We previously listed some means to try to stop screen-scraping, but since it is an ongoing topic for us, it bears revisiting. Any site can be scraped, but some require such an influx of time and resources as to make it prohibitively expensive. Some of the common methods to do so are:
The most common implementation of the Turning Test is the old CAPTCHA that tries to ensure a human reads the text in an image, and feeds it into a form.
We have found a large number of sites that implement a very weak CAPTCHA that takes only a few minutes to get around. On the other hand, there are some very good implementations of Turing Tests that we would opt not to deal with given the choice, but a sophisticated OCR can sometimes overcome those, or many bulletin board spammers have some clever tricks to get past these.
Data as images
Sometimes you know which parts of your data are valuable. In that case it becomes reasonable to replace such text with an image. As with the Turing Test, there is ORC software that can read it, and there’s no reason we can’t save the image and have someone read it later.
Often times, however, listing data as an image without a text alternate is in violation of the Americans with Disabilities Act (ADA), and can be overcome with a couple of phone calls to a company’s legal department.
Recently we’ve encountered some instances where a page has one images containing numbers and letters, and used CSS to display only the characters they desired. This is in effect a combination of the previous 2 methods. First we have to get that master-image and read what characters are there, then we’d need to read the CSS in the site and determine to what character each tag was pointing.
While this is very clever, I suspect this too would run afoul the ADA, though I’ve not tested that yet.
Limit search results
Most of the data we want to get at is behind some sort of form. Some are easy, and submitting a blank form will yield all of the results. Some need an asterisk or percent put in the form. The hardest ones are those that will give you only so many results per query. Sometimes we just make a loop that will submit the letters of the alphabet to the form, but if that’s too general, we must make a loop to submit all combination of 2 or 3 letters–that’s 17,576 page requests.
On occasion, a diligent webmaster will notice a large number of page requests coming from a particular IP address, and block requests from that domain. There are a number of methods to pass requests through alternate domains, however, so this method isn’t generally very effective.
Scraping always keys off of certain things in the HTML. Some sites have the resources to constantly tweak their HTML so that any scrapes are constantly out of date. Therefore it becomes cost ineffective to continually update the scrape for the constantly changing conditions.