fix(server): advertise an https resource behind TLS-terminating load balancers

internal/flashduty/server.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"net"
 	"net/http"
 	"os"
 	"os/signal"
@@ -327,18 +328,37 @@ func httpContextFunc(ctx context.Context, r *http.Request, defaultBaseURL string
 // requestOrigin returns the scheme and host the client used to reach this
 // server, honoring X-Forwarded-Proto/X-Forwarded-Host set by a reverse proxy.
 func requestOrigin(r *http.Request) string {
-	scheme := "http"
-	if r.TLS != nil {
-		scheme = "https"
-	}
-	if proto := firstHeaderValue(r, "X-Forwarded-Proto"); proto != "" {
-		scheme = proto
-	}
 	host := r.Host
 	if fwdHost := firstHeaderValue(r, "X-Forwarded-Host"); fwdHost != "" {
 		host = fwdHost
 	}
-	return scheme + "://" + host
+	return requestScheme(r, host) + "://" + host
+}
+
+// requestScheme trusts X-Forwarded-Proto when it names http or https. Without
+// it (a TLS-terminating load balancer that does not set the header), only a
+// loopback host is plain http: MCP clients require https for any other
+// authorization target, so the public resource is https.
+func requestScheme(r *http.Request, host string) string {
+	switch proto := strings.ToLower(firstHeaderValue(r, "X-Forwarded-Proto")); proto {
+	case "http", "https":
+		return proto
+	}
+	if r.TLS != nil {
+		return "https"
+	}
+	hostname := host
+	if h, _, err := net.SplitHostPort(host); err == nil {
+		hostname = h
+	}
+	hostname = strings.Trim(hostname, "[]")
+	if hostname == "localhost" {
+		return "http"
+	}
+	if ip := net.ParseIP(hostname); ip != nil && ip.IsLoopback() {
+		return "http"
+	}
+	return "https"
 }
 
 // firstHeaderValue returns the first entry of a possibly comma-separated

internal/flashduty/server_test.go

@@ -80,9 +80,28 @@ func TestHTTPMux_MissingCredentialChallenges(t *testing.T) {
 		want    string
 	}{
 		{
-			name: "direct",
+			// A TLS-terminating load balancer that sets no X-Forwarded-Proto.
+			name: "no forwarded proto",
 			path: "/mcp",
-			want: `Bearer resource_metadata="http://mcp.example.com/.well-known/oauth-protected-resource/mcp"`,
+			want: `Bearer resource_metadata="https://mcp.example.com/.well-known/oauth-protected-resource/mcp"`,
+		},
+		{
+			name:    "loopback",
+			path:    "/mcp",
+			headers: map[string]string{"X-Forwarded-Host": "127.0.0.1:11310"},
+			want:    `Bearer resource_metadata="http://127.0.0.1:11310/.well-known/oauth-protected-resource/mcp"`,
+		},
+		{
+			name:    "unknown forwarded proto",
+			path:    "/mcp",
+			headers: map[string]string{"X-Forwarded-Proto": "gopher"},
+			want:    `Bearer resource_metadata="https://mcp.example.com/.well-known/oauth-protected-resource/mcp"`,
+		},
+		{
+			name:    "forwarded http",
+			path:    "/mcp",
+			headers: map[string]string{"X-Forwarded-Proto": "http", "X-Forwarded-Host": "mcp.internal"},
+			want:    `Bearer resource_metadata="http://mcp.internal/.well-known/oauth-protected-resource/mcp"`,
 		},
 		{
 			name:    "behind proxy",
@@ -93,7 +112,7 @@ func TestHTTPMux_MissingCredentialChallenges(t *testing.T) {
 		{
 			name: "legacy path",
 			path: "/flashduty",
-			want: `Bearer resource_metadata="http://mcp.example.com/.well-known/oauth-protected-resource/flashduty"`,
+			want: `Bearer resource_metadata="https://mcp.example.com/.well-known/oauth-protected-resource/flashduty"`,
 		},
 	}
 	for _, tt := range tests {
@@ -161,17 +180,17 @@ func TestHTTPMux_ProtectedResourceMetadata(t *testing.T) {
 		{
 			name:         "path inserted",
 			path:         "/.well-known/oauth-protected-resource/mcp",
-			wantResource: "http://mcp.example.com/mcp",
+			wantResource: "https://mcp.example.com/mcp",
 		},
 		{
 			name:         "root describes /mcp",
 			path:         "/.well-known/oauth-protected-resource",
-			wantResource: "http://mcp.example.com/mcp",
+			wantResource: "https://mcp.example.com/mcp",
 		},
 		{
 			name:         "legacy path",
 			path:         "/.well-known/oauth-protected-resource/flashduty",
-			wantResource: "http://mcp.example.com/flashduty",
+			wantResource: "https://mcp.example.com/flashduty",
 		},
 		{
 			name:         "behind proxy",