feat: Support multiple HTTP headers with the same name

resolver.go

@@ -137,13 +137,13 @@ func newTransport(server string, transportType transport.Type, tlsConfig *tls.Co
 			log.Debugf("Using HTTP(s) transport: %s", server)
 
 			// Parse HTTP headers
-			headers := make(map[string]string)
+			headers := make(map[string][]string)
 			for _, header := range opts.HTTPHeaders {
 				parts := strings.SplitN(header, ":", 2)
 				if len(parts) == 2 {
 					name := strings.TrimSpace(parts[0])
 					value := strings.TrimSpace(parts[1])
-					headers[name] = value
+					headers[name] = append(headers[name], value)
 					log.Debugf("Added header %s: %s", name, value)
 				} else {
 					log.Warnf("Invalid header format: %s (expected 'Name: Value')", header)

transport/http.go

@@ -23,7 +23,7 @@ type HTTP struct {
 	Method       string
 	HTTP2, HTTP3 bool
 	NoPMTUd      bool
-	Headers      map[string]string
+	Headers      map[string][]string
 
 	conn *http.Client
 }
@@ -85,9 +85,11 @@ func (h *HTTP) Exchange(m *dns.Msg) (*dns.Msg, error) {
 
 	// Set custom headers if provided
 	if h.Headers != nil {
-		for name, value := range h.Headers {
-			log.Debugf("Setting custom header %s: %s", name, value)
-			req.Header.Set(name, value)
+		for name, values := range h.Headers {
+			for _, value := range values {
+				log.Debugf("Setting custom header %s: %s", name, value) 
+				req.Header.Add(name, value)
+			}
 		}
 	}