Automatically add canonical header for urls with non empty query strings

Suppose we have simple html site and we still wish to use canonicals for our pages so Google can index them correct.

But here is tricky part - how could we automate adding canonicals for pages?

Seems that google undestands special Link header for canonicals - https://support.google.com/webmasters/answer/139066?hl=en#6

Here is what I end up with:

# If non empty query string add Canonical header
RewriteCond %{QUERY_STRING} ^.+$ [NC]
SetEnvIf Host (.*) HOSTNAME=$1
SetEnvIf Request_URI (.*) URI=$1
RewriteRule .* - [E=SET_CANONICAL_HEADER:1]
Header set Link "<http://%{HOSTNAME}e%{URI}e>; rel=\"canonical\"" env=SET_CANONICAL_HEADER

So we have non empty query string rewrite condition in first line.

Two next lines with SetEnvIf is needed just to duplicate apache variables into environment variables.

Our rewrite rule do nothing except setting SET_CANONICAL_HEADER into 1 only if rewrite condition met.

And if so we are setting header to exactly same url but without query string.