Say for example that I have two web sites - www.contoso.com and www.fabricam.com. Putting aside the Application Gateway for one moment, I could choose to host these as two web sites on one IIS instance - with the bindings set to send the requests to each web site in IIS.
Until the Application Gateway comes into the mix all is fine. But once an Application Gateway is placed in front of the VM* I get an error displayed in the browser which says:
502 - Web server received an invalid response while acting as a gateway or proxy server.
There is a problem with the page you are looking for, and it cannot be displayed. When the Web server (while acting as a gateway or proxy) contacted the upstream content server, it received an invalid response from the content server.
I have tried adding a multi-site binding as per the instructions here:
https://azure.microsoft.com/en-gb/documentation/articles/application-gateway-create-multisite-portal/
But it seems that whilst a host header value is specified in the binding in IIS (i.e. www.contoso.com, www.fabrikam.com) that the Application Gateway will return this response. Without this host header though I don't see how I can make the traffic go to the correct web site in IIS.
It looks to me as the request here (which is marked as completed) is asking for this functionality:
https://feedback.azure.com/forums/217313-networking/suggestions/10140417-allow-iis-hostname-binding-support-in-azure-applic
But as per the comments (one of which I mine I should add) this doesn't seem to be working as asked.
Many thanks
Paul
*I will actually be deploying both web sites to two VMs in an Availability Set, as in two VMs, each with contoso and fabrikam deployed, but that is largely irrelevant to this question.