> Let’s take a look at the robots.txt for census.gov from October of 2018 as a specific example to see how robots.txt files typically work. This document is a good example of a common pattern. The first two lines of the file specify that you cannot crawl census.gov unless given explicit permission. The rest of the file specifies that Google, Microsoft, Yahoo and two other non-search engines are not allowed to crawl certain pages on census.gov, but are otherwise allowed to crawl whatever else they can find on the website. This tells us that there are two different classes of crawlers in the eyes of the operators of census.gov: those given wide access, and those that are totally denied.
> And, broadly speaking, when we examine the robots.txt files for many websites, we find two classes of crawlers. There is Google, Microsoft, and other major search engine providers who have a good level of access and then there is anyone besides the major crawlers or crawlers that have behaved badly in the past that are given much less access. Among the privileged, Google clearly stands out as the preferred crawler of choice. Google is typically given at least as much access as every other crawler, and sometimes significantly more access than any other crawler.