[ipv6hackers] Best Network Tap - for home

Gert Doering gert at space.net
Fri Jul 12 15:12:15 CEST 2013


On Fri, Jul 12, 2013 at 06:02:36AM -0700, Mark ZZZ Smith wrote:
> > http://en.wikipedia.org/wiki/Gigabit_Ethernet
> > 
> > "In a departure from both 10BASE-T and 100BASE-TX, 1000BASE-T uses all 
> > four cable pairs for simultaneous transmission in both directions through 
> > the use of adaptive equalization ..."
> Given that I'm the one in this discussion that has quoted the IEEE spec., what makes you think I don't know that?

The way you insist that a passive tap could work suggests that.  Or that you
still haven't fully understood what "a single wire carriers signals in both
directions, *AT THE SAME TIME*" implies for "listening to it, with no way
to discern signal-from-A from signal-from-B".

> Given that Auto MDI/MDI-X is optional for GigE, and there is an
> official x-over wiring for GigE, isn't is reasonable to speculate
> that the techniques for passively tapping 100BASE-T may also be
> applicable to 1000BASE-T? I'm not making any guarantees, because I
> haven't tested it myself. However, I won't just be be "told",
> because, quite frankly, enough things I have been "told" that were
> supposedly absolute over the years that I've relied on have ended
> up not quite true.

To repeat it: 100BaseTX uses the wires in *one* direction.  1000BasesT
uses the same wire in *both* directions.  Which you can't passively tap,
as you have not enough information to discern the signals from each 
other (the systems talking to each other can do that, because the know
which bits are their own, and which bits are coming from the other side).

But if you don't want to just believe that, please go out and test.

> People should learn to be less absolute about what they think
> they know. Humans and their memory, and communications, (unfortunately)
> aren't binary. Always have an element of doubt. 

This is not the day where you should doubt me :-)

Gert Doering
        -- NetMaster
have you enabled IPv6 on something today...?

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