It is possible to set up a canonical tag that points to an external domain to highlight to Google the page that should be taken into account for ranking. However, Google itself emphasises that this is a recommendation that the search engine usually, but not always, follows. Google recommends using 301 redirects whenever possible.
It is also worth noting that the search engine is good at making its own decision when deciding between two very similar or identical pieces of content. When you set up a canonical tag, make sure that the content of the two pages is similar, otherwise the tag will be ignored by Google.
Instead of HTML tags, you can use a rel=”canonical” http header – provided you can configure your server.
For example, this header looks like this:
One advantage of this method is that it does not increase the size of the page and you can assign an infinite number of duplicated pages. If you don’t want to link to an HTML document, but for example to a PDF, use this method in any case. However, there is also a disadvantage. A rel=”canonical” http header can make it difficult to maintain mappings when dealing with larger websites.
If a page www.example.en is reachable with http:// and https://, you can use a canonical tag to tell Google to index the https:// version.
In the head of the page it looks like this:
<link rel="canonical" href="https://www.example.en "/>
Alternatively, you can use a http header:
< https://www.example.en >; rel="canonical"
Following the same principle, you have the possibility to instruct Google to index the page version with http://:
<link rel=”canonical” href=”https://www.example.en”/>.
The http header will look like this:
< http://www.example.en >; rel=”canonical”
However please note that https ensures more security during data transmission and is therefore preferred by Google.
If you specify more than one canonical URL per page, you risk Google ignoring the canonical tag. Even more: the search engine will ignore all canonical tags on your page. It will be treated like a page without a canonical tag. This means that Google decides which of the similar pages will be used for the ranking.
Therefore, you should try and limit yourself to one URL. Also, do not try to specify multiple pages by using different canonicalisation methods – for example, with one URL in the sitemap and another in an HTML tag. By the way, Google also ignores Canonical tags if the two pages are not similar enough, and a thematic similarity is not enough.