Drop unicast Ethernet frames not destined for the configured address of a tap(4) instance, if IFF_PROMISC is not set.
Drop unicast Ethernet frames not destined for the configured address
of a tap(4) instance, if IFF_PROMISC is not set.

In tap(4), we should emulate the effect IFF_PROMISC would have on
hardware, otherwise we risk introducing layer 2 loops if tap(4) is
used with bridges. This means not even bpf(4) gets to see them.

This patch has been tested in a variety of situations. Multicast and
broadcast frames are correctly allowed through. I have observed this
behaviour causing problems with multiple QEMU instances hosted on
the same FreeBSD machine.

The checks in in ether_demux() [if_ethersubr.c, rev 1.222, line 638]
are insufficient to prevent this bug from occurring, as ifp->if_vlantrunk
will always be NULL for the non-vlan case.

MFC after:	3 weeks
PR:		86429
Submitted by:	Pieter de Boer (with changes)

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