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Subject: cipe won't answer arp requests for its interface?
From: James Ralston <qralston+ml.cipe-l,AT,andrew,DOT,cmu,DOT,edu>
Date: Wed, 21 Mar 2001 06:50:46 +0100

I'm trying to set up cipe to connect my home machine to my work
machine, so I can access things when I'm working from home.  Both
machines are running Red Hat Linux 7.0, and I'm using the default RPM
that comes with it (cipe-1.4snap-1).

Each machine has a single Ethernet interface, which is assigned an IP
address that is not in the private range.  The machines are obviously
not on the same subnet.

Following the setup advice, I picked a private IP address for the cipe
interface on my home machine, and a different private IP address for
the cipe interface on my work machine.  On my home machine, I added a
host route for my work machine to force all traffic to my local
gateway via the ethernet interface, and then added a network route for
the cipe interface for my entire work subnet.

Debugging with tcpdump revealed that my home machine would tunnel
packets through the cipe UDP connection to my work machine, and then
resend them from my work machine.  But the packets cipe on my work
machine would then put on the wire would have a source IP address of
the cipe interface of my home machine.  Because of that, I couldn't
talk to any work machines, because they would send their reply packets
to their gateway (because the 10.* IP address wasn't on their subnet).

So, fine.  I tear down the cipe connections, and this time I set the
IP address of the cipe interface on my home machine to an unused IP
address within my work subnet.  I set up the connections again, and
voilà!  The cipe process on my work machine puts the packets from my
home machine on the eth0 wire with that source IP address.  I see that
the machine I'm trying to contact issues an arp request for the IP
address.  But instead of answering the arp request, my work machine
ignores it.

So, I see from running "ifconfig -a" on my work machine that the cipe
interface has an explicit NOARP on it.  I use ifconfig to remove the
NOARP, and repeat my test.  But my work machine still does not answer
the arp request.

The word "arp" never appears in the cipe documentation.  I cannot
figure out how to get cipe to answer the arp requests for the address
of its cipe peer.  It *must* do this so that other machines on my work
subnet know what ethernet interface to direct responses to.

What am I missing here?  Am I grossly misunderstanding how cipe is
supposed to work?


James Ralston, Information Technology
Software Engineering Institute
Carnegie Mellon University, Pittsburgh, PA, USA

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