Subject: Re: bin/29126: tcpdump leads to packet loss
To: None <>
From: Manuel Bouyer <>
List: netbsd-bugs
Date: 01/26/2005 19:17:34
On Wed, Jan 26, 2005 at 02:03:00PM +0000, wrote:
> Starting tcpdump with or without arguments leads to heavy packet loss, (like freezing all connections, up to 15 seconds) until the first packets are seen on the screen from the dump. During dump flow, no packet loss is detected, but same scenario happens if you stop tcpdump with CTRL-C. Again heavy packet loss. This can lead to break established connections (ssh for example). Maybe this is NIC related. I'm using an ex0 (3com 3c905 txm) Network Card. Verified on 2.99.10 and 2.99.15.

This is because the driver reset the adapter when changing promiscous mode,
which also has the effect of taking the link down (so media settings have to
be renegotiated with the remote end. If you have spanning tree enabled,
you're also hit by a spanning-tree cold restart). I don't know if, for
this particular nic it's possible to enable/disable promiscous mode without
resetting the adapter. Some drivers could possibly be smarter here, but
it's definitively nic-dependant.

Manuel Bouyer, LIP6, Universite Paris VI. 
     NetBSD: 26 ans d'experience feront toujours la difference