resolver: Compress all DNS answers

resolver/resolver.go

@@ -349,10 +349,7 @@ func (res *Resolver) HandleNonMesos(w dns.ResponseWriter, r *dns.Msg) {
 		}
 	}
 
-	err = w.WriteMsg(m)
-	if err != nil {
-		logging.Error.Println(err)
-	}
+	reply(w, m)
 }
 
 // HandleMesos is a resolver request handler that responds to a resource
@@ -463,12 +460,31 @@ func (res *Resolver) HandleMesos(w dns.ResponseWriter, r *dns.Msg) {
 		}
 	}
 
-	err = w.WriteMsg(m)
-	if err != nil {
+	reply(w, m)
+}
+
+// reply writes the given dns.Msg out to the given dns.ResponseWriter,
+// compressing the message first and truncating it accordingly.
+func reply(w dns.ResponseWriter, m *dns.Msg) {
+	m.Compress = true // https://github.com/mesosphere/mesos-dns/issues/{170,173,174}
+	if err := w.WriteMsg(truncate(m, isUDP(w))); err != nil {
 		logging.Error.Println(err)
 	}
 }
 
+// isUDP returns true if the transmission channel in use is UDP.
+func isUDP(w dns.ResponseWriter) bool {
+	return strings.HasPrefix(w.RemoteAddr().Network(), "udp")
+}
+
+// truncate sets the TC bit in the given dns.Msg if its length exceeds the
+// permitted length of the given transmission channel.
+// See https://tools.ietf.org/html/rfc1035#section-4.2.1
+func truncate(m *dns.Msg, udp bool) *dns.Msg {
+	m.Truncated = udp && m.Len() > dns.MinMsgSize
+	return m
+}
+
 func (res *Resolver) configureHTTP() {
 	// webserver + available routes
 	ws := new(restful.WebService)