ACLs vs. full firewalls

Wed Apr 8 02:14:38 UTC 2009

For Defense in depth I would use multi-tiered approach.

Stateless ACL at Border for bound checks
Stateful FW for Checking sessions
Outbound ACLs on Innerchoke points
Application Intelligence and DDOS mitigation by IPS between Border and 
Firewall
Endpoint Security using Enterprise Anti-Virus agents/NAC Agents

Regards

Ubaidali Abdul Razack
+65.65436404 (Office)
+65.65436278 (Fax)

Roland Dobbins <rdobbins at cisco.com> 
04/08/2009 08:28 AM

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NANOG list <nanog at nanog.org>
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Subject
Re: ACLs vs. full firewalls

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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