A Guide to the Robots.txt Exclusion Protocol

Last Updated May 27, 2023 by Tim Trott. First Published in 2013.

Search Engine Optimisation Series

Learn how to use robots.txt to control how search engine crawlers, or spiders, access and crawl your site on the web and what to index.

A Web crawler is sometimes also called a spider or a bot. They are automated Internet robots which systematically browse the World Wide Web, typically for Web indexing, although they can be used for gathering data of any kind. Sometimes these bots can be a bit overzealous in their crawling and generate thousands of hits per hour, a process also known as a denial of service attack.

The robots.txt file is standard to give instructions about a website to the web robots. This standard is called The Robots Exclusion Protocol .

When a robot wants to visit a site to crawl, it firsts checks for the robots.txt to see if is allowed to crawl the site and if there are any areas it should ignore.

The robots.txt is a plain text file which is placed in the root of the website, for example, http://www.example.com/robots.txt

What is the robots.txt File?

The most basic of robots.txt contents looks like this:

User-agent: *

Disallow:

This simple content creates a rule which allows all web crawlers access to the entire site.

User-agent: * indicates that the following rule applies to all spider bots.
Disallow: The empty disallow field indicates nothing is blocked and every link can be crawled.

The opposite would be to block access for all web crawlers and prevent the site from being indexed.

User-agent: *

Disallow: /

User-agent: * indicates that the following rule applies to all spider bots.
Disallow: / indicates that the homepage, and everything under the homepage, is disallowed, or forbidden.

You can specify which URLs are blocked in the Disallow field.

User-agent: *

Disallow: /wp-admin/

Disallow: /private/

Care must be taken when disallowing resources as malicious users may use the robots.txt to locate hidden areas and target them to attack. For example, the above rules would indicate to a hacker that the site is running WordPress and that there is a URL with private resources. They can then tailor an attack to WordPress or investigate those private resources. Read more about internal information disclosure here.

Robots.txt is used to prevent search engines from listing a web page, it should not be used as a security measure.

Limiting Bot Access with robots.txt

You can also limit access on a per-bot basis by specifying them in the User-agent field.

Here are a few of the most popular web crawler user agents to use:

googlebot - Googles own web crawler
Mediapartners-Google - Google Adsense/Adwords
Bingbot - Microsoft Bing
MSNBot - Microsoft's old MSN bot
Slurp - Yahoo! Search
ia_archiver - Internet Archive

You can give access to certain bots while blocking all others:

User-agent: Googlebot
Disallow:

User-agent: Bingbot
Disallow:

User-agent: Slurp
Disallow:

# Everyone Else (NOT allowed)
User-agent: *
Disallow: /

In theory, this should block all "bad bots", i.e. those bots who scrape content and hog bandwidth, but bad bots do not honour the robots.txt rules or even access the file. Bad spiders are not going to follow robots.txt, and search engines and spiders that do follow it are the ones that you want indexing your site.

Validating robots.txt with Google Search Console

Once you have created and uploaded a robots.txt, you can use the Google Search Console (formerly Webmaster Tools) to check for errors and test to see if the rules work against several user agents.

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