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Subject: RE: options config...
From: Amith Varghese <amith,AT,xalan,DOT,com>
Date: Thu, 13 Feb 2003 01:50:43 +0100
In-reply-to: <7DB0958915FDD611961400A0C98F18460BCE76@WINTRIX.thermeon.com>

> It's easier than that: from your peer, ping the ptp address (the CIPE
> address) and then on your local system, do a tcpdump -i eth0 udp port xxxx
> You should see the encrypted packets arriving.

Linux A--------NAT/Firewall---| Internet |-----------Linux B

Pretty interesting...  I did this.  Basically when I tried to ping Linux A 
Linux B, nothing showed up on tcpdump.  Then I tried pinging Linux B from 
A.  All of sudden tcpdump showed lots of activity.  I can even stop the ping
coming from Linux B and the ping from Linux A continues fine (activity showing
up in tcpdump).  If I stop the ping from Linux B to Linux A and wait 10 
I can no longer ping from Linux B to Linux A.

Obviously the NAT box is doing something to the packets.  Now a couple more

1) Someone e-mailed me and suggested that the NAT box is mangling the source
port.  That is Linux B is sending from port xxxx but the NAT box mangles this 
yyyy.  In the config for Linux A I say that Linux B sends from port xxxx.  
CIPE check and see that the source port doesn't match the config?

2) For some reason when I initiate the connection from Linux A, the port seems
to "open up" until Linux A stops sending.  The window seems to close after 10
seconds.  I could understand this if UDP was connection-oriented, but since it
isn't, I dont' see why the NAT box tears down the connection.  Can anyone
explain the config that is being used that would do this with a UDP 

I just wish that I could put my machine out on the Net and let iptables do its
job and not some broken NAT box <sigh>


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