Subject: Re: /dev/tap and tcpdump don't go together very well?
To: Rhialto <>
From: Johnny Billquist <>
List: current-users
Date: 04/08/2007 15:41:13
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This all reminds me of another thing.
Are there any way, or plans to be able to, change the mac address of an 
ethernet interface in NetBSD? I don't think there is, but I might have 
missed something.

When doing stuff like Olaf is doing here, changing the MAC address might 
be desireable (in his case if he wants his PDP-10 to run TOPS-20 with 
DECnet enabled, since DECnet wants specific MAC addresses on interfaces...)


Rhialto wrote:
> On Sun 08 Apr 2007 at 06:47:14 -0400, Greg Troxel wrote:
>>Please be far more clear about how this is all supposed to work.
> I wish I could point to some easy example program, but alas. What I'm
> using involves a pdp-10 emulator with TOPS-20 (
> plus some private patches to add /dev/tap to it.
> ( should be all, if I did it right)
> Is there some system program or possibly something in pkgsrc that I can
> try, that uses /dev/tap, so that we can verify independently? Writing a
> separate program that injects some packets into /dev/tap isn't so
> difficult but I am currently wondering how one would verify that they
> arrive at the other end.
>>Running tcpdump on the host as '-i tap0' will put the tap device in
>>promiscuous mode.  But packets should still function in both directions.
> I looked at the source (of course) and I could find no obvious
> explanation. That's why I was wondering if perhaps it was some
> non-obvious but known problem.
>>Try adding -p to not force promiscuous mode.
> Interesting. I still see (the? all?) packets with tcpdump, but the
> telnet connection still functions. This may be a clue towards finding
> the cause.
>>    Greg Troxel <>
> -Olaf.

Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email:             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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