Defense Against Robots

A typical Fossil website can have millions of pages, and many of those pages (for example diffs and annotations and tarballs) can be expensive to compute. If a robot walks a Fossil-generated website, it can present a crippling bandwidth and CPU load.

A Fossil website is intended to be used interactively by humans, not walked by robots. This article describes the techniques used by Fossil to try to welcome human users while keeping out robots.

The Hyperlink User Capability

Every Fossil web session has a "user". For random passers-by on the internet (and for robots) that user is "nobody". The "anonymous" user is also available for humans who do not wish to identify themselves. The difference is that "anonymous" requires a login (using a password supplied via a CAPTCHA) whereas "nobody" does not require a login. The site administrator can also create logins with passwords for specific individuals.

Users without the Hyperlink capability do not see most Fossil-generated hyperlinks. This is a simple defense against robots, since the "nobody" user category does not have this capability by default. Users must log in (perhaps as "anonymous") before they can see any of the hyperlinks. A robot that cannot log into your Fossil repository will be unable to walk its historical check-ins, create diffs between versions, pull zip archives, etc. by visiting links, because there are no links.

A text message appears at the top of each page in this situation to invite humans to log in as anonymous in order to activate hyperlinks.

But requiring a login, even an anonymous login, can be annoying. Fossil provides other techniques for blocking robots which are less cumbersome to humans.

Automatic Hyperlinks Based on UserAgent

Fossil has the ability to selectively enable hyperlinks for users that lack the Hyperlink capability based on their UserAgent string in the HTTP request header and on the browsers ability to run Javascript.

The UserAgent string is a text identifier that is included in the header of most HTTP requests that identifies the specific maker and version of the browser (or robot) that generated the request. Typical UserAgent strings look like this:

The first two UserAgent strings above identify Firefox 19 and Internet Explorer 8.0, both running on Windows NT. The third example is the robot used by Google to index the internet. The fourth example is the "wget" utility running on OpenBSD. Thus the first two UserAgent strings above identify the requester as human whereas the second two identify the requester as a robot. Note that the UserAgent string is completely under the control of the requester and so a malicious robot can forge a UserAgent string that makes it look like a human. But most robots want to "play nicely" on the internet and are quite open about the fact that they are a robot. And so the UserAgent string provides a good first-guess about whether or not a request originates from a human or a robot.

In Fossil, under the Admin/Robot-Defense menu, there is a setting entitled "Enable hyperlinks based on User-Agent and/or Javascript". If this setting is set to "UserAgent only" or "UserAgent and Javascript", and if the UserAgent string looks like a human and not a robot, then Fossil will enable hyperlinks even if the Hyperlink capability is omitted from the user permissions. This settingn gives humans easy access to the hyperlinks while preventing robots from walking the billions of pages on a typical Fossil site.

If the setting is "UserAgent only", then the hyperlinks are simply enabled and that is all. But if the setting is "UserAgent and Javascript", then the hyperlinks are not enabled directly . Instead, the HTML code that is generated contains anchor tags ("<a>") with "href=" attributes that point to /honeypot rather than the correct link. JavaScript code is added to the end of the page that goes back and fills in the correct "href=" attributes of the anchor tags with the true hyperlink targets, thus enabling the hyperlinks. This extra step of using JavaScript to enable the hyperlink targets is a security measure against robots that forge a human-looking UserAgent string. Most robots do not bother to run JavaScript and so to the robot the empty anchor tag will be useless. But all modern web browsers implement JavaScript, so hyperlinks will show up normally for human users.

Further Defenses

Recently (as of this writing, in the spring of 2013) the Fossil server on the SQLite website ( has been hit repeatedly by Chinese robots that use forged UserAgent strings to make them look like normal web browsers and which interpret JavaScript. We do not believe these attacks to be nefarious since SQLite is public domain and the attackers could obtain all information they ever wanted to know about SQLite simply by cloning the repository. Instead, we believe these "attacks" are coming from "script kiddies". But regardless of whether or not malice is involved, these attacks do present an unnecessary load on the server which reduces the responsiveness of the SQLite website for well-behaved and socially responsible users. For this reason, additional defenses against robots have been put in place.

On the Admin/Robot-Defense page of Fossil, just below the "Enable hyperlinks using User-Agent and/or Javascript" setting, there are now two additional sub-settings that can be optionally enabled to control hyperlinks.

The first new sub-setting is a delay (in milliseconds) before setting the "href=" attributes on anchor tags. The default value for this delay is 10 milliseconds. The idea here is that a robots will try to interpret the links on the page immediately, and will not wait for delayed scripts to be run, and thus will never enable the true links.

The second sub-setting waits to run the JavaScript that sets the "href=" attributes on anchor tags until after at least one "mousedown" or "mousemove" event has been detected on the <body> element of the page. The thinking here is that robots will not be simulating mouse motion and so no mouse events will ever occur and hence the hyperlinks will never become enabled for robots.

See also Managing Server Load for a description of how expensive pages can be disabled when the server is under heavy load.

The Ongoing Struggle

Fossil currently does a very good job of providing easy access to humans while keeping out troublesome robots. However, robots continue to grow more sophisticated, requiring ever more advanced defenses. This "arms race" is unlikely to ever end. The developers of Fossil will continue to try improve the robot defenses of Fossil so check back from time to time for the latest releases and updates.

Readers of this page who have suggestions on how to improve the robot defenses in Fossil are invited to submit your ideas to the Fossil Users forum: