When should I set cid-localhost?

Question

In the web.config file of DXA you can specify a cid-localhost setting. This setting is commented as follows:

Use the setting below if you need to map localhost to a specific hostname so the CID service can access resources. On some environments localhost may not resolve correctly to the external hostname.

I don't understand why you would need to map localhost to something else. Why does the CID service need to access resources on the application server? Surely it just returns the requested binaries? Or is there more to it?

How does this setting get used? When would you configure it, and what value would it make sense to use?

Answer 1

The CID Service obtains the original images from the Web Application; for that purpose, the Web Application passes a reference to itself in the CID request URLs.

By default, it passes the authority used in the original request (i.e. the request for the Page which contains CID image URLs), but this strategy won't work if the original request is on localhost, but the CID service runs on a different box.

To make such scenarios work, there is special handling for localhost requests: you can configure the hostname to send to the CID service if the original Request is for localhost.