RE: ICMP pokes holes in firewalls...

["Daniel Chemko" <dchemko () smgtec com>]

> 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
protocol.

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

> to requests from an alternative interface using a different IP address 
> entirely.

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.