Googlebot is Google’s web crawling robot, which finds and retrieves pages on the web and hands them off to the Google indexer. It’s easy to imagine Googlebot as a little spider scurrying across the strands of cyberspace, but in reality Googlebot doesn’t traverse the web at all. It functions much like your web browser, by sending a request to a web server for a web page, downloading the entire page, then handing it off to Google’s indexer.
Googlebot consists of many computers requesting and fetching pages much more quickly than you can with your web browser. In fact, Googlebot can request thousands of different pages simultaneously. To avoid overwhelming web servers, or crowding out requests from human users, Googlebot deliberately makes requests of each individual web server more slowly than it’s capable of doing.
Googlebot finds pages in two ways: through an add URL form, www.google.com/addurl.html, and through finding links by crawling the web.
How Googlebot accesses your site
For most sites, Googlebot shouldn't access your site more than once every few seconds on average. However, due to network delays, it's possible that the rate will appear to be slightly higher over short periods. In general, Googlebot should download only one copy of each page at a time. If you see that Googlebot is downloading a page multiple times, it's probably because the crawler was stopped and restarted.
Googlebot was designed to be distributed on several machines to improve performance and scale as the web grows. Also, to cut down on bandwidth usage, we run many crawlers on machines located near the sites they're indexing in the network. Therefore, your logs may show visits from several machines at google.com, all with the user-agent Googlebot. Our goal is to crawl as many pages from your site as we can on each visit without overwhelming your server's bandwidth. Request a change in the crawl rate.
Blocking Googlebot from content on your site
It's almost impossible to keep a web server secret by not publishing links to it. As soon as someone follows a link from your "secret" server to another web server, your "secret" URL may appear in the referrer tag and can be stored and published by the other web server in its referrer log. Similarly, the web has many outdated and broken links. Whenever someone publishes an incorrect link to your site or fails to update links to reflect changes in your server, Googlebot will try to download an incorrect link from your site.
Once you've created your robots.txt file, there may be a small delay before Googlebot discovers your changes. If Googlebot is still crawling content you've blocked in robots.txt, check that the robots.txt is in the correct location. It must be in the top directory of the server (e.g., www.myhost.com/robots.txt); placing the file in a subdirectory won't have any effect.
If you just want to prevent the "file not found" error messages in your web server log, you can create an empty file named robots.txt. If you want to prevent Googlebot from following any links on a page of your site, you can use the nofollow meta tag. To prevent Googlebot from following an individual link, add the
rel="nofollow" attribute to the link itself.
Here are some additional tips:
- Test that your robots.txt is working as expected. The Test robots.txt tool on the Blocked URLs (robots.txt) tab of the Crawler Access page lets you see exactly how Googlebot will interpret the contents of your robots.txt file. The Google user-agent is (appropriately enough)
- The Fetch as Googlebot tool in Webmaster Tools helps you understand exactly how your site appears to Googlebot. This can be very useful when troubleshooting problems with your site's content or discoverability in search results.