[HECnet] Multinet Tunnel Connections to SG1::

[Dave McGuire]

On 06/07/2012 03:28 AM, Johnny Billquist wrote:
  SIMH uses libpcap and implements networking for emulated machines by
kicking the interface into promiscuous mode.   That's a bit dangerous
from a traffic perspective; I'm not certain that'd be a good idea for
anything other than the most casual use on a quiet network.

Dangerous in which way?

  Traffic.   My main desktop machine sits on a busy network, and I am
uncomfortable with the notion of it sitting in promiscuous mode.
Remember that I also do all of my work (meaning "make money for food"
work) on this system.

It will create a larger load on the system, but that's about it. And
todays machines are fast enough that you really need a lot of traffic
before it will become a serious problem from that point of view.
Remember that most of the filtering out of unwanted packets are done in
the kernel anyway, and not in user space. That's what the bpf interface
and filter programs do for you.

  Yes I understand, but while not in promiscuous mode, the filtering is
done by the network interface chip, rather than by one of my cores which
are typically pretty busy doing other things.   I have a very fast
machine on my desk, but that doesn't mean I want to waste the cycles...I
use those cycles for lots of stuff. ;)

The main reason in the past for changing the MAC address has been that
you want to control the source MAC address. However, most systems now
allows you to spoof the source MAC when outputting packets on the
ethernet, so that problem is solved.

  Well, and to receive those packets!

Load is the one remaining reason to even worry, and that is a rather
small worry for most people.

  I agree, but it is a bigger concern for me as above.   There is also a
"purist" aspect...I also eschew things like "winprinters" (if you
remember those!)...my main CPU cores have other work to be doing.

With todays network gear, you will not get most packets to your machine
anyway, the switches are already doing the filtering for you, so
promiscuous mode don't hurt much from that point either.

  ...and this is the only thing that makes me more comfortable with the
idea.   The switch will just have more than one entry in its MAC table
for this port.

  But I still don't like it. ;)

                    -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA