CrawlBot Wars

This article was originally published on the Axant Technical Blog.

Everybody who ever wanted to write a “successful website” (or more recently, thanks to the Web 2.0 hype, a “successful blog”) knows the bless and curse of crawlers, or bots, that are unleashed by all kind of entities to scan the web, and report the content back to their owners.

Most of these crawlers are handled by search engines, such as Google, Microsoft Live Search, Yahoo! and so on. With the widespread use of feeds, at least Google and Yahoo! added to their standard crawler bots also feed-specific crawlers that are used to aggregate blogs and other feeds into nice interfaces for their users (think Google Reader). Together with this kind of crawlers, though, there are less useful, sometimes nastier crawlers that either don’t respond to search engines, or respond to search engines whose ethical involvement makes somewhat wonder.

Good or bad, at the end of the day you might not want some bots to crawl your site; some Free Software -bigots- activists some time ago wanted, for instance, to exclude the Microsoft bot from their sites (while I have some other ideas), but there are certain bots that are even more useful to block, like the so-called “marketing bots”.

You might like Web 2.0 or you might not, but certainly lots of people found the new paradigm of Web as a gold mind to make more money out of content others have written – incidentally these are not, like RIAA, MPAA and SIAE insist, the “pirates” that copy music and movies, but rather companies whose objective is to provide other companies with marketing research and data based on content of blogs and similar services. While some people might be interested in getting their blog scanned by these crawlers either way, I’d guess that for most users who host their own blog this is just a waste of bandwidth: the crawlers tend to be quite pernicious since they don’t use If-Modified-Since or Etag headers in their request, and even when they do, they tend to make quite a few requests on the feeds per hour (compare this with Google’s Feedfetcher bot that requires at most one copy of the same feed per hour – well, if it isn’t confused by multiple compatibility redirects like it unfortunately is with my main blog).

While there is a voluntary exclusion protocol (represented by the omni-present robots.txt file), only actually “good” robots do consider that, while evil or rogue robots can simply ignore it. Also, it might be counter-productive to block rogue robots even when they do look at it. Say that a rogue robot wants your data, and to pass as a good one is advertising itself in the User-Agent string, complete with a link to a page explaining what it’s supposedly be doing, and accepting the exclusion. If you exclude it in robots.txt you can give it enough information to choose a different User-Agent string that is not listed in the exclusion protocol.

One way to deal with the problem is by blocking the requests at the source, answering straight away with an HTTP 403 (Access Denied) on the web server when making a request. When using the Apache web server, the easiest way to do this is by using modsecurity and a blacklist rule for rogue robots, similar to the antispam system I’ve been using for a few months already. The one problem I see with this is that Apache’s mod_rewrite seem to be executed before mod_security, which means that for any request that is rewritten by compatibility rules (moved, renamed, …) there is first a 301 response and just after that an actual 403.

I’m currently working on compiling such a blacklist by analysing the logs of my server, the main problem is deciding which crawlers to block and which to keep. When the description page explicitly states they are marketing research, blocking them is quite straightforward; when they seem to provide an actual search service, that’s more shady, and it turns down to checking the behaviour of the bot itself on the site. And then there are the vulnerability scanners.

Still, it doesn’t stop here: given that in the Google description of GoogleBot they provide a (quite longish to be honest) method to verify that a bot is actually GoogleBot as it advertises itself to be, one has to assume that there are rogue bots out there trying to pass for GoogleBot or other good and lecit bot. This is very likely the case because some website that are usually visible only by registered users make an exception for search engine crawlers to access and index their content.

Especially malware, looking for backdoors into a web application, is likely to forge the User-Agent of a known good search engine bot (that is likely not blocked by the robots.txt exclusion list), so that it doesn’t fire up any alarm in the logs. So finding “fake” search engine bots is likely to be an important step in securing a webserver running webapplications, may them be trusted or not.

As far as I know there is currently no way in Apache to check that a request actually does come from the bot it’s declared to come from. The nslookup method that Google suggests works fine for a forensic analysis but it’s almost impossible to perform properly with Apache itself, and not even modsecurity, by itself, can do much about that. On the other hand, there is one thing in the recent 2.5 versions of modsecurity that can be probably used to implement an actually working check: the LUA scripts loading. Which is what I’m going to work on as soon as I find some extra free time.

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