I don't need no stinking firewall!

Dobbins, Roland rdobbins at arbor.net
Tue Jan 5 20:29:01 UTC 2010

On Jan 6, 2010, at 3:16 AM, Brian Johnson wrote:

>  Given this information, and not prejudging any responses, exactly what is a
> firewall for and when is statefull inspection useful?

In the most basic terms, a stateful firewall performs bidirectional classification of communications between nodes, and makes a pass/fail determination on each packet based on a) whether or not a bidirectional communications session is already open between the nodes and b) any policy rules configured on the firewall as to what ports/protocols should be allowed between said nodes.

Stateful firewalls make good sense in front of machines which are primarily clients; the stateful inspection part keeps unsolicited packets away from the clients.

Stateful firewalls make absolutely no sense in front of servers, given that by definition, every packet coming into the server is unsolicited (some protocols like ftp work a bit differently in that there're multiple bidirectional/omnidirectional communications sessions, but the key is that the initial connection is always unsolicited).

Putting firewalls in front of servers is a Really Bad Idea - besides the fact that the stateful inspection premise doesn't apply (see above), rendering the stateful firewall superfluous, even the biggest, baddest firewalls out there can be easily taken down via state-table exhaustion; an attacker can craft enough programmatically-generated, well-formed traffic which conforms to the firewall policies to 'crowd out' legitimate traffic, thus DoSing the server.  Addtionally, the firewall can be made to collapse far quicker than the server itself would collapse, as the overhead on the state-tracking is less than what the server itself could handle on its own.

Roland Dobbins <rdobbins at arbor.net> // <http://www.arbornetworks.com>

    Injustice is relatively easy to bear; what stings is justice.

                        -- H.L. Mencken

More information about the NANOG mailing list