Subject: Re: /dev/tap and tcpdump don't go together very well?
To: Greg Troxel <>
From: Johnny Billquist <>
List: current-users
Date: 04/10/2007 12:40:44
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Greg Troxel wrote:
> Johnny Billquist <> writes:
>>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.
> I think someone just needs to write device-indenpendent support and then
> modify various drivers.
>>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
> I don't think the tap's Ethernet address is relevant.  The PDP-10's
> emulated Ethernet interface (how modern to plug one of those in, instead
> of CHAOSnet or an IMP....) will have an address, but that's controlled
> entirely within the emulator.  Packets with that source address are
> generated, and then written to /dev/tapN to make them show up on the
> virtual interface on the host.

Are you suggesting that we'll always be in promiscous mode then?
I'm not just thinking of source address information, but also reception 
of packets from the outside destined to the virtual machine.
A PDP-10 can be running both DECnet and TCP/IP, in which case you'll 
want it's ethernet address to show up in other machines arp tables as 
well, and so on. And maybe you'd like to dedicate a specific ethernet 
interface to the virtual machine, so that it's actually directly hooked 
into the physical network. So you'd really need the ability to change 
the mac address of specific interfaces.


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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