v6 & DSL / Cable modems [was: Private use of non-RFC1918 IP space

Stephen Sprunk stephen at sprunk.org
Mon Feb 9 23:24:51 UTC 2009

Ricky Beam wrote:
> On Sat, 07 Feb 2009 14:31:57 -0500, Stephen Sprunk 
> <stephen at sprunk.org> wrote:
>> Non-NAT firewalls do have some appeal, because they don't need to 
>> mangle the packets, just passively observe them and open pinholes 
>> when appropriate.
> This is exactly the same with NAT and non-NAT -- making any anti-NAT 
> arguments null.
> In the case of NAT, the "helper" has to understand the protocol to 
> know what traffic to map.
> In the case of a stateful firewalling ("non-NAT"), the "helper" has to 
> understand the protocol to know what traffic to allow.
> Subtle difference, but in the end, the same thing... if your gateway 
> doesn't know what you are doing, odds are it will interfere with it.  
> In all cases, end-to-end transparency doesn't exist. (as has been the 
> case for well over a decade.)

Yes, an ALG needs to understand the packet format to open pinholes -- 
but with NAT, it also needs to mangle the packets.  A non-NAT firewall 
just examines the packets and then passes them on unmangled.

This mangling can be a serious source of problems.  With UDP, it can 
introduce checksum errors.  With TCP, not only do you have possible 
checksum errors, you also have to mangle the sequence numbers in both 
directions if the length of the payload changes.  The mangling will 
inherently break standard IPsec and other "shim" layers like HIP.  And 
let's not forget that NAT makes widespread deployment of any L4 
alternative to TCP and UDP (e.g. SCTP) virtually impossible, forcing 
every new transport or shim protocol to inefficiently ride on top of TCP 
or UDP...

Some protocols, e.g. SIP/RTP, also work fine through a stateful firewall 
even without an ALG in most cases -- but not when you add in NAT.


Stephen Sprunk         "God does not play dice."  --Albert Einstein
CCIE #3723         "God is an inveterate gambler, and He throws the
K5SSS        dice at every possible opportunity." --Stephen Hawking

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