On Sat, 24 Oct 1998, Mikkel L. Ellertson wrote:
> At 11:57 AM 10/24/98 -0400, you wrote:
> > Is it possible to telnet into a third box using PLIP
> >through ethernet??
> >____ ____ ____
> >| |----------------| |---------------| |
> >|__| PLIP Cable |___| Ethernet |___|
> >laptop shortline mainline
> >192.168.192.4 eth0 192.168.192.3 192.168.192.1
> >plip1 plip1 192.168.192.10
> Make sure you have IP forwarding turned on for shortline.
> It has to forward the packets from plip1 to eth0. Redhat
> has forwrding turned off by default. I am not sure about
> other versions...
Also, the middle box may have to Proxy ARP on behalf of the laptop.
What happens is that mainline tries to send a packet to ``192.168.192.4'',
and knows that it can reach this adddress through its ethernet, because
its routing tables tell it that 192.168.192.x is the ethernet.
But before doing that, it has to know the *ethernet* address corresponding
to ``192.168.192.4'', so it sends out an ARP request. Guess what, there isn't
anyone on the ethernet who has that address, so the ARP request goes
unanswered, and thus mainline can never reply to packets coming
from the laptop.
You have to set up shortline to answer ARP requests on behalf of
``192.168.192.4''. This is called Proxy ARP, or ``the ARP hack''.
RTFM on the subject of ARP and Proxy ARP.
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