<title>Defense Against Spiders</title>
The website presented by a Fossil server has many hyperlinks.
Even a modest project can have millions of pages in its
tree, and many of those pages (for example diffs and annotations
and ZIP archive of older check-ins) can be expensive to compute.
If a spider or bot tries to walk a website implemented by
Fossil, it can present a crippling bandwidth and CPU load.
The website presented by a Fossil server is intended to be used
interactively by humans, not walked by spiders. This article
describes the techniques used by Fossil to try to welcome human
users while keeping out spiders.
<h2>The "hyperlink" user capability</h2>
Every Fossil web session has a "user". For random passers-by on the internet
(and for spiders) 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.
The "h" or "hyperlink" capability is a permission that can be granted
to users that enables the display of hyperlinks. Most of the hyperlinks
generated by Fossil are suppressed if this capability is missing. So
one simple defense against spiders is to disable the "h" permission for
the "nobody" user. This means that users must log in (perhaps as
"anonymous") before they can see any of the hyperlinks. Spiders do not
normally attempt to log into websites and will therefore
not see most of the hyperlinks and will not try to walk the millions of
historical check-ins and diffs available on a Fossil-generated website.
If the "h" capability is missing from user "nobody" but is present for
user "anonymous", then a message automatically appears at the top of each
page inviting the user to log in as anonymous in order to activate hyperlinks.
Removing the "h" capability from user "nobody" is an effective means
of preventing spiders from walking a Fossil-generated website. But
it can also be annoying to humans, since it requires them to log in.
Hence, Fossil provides other techniques for blocking spiders which
are less cumbersome to humans.
<h2>Automatic hyperlinks based on UserAgent</h2>
Fossil has the ability to selectively enable hyperlinks for users
that lack the "h" 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 spider) that generated the request. Typical UserAgent
strings look like this:
<ul>
<li> Mozilla/5.0 (Windows NT 6.1; rv:19.0) Gecko/20100101 Firefox/19.0
<li> Mozilla/4.0 (compatible; MSIE 8.0; Windows_NT 5.1; Trident/4.0)
<li> Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
<li> Wget/1.12 (openbsd4.9)
</ul>
The first two UserAgent strings above identify Firefox 19 and
Internet Explorer 8.0, both running on Windows NT. The third
example is the spider 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 requestor
as human whereas the second two identify the requestor as a spider.
Note that the UserAgent string is completely under the control
of the requestor and so a malicious spider can forge a UserAgent
string that makes it look like a human. But most spiders truly
seem to desire to "play nicely" on the internet and are quite open
about the fact that they are a spider. And so the UserAgent string
provides a good first-guess about whether or not a request originates
from a human or a spider.
In Fossil, under the Admin/Access menu, there is a setting entitled
"<b>Enable hyperlinks for "nobody" based on User-Agent and Javascript</b>".
If this setting is enabled, and if the UserAgent string looks like a
human and not a spider, then Fossil will enable hyperlinks even if
the "h" capability is omitted from the user permissions. This setting
gives humans easy access to the hyperlinks while preventing spiders
from walking the millions of pages on a typical Fossil site.
But the hyperlinks are not enabled directly with the setting above.
Instead, the HTML code that is generated contains anchor tags ("<a>")
without "href=" attributes. Then, javascript code is added to the
end of the page that goes back and fills in the "href=" attributes of
the anchor tags with the hyperlink targets, thus enabling the hyperlinks.
This extra step of using javascript to enable the hyperlink targets
is a security measure against spiders that forge a human-looking
UserAgent string. Most spiders do not bother to run javascript and
so to the spider the empty anchor tag will be useless. But all modern
web browsers implement javascript, so hyperlinks will show up
normally for human users.
<h2>Further defenses</h2>
Recently (as of this writing, in the spring of 2013) the Fossil server
on the SQLite website ([http://www.sqlite.org/src/]) has been hit repeatedly
by Chinese spiders 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
spiders have been put in place.
On the Admin/Access page of Fossil, just below the
"<b>Enable hyperlinks for "nobody" based on User-Agent and Javascript</b>"
setting, there are now two additional subsettings that can be optionally
enabled to control hyperlinks.
The first subsetting waits to run the
javascript that sets the "href=" attributes on anchor tags until after
at least one "mouseover" event has been detected on the <body>
element of the page. The thinking here is that spiders will not be
simulating mouse motion and so no mouseover events will ever occur and
hence the hyperlinks will never become enabled for spiders.
The second new subsetting 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 spider will try to
render the page immediately, and will not wait for delayed scripts
to be run, thus will never enable the hyperlinks.
These two subsettings can be used separately or together. If used together,
then the delay timer does not start until after the first mouse movement
is detected.
See also [./server.wiki#loadmgmt|Managing Server Load] for a description
of how expensive pages can be disabled when the server is under heavy
load.
<h2>The ongoing struggle</h2>
Fossil currently does a very good job of providing easy access to humans
while keeping out troublesome robots and spiders. However, spiders and
bots 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 spider 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 spider
defenses in Fossil are invited to submit your ideas to the Fossil Users
mailing list:
[mailto:fossil-users@lists.fossil-scm.org | fossil-users@lists.fossil-scm.org].