Re: Firewall Best Practices

From: Eirik Seim (
Date: 02/07/02

From: (Eirik Seim)
Date: 7 Feb 2002 10:34:51 GMT

On Wed, 06 Feb 2002 20:09:29 GMT, Roger Marquis <> wrote:
>Comments appreciated.

And comments you'll get. This looks like an interesting project, feel
free to contact me by email if you'd like. This NG gets awfully crowded

> Firewall Best Practices (draft)
> 1 Know what every rule does
> 1.1 Be sure you know exactly what every single rule does

Of cause, everyone here on this NG understands that this means "Be sure
you know exactly what each and every rule does, and how it affects the
ruleset as a whole."

I'm thinking of instances where several people with a various degree of
knowledge has mocked around in the firewall rules, making "holes" in existing
rules to get that important application available, not having the time
to document the change, the extra rules, and so on, and so on.

> 1.2 Know your TCP and UDP ports

..both inbound and outbound ports and port ranges..

> 2 If you cannot figure out what a rule does delete it (and see whether
> anyone notices)
> 2.1 Never violate rule #2

Problem is if this rule was important, the moment anyone notices, or should
I say "notices enough to tell anyone", the network behind the firewall might
already be compromised.

If you cannot figure out what a rules does, then study the documentation,
both the vendor's (if any) and whatever the people created that rule might
have produced (if any).

If this does not help, test the rule or preferably the complete ruleset in an
isolated environment, and if that won't give you a clue either, start
studying the documentation again.

At least, that's my opinion. Feel free to disagree :)

> 3 Know your applications and risks (active-x for example)
> 3.1 Get management buy-in for any controversial or potentially
> disruptive filtering
> 3.2 If you cannot get management buy-in be sure to CYA by
> documenting the risk (always assign dollar values)
> 3.3 If you have to CYA be sure the risk analysis is adequately
> distributed, beyond your direct manager
> 4 Know what the firewall is protecting

..and who it is that you're protecting this from.

> 5 Always log and read the syslogs frequently

..and keep those logs as long as possible.

> 5.1 Consider increased rule-specific logging (at least temporarily)
> when changing rules

And increased logging is also a good thing when optimizing rulesets.

> 5.2 Use Intrusion Detection Software (NIDS, HIDS, AIDS) wherever
> the business case permits
> 5.3 Configure IDS alerts and alarms carefully

..and keep those logs as well :)

> 7 Audit thoroughly and often
> 7.1 Have someone else perform audits periodically
> 7.2 Use the most experienced and knowledgeable auditors /
> consultants / engineers possible (this is not an area where
> it is typically worth considering a low bidder)

But don't take it for granted that the guy who's asking $300 an hour is
going to make your network more secure than the guy who'll do the job for
$100 an hour. I believe in using experienced and knowledgeable auditors, but
also _different_ auditors. Maybe not for each audit, but switching often
enough that you won't allow this one guy ignore the same problem month after

> 8 Read several security newsgroups and mailing lists daily

Especially the security alert lists many vendors provide..

- Eirik

Eirik Seim                        System Administrator              Math. Department       University of Bergen