Preview server appears to redirect preview files to bound hostname

[daveaglick]

For example when using GitHub Codespaces to forward a port, the host name of the proxy is not localhost. When a URL that relies on default filenames (I.e. an /foo/index.html is requested as /foo) the preview server redirects/rewrites the URL to https://localhost/foo. This doesn’t appear to be happening with other middleware like default extensions (probably because the default file name support is built-in to Kestrel while the other stuff is custom).

[daveaglick]

I'm having trouble replicating this locally, so might be something about the way GitHub Codespaces forwards ports.

[daveaglick]

It appears the 301 is being issued by Kestrel - I can now reproduce (I always could, just didn't realize it). When a URL like /foo/bar is requested and there's no /bar file, a 301 is issued to /foo/bar/ and then the default file logic kicks in. The trick will be to figure out what's issuing the 301 and have it use the current host header instead of localhost.

Possibly related: https://docs.microsoft.com/en-us/aspnet/core/host-and-deploy/proxy-load-balancer?view=aspnetcore-3.1

I imagine this doesn't happen when testing with an alternate DNS name locally because the alternate DNS is actually set by the client in the request. When issued through a proxy, the proxy issues the request for localhost and the server never sees the original host header.

I'll try adding UseForwardedHeaders() and we'll see if that resolves it.

[daveaglick]

It appears to be caused by UseDefaultFiles:

Here's the flow:

• UseDefaultFiles triggers a redirect using the current host header
• Because nginx is proxying traffic, the host header UseDefaultFiles sees is localhost
• The triggers the redirect to localhost in the client

[daveaglick]

Confirmed working now