Last time I checked this is how it worked. I know there was talks of having a parent.config option which forced the child to send a relative URL request. Instead of the full URL GET currently sent to the parent.
That used to be an option in the remap configuration setting, but was removed (because it was crippled). John Rushford is working on a replacement for this feature, such that parent request follow normal (“origin”) request semantics, and not the way parent proxying works today. I’m hoping that it’ll be configurable per rule in parent.config (and not a global setting as it was before). Alternatively, maybe it could be an overridable records.config setting.
That much said, I believe Jeremy is right, in that pristine host headers don’t apply to parent selection. It looks like parent selection happens right after remapping, and it does not consider pristine host headers. John and Phil, can you confirm?
Note that the parent.config selection happens on the remapped host name (I’m pretty sure, I’m sure someone will correct me :).