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RE: ICMP pokes holes in firewalls...
From: "Daniel Chemko" <dchemko () smgtec com>
Date: Thu, 25 Sep 2003 15:05:07 -0700

NAT gateway has been 
detected as a ignore-the-source UDP forwarder

2.4 kernels: NAT doesn't work without ip_conntrack, and ip_conntrack
always keeps track of source IP addresses (hence its function). I can't
think of a situation for any Linux machine which allows inbound UDP
replies from other sources. Spoofing the original sender's address is a
different story, but that is pandemic of any stateless AND insecure

I posted about this in March of 2000, the kernel development team
was that many RPC services require this functionality and it would not
fixed. The reason is that many UDP-based RPC services will respond back

to requests from an alternative interface using a different IP address 

Just recently someone has written a conntrack handler to traverse
firewalls with RPC as you describe. No leaks to my knowledge, although I
am not too familiar with this module.

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