a stupid question about ethernet addresses

jonathandsg.stanford.edu writes:

> In message <87u096liq4.fsfphi.internal.fnop.net>Rui Paulo writes
>
>>"Konstantin KABASSANOV"
<Konstantin.Kabassanovlip6.fr> writes:
>>
>>> If there is a packet with the same source and
destination mac address
>>> (typically a packet wrongly sent over an
interface different from the
>>> loopback), what could be the right behaviour of
the kernel? Is the packet
>>> treated twice (no way to make difference
between incoming and outgoing
>>> direction)?
>>
>>If I understad you correctly you are asking what
should a system do if
>>it recives a packet with the same src and dst MAC
address ?
>>Then I guess, it should be dropped.
>
> Rui,
>
> I can't see any reason why such packets should be
dropped, given that
> the local host decided to emit them?.

I didn't understand that they were emitted by the local
host. My
mistake.

-- 
  Rui Paulo			<rpaulo{NetBSD{,-PT}.org,fnop.net}>

In message <87ek0alcg0.fsfphi.internal.fnop.net>Rui Paulo writes
>jonathandsg.stanford.edu writes:
>
>> In message <87u096liq4.fsfphi.internal.fnop.net>Rui Paulo writes
>>
>>>"Konstantin KABASSANOV"
<Konstantin.Kabassanovlip6.fr> writes:
>>>
>>>> If there is a packet with the same source
and destination mac address
>>>> (typically a packet wrongly sent over an
interface different from the
>>>> loopback), what could be the right
behaviour of the kernel? Is the packet
>>>> treated twice (no way to make difference
between incoming and outgoing
>>>> direction)?
>>>
>>>If I understad you correctly you are asking what
should a system do if
>>>it recives a packet with the same src and dst
MAC address ?
>>>Then I guess, it should be dropped.
>>
>> Rui,
>>
>> I can't see any reason why such packets should be
dropped, given that
>> the local host decided to emit them?.
>
>I didn't understand that they were emitted by the local
host. My
>mistake.

Oh, not necessarily your mistake at all.  It's more that I
don't see
how a network stack or driver can distinguish the
odd-but-legal case
of a self-addressed MAC frame, from case where a remote host
sends a
packet to the local MAC but with the same MAC as a (spoofed)
source
MAC address (which I confess I didn't really consider).

I deal regularly with multi-homed machines with mutilple
interfaces,
all on the same Ethernet domain. Think about the
implications of
trying to filter based on "is this source MAC address
one of my local
MACs?", and consider multicast and broadcast traffic.
That'll give you
some sense of where I'm coming from.

I concede that's a rather unusual viewpoint :-/.