Updated urlbuilder X-Forwarded-Host logic

According to the Apache mod_proxy docs, X-Forwarded-Host can be a
comma-separated list of hosts, to which each proxy appends the requested
host. We want to grab only the first from this comma-separated list
to get the original requested Host when building URLs.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

registry/api/v2/urls.go

@@ -62,7 +62,12 @@ func NewURLBuilderFromRequest(r *http.Request) *URLBuilder {
 	host := r.Host
 	forwardedHost := r.Header.Get("X-Forwarded-Host")
 	if len(forwardedHost) > 0 {
-		host = forwardedHost
+		// According to the Apache mod_proxy docs, X-Forwarded-Host can be a
+		// comma-separated list of hosts, to which each proxy appends the
+		// requested host. We want to grab the first from this comma-separated
+		// list.
+		hosts := strings.SplitN(forwardedHost, ",", 2)
+		host = strings.TrimSpace(hosts[0])
 	}
 
 	basePath := routeDescriptorsMap[RouteNameBase].Path

registry/api/v2/urls_test.go

@@ -151,6 +151,12 @@ func TestBuilderFromRequest(t *testing.T) {
 	forwardedProtoHeader := make(http.Header, 1)
 	forwardedProtoHeader.Set("X-Forwarded-Proto", "https")
 
+	forwardedHostHeader1 := make(http.Header, 1)
+	forwardedHostHeader1.Set("X-Forwarded-Host", "first.example.com")
+
+	forwardedHostHeader2 := make(http.Header, 1)
+	forwardedHostHeader2.Set("X-Forwarded-Host", "first.example.com, proxy1.example.com")
+
 	testRequests := []struct {
 		request *http.Request
 		base    string
@@ -163,6 +169,14 @@ func TestBuilderFromRequest(t *testing.T) {
 			request: &http.Request{URL: u, Host: u.Host, Header: forwardedProtoHeader},
 			base:    "https://example.com",
 		},
+		{
+			request: &http.Request{URL: u, Host: u.Host, Header: forwardedHostHeader1},
+			base:    "http://first.example.com",
+		},
+		{
+			request: &http.Request{URL: u, Host: u.Host, Header: forwardedHostHeader2},
+			base:    "http://first.example.com",
+		},
 	}
 
 	for _, tr := range testRequests {