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Flameeyes's Ruleset for ModSecurity

This is a rule set for ModSecurity developed by Diego Elio Pettenò.

The primary aim of this rule set is to reduce bandwidth waste on hosts that might even be static by disallowing certain known marketing, broken or abusive bots from hitting the site at all. This becomes very important when the requests are going to generate dynamic content by starting up applications such as Rails, which pose a load on the webserver, especially if invoked serially.

Even if you do not pay for bandwidth and your website is not going to suffer from the requests, keeping spammers and malicious crawlers at bay is important for the general health of the Network, so please be mindful.

Additionally, it provides antispam measure for active requests (HEAD and GET are passive requests, everything else is active as it sends or modify content). This antispam is based on what I developed for my own blog and employs a number of techniques, including DNSBL and User-Agent validation.

Prerequisites

The ruleset won't work without, obviously, ModSecurity; it has been tested with ModSecurity 2.7.1 as of February 2013.

Some of the rules make use of global collections and other configuration directives that are provided by the ModSecurity Core Rule Set. The file optional/flameeyes_init.conf contains the required directives, in case you explicitly don't want to use CRS.

If you do have CRS installed, there are also a few rules that re-use its tests.

Note: as of ModSecurity CRS 2.1.1, the IP global collection is hashed on remote address and User-Agent value, to discriminate between different sessions from the same computer or NAT-routed network. This might not be desirable for the antispam measures, and might require creating a different collection for this rule set in the future.

A few rules require you to enable forward-confirmed hostname lookups, to do so you need to set HostnameLookups double in your configuration files.

For both the rules requiring FcRNDS and those using DNSBL, it is recommended that your server has access to some namecache, either in form of caching resolver or a nscd (Name Service Cache Daemon) instance, to avoid repeating the same requests for long times.

Development and contribution

The rules are developed via a GitHub repository which also hosts the issue tracker. If you want to make fixes or improvements to the rules, you're invited to fork the project and send a pull request there.

Starting from ModSecurity 2.7 rule IDs are mandatory. To avoid colliding with other rulesets, this ruleset is assigned, as of 2012-03-07, the range 430,000-439,999.

Asked Questions

  • My website/browser is not a spammer, where do I complain?

    The ruleset is, for the most part, hand-compiled, so mistakes may happen. Most of the data is gathered through my websites, which receive about 30MB of traffic per day, and which I analyse over time. In case I made a mistake, please let me know by sending an email to flameeyes@flameeyes.eu — or open an issue on the GitHub Project.

  • Are you trying to stop all the crawlers? Don't you want to be found on search engines?

    I'm not; this ruleset is designed to stop the “bad” crawlers. There are a number of companies and individuals out there that are using crawlers for shady (or simply useless) business. We range from email address harvester (which will be used by spammers) to companies doing “marketing research” searching for mentions of their clients' products, brands or logos.

  • So are you trying to inconvenience marketeers?

    Again, no. I'm asking them to play by the rules. I have no problem with search companies doing marketing work on the side; or using the indexes of other search engines like Google, Yahoo or Bing. Fetching pages just for the sake of marketing is, though, a waste of our network bandwidth.

  • Some legit crawler is getting a 406 “Not Acceptable” reply; what's that?

    Even if your crawler is totally legit and doing a good job, you should abide to rules; one of these is “request (and accept) compressed responses”. If a crawler is not sending Accept-Encoding: gzip,deflate (or something along those lines), then it is not requesting compressed responses, which can use even over three times the bandwidth. While a single client requesting non-compressed data is not much of an issue, a crawler requesting multiple pages without it, is wasted effort.

  • Why do you try to validate my bot as if it was a browser?

    Some crawlers seem to like declaring themselves as browsers, for the most part faking a Safari or (more rarely) Chrome User-Agent string. More often than not, in these cases, the crawlers then get an access denied error.

    If you're writing a crawler, make sure that its User-Agent is proper, stating where it's coming from, and stop trying to pass for a browser, if you're not. With the advent of "optimizing" proxies and tricks like PageSpeed, lying on your User-Agent can easily give you wrong data to begin with.

  • Can you add a rule to validate a specific crawler?

    It depends; most search engines use a pre-defined network range to run their crawlers; Google does it, Yahoo does it; Microsoft does it. To verify the veracity of their crawlers, you just have to apply a FcRDNS check, to ensure that the bot is in the correct domain range. Unfortunately, well-intentioned crawlers, especially coming from start-ups, not always have a way to set their reverse DNS, for instance when using Amazon EC2 services. We don't currently have a standardised way to otherwise ensure the provenience of crawlers.

  • I'm anonymising myself by replacing my browser's User-Agent with a funny string, why are you banning me?

    People seem to think they are pretty smart by trying to hide behind User-Agent strings reporting Windows 3.1, DOS, or a Commodore 64 as the system used, but not only these can be easily used by spammers to go around most veracity checks on User-Agent strings, they also they work against anonymity. EFF's Panopticlick will show how your unique User-Agent is going to make you much more easily trackable, as it provides a very high entropy for browser fingerprinting. So stop trying to be smart, and be smart.

  • Why does Munin stop monitoring Apache after setting up the Ruleset?

    Up to March 2011, Munin didn't present itself with a specific User-Agent string, using instead the default libwww-perl header. This behaviour, though, conforms to that of a number of unknown scanners, and is thus blocked by the ruleset.

    You can work around this limitation by adding a specific rule allowing access from localhost only:

    SecRule REMOTE_ADDR "@ipMatch 127.0.0.1" "ctl:ruleEngine=Off"

Debugging rules

While the ruleset strive to avoid false positives whenever possible, it can happen that legit requests are denied reply when using these rules. It can thus be quite important to debug the encountered issues.

The quickest way is to define a debug entrypoint, which can be something like http://yourhost/modsec-debug and then set it up so that the requests coming to that address are logged in full into the audit log.

Enabling the audit log on all denied requests is definitely a bad idea, since it could easily fill up the logs directory; to solve the issue, you should add noauditlog to the SecDefaultAction list and then configure it as follows:

SecAuditEngine RelevantOnly
SecAuditLog /var/log/apache2/modsec_audit.log
SecAuditLogType serial
SecAuditLogParts ABCFHZ

<Location /modsec-debug>
    SecAction "auditlog"
</Location>

License

The rules are released under CreativeCommons Attribution-ShareAlike license (CC-BY-SA). If you're interested in making use of these rules on commercial products where CC-BY-SA is not acceptable, you can contact me at flameeyes@flameeyes.eu

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