Accessing an instance internally or across instances via proxy

[tbongers-cat]

When trying to access an URL via SOAP from a testscript on the same instance, I had the issue that I couldn't access the WSDL at http://doil/instancename/webservice/soap/server.php?wsdl=1, but had to use the IP address instead (http://1.2.3.4/instancename/webservice/soap/server.php?wsdl=1). The same seems to be the case when I try this from a script running on a different instance.

I would like to figure out if this is only limited to SOAP calls or if the access via the proxy name is not possible at all from "inside" an instance and how we can possibly fix this.
@daniwe4 Let's have a look at this together when we have some time.