A note on unsupported rules in robots.txt

Yesterday we announced that we’re open-sourcing Google’s production robots.txt parser. It was an exciting moment that paves the road for potential Search open sourcing projects in the future! Feedback is helpful, and we’re eagerly collecting questions from developers and webmasters alike. One question stood out, which we’ll address in this post:
Why isn’t a code handler for other rules like crawl-delay included in the code?
The internet draft we published yesterday provides an extensible architecture for rules that are not part of the standard. This means that if a crawler wanted to support their own line like “unicorns: allowed”, they could. To demonstrate how this would look in a parser, we included a very common line, sitemap, in our open-source robots.txt parser.
While open-sourcing our parser library, we analyzed the usage of robots.txt rules. In particular, we focused on rules unsupported by the internet draft, such as crawl-delay, nofollow, and noindex. Since these rules were never documented by Google, naturally, their usage in relation to Googlebot is very low. Digging further, we saw their usage was contradicted by other rules in all but 0.001% of all robots.txt files on the internet. These mistakes hurt websites’ presence in Google’s search results in ways we don’t think webmasters intended.
In the interest of maintaining a healthy ecosystem and preparing for potential future open source releases, we’re retiring all code that handles unsupported and unpublished rules (such as noindex) on September 1, 2019. For those of you who relied on the noindex indexing directive in the robots.txt file, which controls crawling, there are a number of alternative options:

  • Noindex in robots meta tags: Supported both in the HTTP response headers and in HTML, the noindex directive is the most effective way to remove URLs from the index when crawling is allowed.
  • 404 and 410 HTTP status codes: Both status codes mean that the page does not exist, which will drop such…

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