Subject: SIMPLEX interfaces, broadcast packets and BPF
To: None <tech-net@NetBSD.ORG>
From: Ignatios Souvatzis <>
List: tech-net
Date: 03/20/1997 15:25:43
While looking through old PRs, searching for something else:

PR's 1313, 1693 and 2793 describe the same problem:

Broadcast Packets sent to SIMPLEX interfaces are loopedback to
ourselves through looutput(). If BPF listens to "lo", these packets
are shown to it (possibly again), but with an incorrect header... BPF
thinks they are lo packets, which get an artificial header containing
only the address family ID, while in reality they have (if coming from
Ethernet, depending on the address family) no header, or a 14-byte
Ethernet header.

The author of PR 1693 suggests to not show these packets to BPF at all
(that is, only bpf_mtap() them if they come from a IFF_LOOPBACK

On the other hand, the author of PR 2973 suggests ways to show them

Which behaviour is the intended one?

	Ignatios Souvatzis