Types of links followed
- anchors - <a>
- images - <img>
- image maps - <area>
- stylesheets - <link>
- scripts - <script>
- frames - <frame>
- iframes - <iframe>
Where links are followed
All links on internal pages are followed, even those which point to external domains. Also, redirects are followed regardless of the domain.
Any other links on external domains are not followed. Those links are not directly related to your site.
Both your domain with and without the www prefix is considered internal, and we link both versions of your domain and check whether one of them redirects to the other - which it should. You can read more on www/non-www domains
To make sure your website handles missing documents properly, we generate a random URL, and request it from your server. It is practically impossible that your website actually has such a document, so it should return a 404 - Not Found error. This is the only way robots can detect broken links on your website.
We could not connect to the URL that was linked. Either it is not a valid domain, the domain didn't exist or it simply could not be reached.
We successfully connected to your server but had no proper response. This is usually due to server overload, which is temporary. Other times it is a programming error.
You server may use an unknown (eg. typo) or invalid Transfer-Encoding.
Textual responses, like HTML code is usually compressed so that it travels faster on the wires. Your server may have used an invalid Content-Encoding or applied it badly.
Character Set Error
At lower levels, your server is not sending documents per se, it is really sending ones and zeroes (010110110101010..). There is a standard on how these bits must be interpreted, but sometimes the contents of the response are contradictive, which causes such an error. It means the response simply can not be interpreted, no matter how hard we tried. Fatal.
Hop limit error
You see this error if a visitor needs more then 16 hops (~ clicks) to reach a document. There is no way your users will click so many times, you could as well delete those documents.
Also, some websites have infinite URL loops, like a gallery with pagination that displays a next page button even when there are no more images. Sometimes there are indeed so many pages, in which case you should create an index to make them easier to reach.
URL limit error
This is an error related to the testing process itself, not your website, but it may also be informative to uncover actual real issues. Sometimes, websites include infinite URL structures by mistake. If you bump into an URL limit error, please check what URLs were not followed, you may want to clean up your site. (Note: because there are sometimes tens of thousands of such URLs, I limited that list to 100 URLs, which should be plenty for debugging purposes.)
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