I don't need no stinking firewall!

Joel Snyder Joel.Snyder at Opus1.COM
Fri Jan 8 15:21:52 UTC 2010

> On Thu Jan 07, 2010 at 01:04:01PM -0800, Jay Hennigan <jay at west.net> wrote:
>>> Or better:
>>>     - Allow from anywhere port 80 to server port > 1023 established
>>  Adding "established" brings us back to stateful firewall!
> Not really.  It only looks to see if the ACK or RST bits are set.  This 
> is different from a stateful firewall which memorizes each outbound 
> packet and checks the return for a match source/destination/sequence.

Actually, most firewalls don't check TCP sequence numbers.  You are 
totally correct in that stateless packet filters with "established" are 
only looking for TCP bits, but the main difference that stateful 
firewalls add is watching the TCP state machine.  Sequence number 
watching is a bonus, something you can enable on some firewalls, but 
most of the common ones don't do it by default.


Joel M Snyder, 1404 East Lind Road, Tucson, AZ, 85719
Senior Partner, Opus One       Phone: +1 520 324 0494
jms at Opus1.COM                http://www.opus1.com/jms

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