Radv proto sending adverts on wrong interface

[dxld at darkboxed.org]

Hi Ondrej,

It looks like made a mistake when testing my patch. It does in fact not fix
the problem. I then did some more reading of the linux scriptures and it
turns out PACKET_OUTGOING ("Out" in tcpdump) should actually be reliable so
that meant that the "M" means that packet is actually coming in from the
outside.

Lo and behold I had an unintentonal, but at glance harmless, vlan
configuration on the switch both enp1s0 and enp2s0 are connected
to.

Essentially enp2 is untagged vlan 1 and enp1 is untagged vlan 4 and tagged
vlan 1 on the switch side. When sending the (untagged) RA on enp2 then I
would expect to receive this with a vlan 1 tag on enp1 which would have
made it obvious what is going on, but no it was coming in untagged.

Smells like a switch bug[1] to me or maybe I don't understand 802.1Q VLANs as
well as I thought...

Sorry for the noise.

Thanks,
--Daniel

[1]: This is with a Brocade ICX 6450 running R08030u. Relevant config
snippets:

    vlan 1 by port
     tagged ethe 1/1/1 1/1/3
     router-interface ve 1
    
    vlan 4 by port
     tagged ethe 1/1/1 1/1/3
    
    interface ethernet 1/1/3
     dual-mode  1

I can see untagged multicast going into 1/1/3 (enp2s0) coming out 1/1/1 as
untagged despite 1/1/3 being in dual-mode.

Interestingly this also happens for unicasts but only in one direction. If
I add the enp1s0 lladdr to the neighbour table I can see pings through
enp2s0 come in untagged on enp1s0, but the return seems to be filtered
which is why ND doesn't work (remember: ND responses are sent as unicast).

Here's to hoping affordable open Linux NOS switches to come onto the second
hand market eventually...