On Apr 8, 2009, at 4:05 AM, Michael Helmeste wrote:
However, I wanted to get other opinions of what packet filtering
solutions people use in the border and in the
core, and why.
Stateless ACLs in hardware at the edge are important both for
infrastructure self-protection (i.e., iACLs) and for policy
enforcement of the type you indicate. As others on this thread have
pointed out, do understand your platform characteristics and craft
your ACLs accordingly.
Stateful - i.e., context-aware bidirectional - filtering via a
firewall makes sense in situations in which a) the nodes 'behind' the
firewall aren't typically operating as servers and/or b) the
bidirectional communications patterns which should be observed are
well-known, and in which the participation of hosts is under the
control/influence of the network operator. For example, in front of a
corporate LAN, or between the tiers of a multi-tiered application, one
can craft quite specific stateful inspection rules which can be used
to explicitly allow and disallow certain types of traffic.
For front-end, publicly-accessible conventional servers, stateful
inspection may not add as much value, as basically every connection
which comes into those servers is unsolicited (i.e., no existing
stateful communications context against which to measure pass/fail
decisions); this is where high-speed stateless ACLs, coupled with host
OS/app/service hardening play a key role. It's very important to
avoid the instantiation of unnecessary state in front of public-facing
assets, as DDoS attacks are essentially attacks against capacity and
against state.
One should also look into implementing DDoS mitigation techniques such
as S/RTBH, in conjunction with the chosen policy-enforcement regime.
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