We are fighting some duplicate content issues across multiple domains. We have a few magento stores that have different country codes. For example: domain.com and domain.ca, domain.com is the "main" domain. We have set up different rel="alternative codes like: The question is, do we need to add custom rel="canonical" tags to domain.ca that points to domain.com? For example for domain.ca/product.html to point to: Also how far does rel="canonical" follow?  For example if we have:
domain.ca/sub/product.html canonical to domain.com/sub/product.html
then,
domain.com/sub/product.html canonical to domain.com/product.html