Busting Some Myths Concerning robots.txt

After all these years, there still seems to be quite a lot of confusion and misinformation floating around about "robots.txt" files, how they work, and what they do, so I'd like to go some way towards clearing up that confusion here.

What is robots.txt?

robots.txt is a small text file placed in the document root of a web site in order to signal that the site's owner does not want certain parts of the site to be crawled or listed by search engines. For example:

User-agent: *
Disallow: /my-private-stuff/

This signals that the site's owner does not want anything in the /my-private-stuff/ directory to be spidered by search engines. In this example that intention applies to all search engines and robots, as signified by the wildcard "*". You can read a lot more about robots.txt on the robotstxt.org site, so I won't dwell too much here.

The concept is very simple, and yet is often misunderstood. I come across three quite severe misunderstandings quite regularly, so let's tackle them one by one.

Myth 1: robots.txt Can Prevent Crawlers from Accessing Pages

This is the most prevalent misconception concerning robots.txt: that it somehow prevents pages and files from being viewed by search engine crawlers. It does not. Robots can and do ignore robots.txt. By listing content in robots.txt you signal that you would rather it was not indexed, but it is impossible for you to prevent access to it purely through robots.txt files. The mechanism is not intended for access control and neither should be nor can be used for that purpose.

Worse, it's fairly easy to see that attempting to do so is more likely be counterproductive: by listing the URLs of "secret" files and web content in robots.txt, you are advertising to the world that it's there, and that it might be of interest. This clearly achieves quite the opposite of what you intended it to.

If you don't want people to see something, do not put it on the web, or at least learn to use HTTP authentication.

Myth 2: robots.txt Can Prevent Sites Deep-linking to Pages

Hard as it is to believe, there are still people out there who believe that a robots.txt file can prevent other sites or services deep-linking to pages on their site. The belief is exemplified by this post, which describes robots.txt as a program that prevents robots from linking to any page.

This too, is bogus. A search engine, or a service such as Google News - which the blog post discusses - may choose to interpret certain robots.txt files as a request not to link to your site, and they may even subsequently choose to honour that request. But the fact remains that no robots.txt file in the work can prevent that deep-linking happening.

Myth 3: robots.txt Is Good for Search Engine Optimisation

I've occasionally been asked to add a robots.txt file to a website under the misunderstanding that it is somehow an aid to SEO. This is entirely untrue, and having a robots.txt file is not inherently good for your search engine rankings.

Not having a robots.txt file, having an empty or malformed one, or having one containing the content in the following listing are all equivalent:

User-agent: *
Disallow:

All of those states will signal that you're happy for search engines to index your site. From an SEO perspective, this is generally what you want. Since this is the default behaviour if robots.txt does not exist, there's clearly no real reason to add one purely for SEO purposes.