cmd/ctrld: use RFC1918 addresses as nameservers if required

cmd/ctrld/dns_proxy.go

@@ -80,7 +80,6 @@ func (p *prog) serveDNS(listenerNum string) error {
 		}
 	})
 
-	needRFC1918Listeners := listenerConfig.IP == "127.0.0.1" && listenerConfig.Port == 53
 	g, ctx := errgroup.WithContext(context.Background())
 	for _, proto := range []string{"udp", "tcp"} {
 		proto := proto
@@ -101,17 +100,11 @@ func (p *prog) serveDNS(listenerNum string) error {
 		}
 		// When we spawn a listener on 127.0.0.1, also spawn listeners on the RFC1918
 		// addresses of the machine. So ctrld could receive queries from LAN clients.
-		if needRFC1918Listeners {
+		if needRFC1918Listeners(listenerConfig) {
 			g.Go(func() error {
-				interfaces.ForeachInterface(func(i interfaces.Interface, prefixes []netip.Prefix) {
+				for _, addr := range rfc1918Addresses() {
-					addrs, _ := i.Addrs()
-					for _, addr := range addrs {
-						ipNet, ok := addr.(*net.IPNet)
-						if !ok || !ipNet.IP.IsPrivate() {
-							continue
-						}
 					func() {
-							listenAddr := net.JoinHostPort(ipNet.IP.String(), "53")
+						listenAddr := net.JoinHostPort(addr, strconv.Itoa(listenerConfig.Port))
 						s, errCh := runDNSServer(listenAddr, proto, handler)
 						defer s.Shutdown()
 						select {
@@ -124,7 +117,6 @@ func (p *prog) serveDNS(listenerNum string) error {
 						}
 					}()
 				}
-				})
 				return nil
 			})
 		}
@@ -556,3 +548,22 @@ func (p *prog) getClientInfo(ip, mac string) *ctrld.ClientInfo {
 	ci.Hostname = p.ciTable.LookupHostname(ci.IP, ci.Mac)
 	return ci
 }
+
+func needRFC1918Listeners(lc *ctrld.ListenerConfig) bool {
+	return lc.IP == "127.0.0.1" && lc.Port == 53
+}
+
+func rfc1918Addresses() []string {
+	var res []string
+	interfaces.ForeachInterface(func(i interfaces.Interface, prefixes []netip.Prefix) {
+		addrs, _ := i.Addrs()
+		for _, addr := range addrs {
+			ipNet, ok := addr.(*net.IPNet)
+			if !ok || !ipNet.IP.IsPrivate() {
+				continue
+			}
+			res = append(res, ipNet.IP.String())
+		}
+	})
+	return res
+}

cmd/ctrld/prog.go

@@ -239,7 +239,11 @@ func (p *prog) setDNS() {
 		// so we could just use lc.IP as nameserver.
 	}
 
-	if err := setDNS(netIface, []string{ns}); err != nil {
+	nameservers := []string{ns}
+	if needRFC1918Listeners(lc) {
+		nameservers = append(nameservers, rfc1918Addresses()...)
+	}
+	if err := setDNS(netIface, nameservers); err != nil {
 		logger.Error().Err(err).Msgf("could not set DNS for interface")
 		return
 	}