From: Rich Kulawiec <rsk@gsp.org> On Wed, May 06, 2015 at 03:30:01PM -0700, Scott Weeks wrote:
From: Rich Kulawiec <rsk@gsp.org>
The first rule in every firewall is of course "deny all" and subsequent rulesets permit only the traffic that is necessary. ------------------------------------
I think you got this backward? That way all traffic is blocked, so none is allowed through.
Nope, I said exactly what I intended (and what I do, in practice). Doing so forces one to understand in detail what traffic actually needs to pass in/out and to craft specific rules for it. This in turn helps avoid making mistake #1: The Six Dumbest Ideas in Computer Security http://www.ranum.com/security/computer_security/editorials/dumb/ ----------------------------------------------------- After reading your emails all these years, I figured you meant it the way you wrote it. When you wrote "...subsequent rulesets permit only the traffic that is necessary" I misunderstood and thought you meant rules put in after the default deny, which are useless. But by subsequent rulesets you meant rule sets put in later in time and above the deny all not after the deny all. Small confusion over wording... :-) scott
Ah...got it, this was sloppy phrasing on my part. I meant "first" in the sense of "first rule that one should write". Depending on the firewall type/implementation, that might be the rule that's lexically first or last (or maybe somewhere else). ---rsk
On Thu, May 7, 2015 at 7:12 AM, Rich Kulawiec <rsk@gsp.org> wrote:
Ah...got it, this was sloppy phrasing on my part. I meant "first" in the sense of "first rule that one should write". Depending on
Security best practice to always have an active "cleanup" rule for every traffic direction applicable to every pair of zones (or interfaces) with a default DROP, to catch traffic matching no accept rule. In practice... however.... in the real world, many firewalls get configured with this only in the INBOUND direction (Default deny Write packet to Higher integrity level zone from lower level security zone), and Default Accept for packet from more secure zone to less secure zone, Since this has superior usability and is lower maintenance. And for client devices, in a low security environment: with just a simple Layer4 stateful inspection firewall, this is probably the right solution. "Permit only traffic that is necessary" Only works out if you are able to rigidly define what exactly that traffic is in advance. Which is feasible to do for servers and other single-purpose devices, but very expensive to do for clients, at least without a firewall aware of the communications at the application layer that can look at those UDP connections and say "OKAY, This is skype... allow it", Or... "This connection going out on port 80.. it's not a valid HTTP request, Drop the connection now and cache a rule to Deny further connections to that IP:Port number pair.".
the firewall type/implementation, that might be the rule that's lexically first or last (or maybe somewhere else). ---rsk -- -JH
participants (3)
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Jimmy Hess -
Rich Kulawiec -
Scott Weeks