Subject: Re: bpf write() to local host
To: der Mouse <mouse@Rodents.Montreal.QC.CA>
From: Andrew Brown <>
List: tech-net
Date: 01/20/2003 17:38:11
>>> Hum, reading the sources I can't see how adding a second NIC would
>>> help.  Maybe it's hardware-dependant ? Does ifconfig have the
>>> SIMPLEX flag for the ethernet you use with simh ?
>> If the second nic is dedicated to the sim, and you don't configure an
>> IP address on it, then it all works fine (the packets will go out to
>> the Ethernet switch and back again to the other interface on the
>> system :-)
>Except if you're on something like a Sun which uses the same MAC
>address for all interfaces, in which case the poor switch will get
>Terribly Confused.  (And if you don't have a switch, what happens?  I'm
>not sure what NetBSD does if it sees its own MAC address coming from
>what looks like another host; I've never tried it....)

first of all, you can probably make the sun not do that.

anyway, i imagine it would all work out just fine, barring a switch
that insisted that the same mac address *not* appear on two different

just because the hardware addresses match doesn't mean it's all gonna
fall apart.  suppose the sim sends a packet to the "other" local ip
address/hardware address?  presumably the switch/hub would send it
back to the other port (the one that isn't transmitting).  there it
would match the destination hw address, the destination ip address,
etc.  all seems fine, no?

hmm...this makes me think that looping the stuff back in software is
wrong, unless we can guarantee that the switch/hub/whatever won't send
our broadcast/multicast packet back to us.  otherwise we'll get two
copies (one via software and one via the wire).

|-----< "CODE WARRIOR" >-----|             * "ah!  i see you have the internet (Andrew Brown)                that goes *ping*!"       * "information is power -- share the wealth."