Check the canonical tag of a page and find canonical problems.
The canonical URL checker downloads a page and reads its rel="canonical" link tag and Link HTTP header. It tells you whether the page points to itself or to another URL, and checks the canonical URL itself: does it work, does it redirect, is it on https?
Canonical tags tell Google which version of a page to index when the same content is reachable at several URLs – with and without www, with tracking parameters, or in print versions. A wrong canonical can remove a page from Google entirely.
Reads both the <link> tag and the HTTP Link header.
Tells whether the canonical points to the page itself.
Tests that the canonical URL returns 200 without redirects.
Finds several canonicals, noindex + canonical and og:url mismatches.
A <link rel="canonical" href="…"> tag in the head that names the preferred URL of a page.
It's a good practice: a self-referencing canonical protects the page against duplicate URLs with parameters.
It's a strong hint. Google can choose another URL when signals conflict, e.g. the canonical redirects or isn't in the sitemap.