Allow for a proxy Host header for requests to master

In certain situations it's possible that the backend server doesn't have access to the same DNS host values as the client. For example, this can happen when servers are behind a firewall, or in a private infrastructure. In situations like this, the webserver itself will issue pages, but not be able to make requests to that domain.

Pantheon hosting is an example. If i create two sites, they will be hosted as dev-site1.pantheonsite.io and dev-site2.pantheonsite.io. When i then apply actual domain urls to these sites eg: site1.example.com and site2.example.com the sites are reachable after I either modify the example.com dns record, or add those entries to my /etc/hosts file.

The problem is most pronounced when the /etc/hosts file is modified though. Now, My browser knows how to reach site1 and site2, but those two sites don't know how to reach each other, because they only know the 'pantheonsite.io' dns entries. Changing the bakery master url won't suffice either because it can only be changed to a completely different domain scope.

My recommendation here is to leverage the fix added in core → (specifically for situations like this). and allow for a bakery proxy url. With the attached patch, bakery can be configured to make the backend request to master using the known internal domain name in order to connect with the right server, but then change the Host header to be the domain that wouldn't normally resolve.

I configured this in my settings.php file directly, but you could also use drush vset, or other tools for this. Here's what it looks like:

$conf['bakery_master'] = "site1.example.com";
$conf['bakery_master_proxy'] = "dev-site1.pantheonsite.io";

Hope this helps!