Subject: Re: /dev/tap and tcpdump don't go together very well?
To: Greg Troxel <>
From: Rhialto <>
List: current-users
Date: 04/08/2007 15:24:46
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 'Rhialto' Seibert      -- You author it, and I'll reader it.
\X/ rhialto/at/        -- Cetero censeo "authored" delendum esse.