Today, for reasons outside the scope of this entry, I decided to spend some time maintaining and pruning the access control rules for Wandering Thoughts , this blog. Due to the ongoing crawler plague (and past abuses), Wandering Thoughts has had to build up quite a collection of access control rules, which are mostly implemented as a bunch of things in an Apache .htaccess file (partly 'Deny from ...' for IP address ranges and partly as rewrite rules based on other characteristics). The experience has left me with a renewed view of something, which is that systems with access control rules need some way of letting you see which rules are still being used by your traffic .

It's in the nature of systems with access control rules to accumulate more and more rules over time. You hit another special situation, you add another rule, perhaps to match and block something or perhaps to exempt something from blocking. These rules often interact in various ways, and over time you'll almost certainly wind up with a tangled thicket of rules (because almost no one goes back to carefully check and revisit all existing rules when they add a new one or modify an existing one). The end result is a mess, and one of the ways to reduce the mess is to weed out rules that are now obsolete. One way a rule can be obsolete is that it's not used any more, and often these are the easiest rules to drop once you can recognize them.

(A rule that's still being matched by traffic may be obsolete for other reasons, and rules that aren't currently being matched may still be needed as a precaution. But it's a good starting point.)

If you have the necessary log data, you can sometimes establish if a rule was actually ever used by manually checking your logs. For example, if you have logs of rejected traffic (or logs of all traffic), you can search it for an IP address range to see if a particular IP address rule ever matched anything. But this requires tedious manual effort and that means that only determined people will go through it, especially regularly. The better way is to either have this information provided directly, such as by counters on firewall rules, or to have something in your logs that makes deriving it easy.

(An Apache example would be to augment any log line that was matched by some .htaccess rule with a name or a line number or the like. Then you could go readily through your logs to determine which lines were matched and how often.)

The next time I design an access control rule system, I'm hopefully going to remember this and put something in its logging to (optionally) explain its decisions.

(Periodically I write something that has an access control rule system of some sort. Unfortunately all of mine to date have been quiet on this, so I'm not at all without sin here.)