breaking the rest of neighbor-discovery, so that I can prove whether or not it is the cause of the duplicate-address problem?

  • LAYER 8 Netgate

    You will probably have to provide a packet capture file pointing out exactly what you think is being done incorrectly.

    SLAAC works fine on a pfSense interface.

    When you connect a host it should send a Router Solicitation. In response it will get a Router Advertisement. From the information in that RA the host will choose a SLAAC address, set the gateway, DNS server(s), etc.

    It could also just sit silently and wait for a periodic RA.

    Neighbor discovery is more like ARP and shouldn't be adding addresses to interfaces in any case.


  • pfsense-ns-retransmit.pcap

    I've attached the two packets that show the unexpected behaviour.

  • LAYER 8 Netgate

    Exactly what frame(s) do you believe to be incorrect and why?


  • There are only two frames in the capture. The first is the NS from the client, with IPs that are within the local link's prefix list. The second is the NS from PFSense that I believe shouldn't be sent at all.

  • LAYER 8 Netgate

    I am unfamiliar with this fec0:: prefix. What is that?


  • fec0::/10 is the (deprecated) site-local prefix; as this is a local test network (as this is a test environment, it isn't using a globally-routable address).

  • LAYER 8 Netgate

    Yeah I just found that. Why use something known to be deprecated.

  • LAYER 8 Netgate

    I would use a current, proper ULA /48 for your test network to avoid people asking you silly questions when you raise a potential issue.

    https://cd34.com/rfc4193/


  • pfsense-ns-retransmit2.pcap

    Fine - I've reconfigured the test environment to use a proper ULA /48; as I expected, the behaviour is exactly the same (PFSense sending the multicast solicitation, and the client reporting a duplicate IP address from the PFSense MAC address and the target IP address).


  • @csmith

    You should be using unique local addresses, which are the entire fc:: /block. You should be able to find something in there. 😉

  • LAYER 8 Netgate

    You'll have to help me out in understanding why node fe80::20c:29ff:fe95:8b31 should pay any attention to a Neighbor Solicitation from Src: fd8c:60d0:4040:64:204:2ee:304d:132, Dst: fd8c:60d0:4040:64:204:2ee:304d:132 asking what the link address of fd8c:60d0:4040:64:204:2ee:304d:132 is.

    Node fe80::20c:29ff:fe95:8b31 is sending a properly-formatted Neighbor Solicitation to node fd8c:60d0:4040:64:204:2ee:304d:132's proper solicited-node multicast address.


  • @Derelict

    Unlike IPv4, IPv6 depends on link local addresses for things like RAs & RSs, among many others. A device will accept packets sent to it's link local, ULA or GUA addresses. They all contain the same MAC address, which is what determines whether a NIC accepts the packet. Multicasts, such as neighbour solicitation, use a multicast address where the lower 24 bits (IIRC) of the destination address will form part of that address.

    BTW, if you want to learn about this sort of thing, I recommend IPv6 Essentials.

  • LAYER 8 Netgate

    How does that explain the first frame in the packet capture that has been posted? What should the receiving host do with such an NS?

    You seem to think nobody has any clue about how IPv6 works in every answer you give which have been pointing people to completely irrelevant information.


  • @Derelict

    The first frame would appear to be duplicate address detection, where a device checks to see if it's address is in use. The 2nd is just one device trying to find the MAC of another. In this case the destination is a multicast address, as I described above, as used with neighbour solicitation. Entirely normal.

  • LAYER 8 Netgate

    Right. So we can wait for OP to outline exactly what is wrong with such behavior.


  • @Derelict

    BTW, you can fire up Wireshark to see the RAs, NSs, etc. You will often see them coming from a link local address going to a ULA or GUA address.

  • LAYER 8 Netgate

    Really?