The allow_aboslute_url option on the http1 codec options on the HCM imply that leaving at the default of false will mean that absolute URLs are not allowed. Instead, what happens is that the url is passed unchanged as the :path header to the HCM, which results in the full URI being used for route matching. As a result, the common idiom of using prefix: / as "match any" doesn't work because it doesn't match the scheme-prefixed URL.

I'm not sure if this warrants any behavioral changes, but at the very least the docs should be clearer about what the implication of this option is. Maybe even renaming it to handle_absolute_url with V3?

Relevant code: https://github.com/envoyproxy/envoy/blob/master/source/common/http/http1/codec_impl.cc#L544-L572