Add req.host to targetURL when redirecting to https (#668)

* Add req.host to targetURL when redirecting to https

The req.URL.Host might not be present when redirecting to https if the
given req.URL is something like "/". In such scenario, the req.Host is
still present and valid.

This commit adds the original req.Host to the targetURL before returning
the 308 status, to avoid having a `Location: https:///` in the response.

* Bring back empty line

* Wrap the setting of targetURL.Host in a condition

* Add a comment to the test explaining why the redirectURL includes example.com

* Add changelog entry

CHANGELOG.md

@@ -8,6 +8,7 @@
 
 ## Changes since v6.0.0
 
+- [#668](https://github.com/oauth2-proxy/oauth2-proxy/pull/668) Use req.Host in --force-https when req.URL.Host is empty (@zucaritask)
 - [#660](https://github.com/oauth2-proxy/oauth2-proxy/pull/660) Use builder pattern to simplify requests to external endpoints (@JoelSpeed)
 - [#591](https://github.com/oauth2-proxy/oauth2-proxy/pull/591) Introduce upstream package with new reverse proxy implementation (@JoelSpeed)
 - [#576](https://github.com/oauth2-proxy/oauth2-proxy/pull/576) Separate Cookie validation out of main options validation (@JoelSpeed)

pkg/middleware/redirect_to_https.go

@@ -38,6 +38,11 @@ func redirectToHTTPS(httpsPort string, next http.Handler) http.Handler {
 		// Set the scheme to HTTPS
 		targetURL.Scheme = httpsScheme
 
+		// Set the req.Host when the targetURL still does not have one
+		if targetURL.Host == "" {
+			targetURL.Host = req.Host
+		}
+
 		// Overwrite the port if the original request was to a non-standard port
 		if targetURL.Port() != "" {
 			// If Port was not empty, this should be fine to ignore the error

pkg/middleware/redirect_to_https_test.go

@@ -154,5 +154,15 @@ var _ = Describe("RedirectToHTTPS suite", func() {
 			expectedStatus: 200,
 			expectedBody:   "test",
 		}),
+		// By using newRequest from httptest we get example.com as a Host
+		// when the target is just a path.
+		// For details: https://golang.org/pkg/net/http/httptest/#NewRequest
+		Entry("without TLS with a path as request", &requestTableInput{
+			requestString:    "/",
+			useTLS:           false,
+			expectedStatus:   308,
+			expectedBody:     permanentRedirectBody("https://example.com/"),
+			expectedLocation: "https://example.com/",
+		}),
 	)
 })