Update port, ip choice logic in DNS service

client/internal/dns/service_listener.go

@@ -28,7 +28,7 @@ type serviceViaListener struct {
 	customAddr        *netip.AddrPort
 	server            *dns.Server
 	listenIP          string
-	listenPort        int
+	listenPort        uint16
 	listenerIsRunning bool
 	listenerFlagLock  sync.Mutex
 	ebpfService       ebpfMgr.Manager
@@ -66,15 +66,6 @@ func (s *serviceViaListener) Listen() error {
 		return fmt.Errorf("eval listen address: %w", err)
 	}
 	s.server.Addr = fmt.Sprintf("%s:%d", s.listenIP, s.listenPort)
-
-	if s.shouldApplyPortFwd() {
-		s.ebpfService = ebpf.GetEbpfManagerInstance()
-		err = s.ebpfService.LoadDNSFwd(s.listenIP, s.listenPort)
-		if err != nil {
-			log.Warnf("failed to load DNS port forwarder, custom port may not work well on some Linux operating systems: %s", err)
-			s.ebpfService = nil
-		}
-	}
 	log.Debugf("starting dns on %s", s.server.Addr)
 	go func() {
 		s.setListenerStatus(true)
@@ -128,7 +119,7 @@ func (s *serviceViaListener) RuntimePort() int {
 	if s.ebpfService != nil {
 		return defaultPort
 	} else {
-		return s.listenPort
+		return int(s.listenPort)
 	}
 }
 
@@ -140,55 +131,112 @@ func (s *serviceViaListener) setListenerStatus(running bool) {
 	s.listenerIsRunning = running
 }
 
-func (s *serviceViaListener) getFirstDNSListenerAvailable() (string, int, error) {
-	ips := []string{defaultIP, customIP}
+// evalListenAddress figure out the listen address for the DNS server
+// first check the 53 port availability on WG interface or lo, if not success
+// pick a random port on WG interface for eBPF, if not success
+// check the 5053 port availability on WG interface or lo without eBPF usage,
+func (s *serviceViaListener) evalListenAddress() (string, uint16, error) {
+	if s.customAddr != nil {
+		return s.customAddr.Addr().String(), s.customAddr.Port(), nil
+	}
+
+	ip, ok := s.testFreePort(defaultPort)
+	if ok {
+		return ip, defaultPort, nil
+	}
+
+	ebpfSrv, port, ok := s.tryToUseeBPF()
+	if ok {
+		s.ebpfService = ebpfSrv
+		return s.wgInterface.Address().IP.String(), port, nil
+	}
+
+	ip, ok = s.testFreePort(customPort)
+	if ok {
+		return ip, customPort, nil
+	}
+
+	return "", 0, fmt.Errorf("failed to find a free port for DNS server")
+}
+
+func (s *serviceViaListener) testFreePort(port int) (string, bool) {
+	var ips []string
 	if runtime.GOOS != "darwin" {
-		ips = append([]string{s.wgInterface.Address().IP.String()}, ips...)
-	}
-	ports := []int{defaultPort, customPort}
-	for _, port := range ports {
-		for _, ip := range ips {
-			addrString := fmt.Sprintf("%s:%d", ip, port)
-			udpAddr := net.UDPAddrFromAddrPort(netip.MustParseAddrPort(addrString))
-			probeListener, err := net.ListenUDP("udp", udpAddr)
-			if err == nil {
-				err = probeListener.Close()
-				if err != nil {
-					log.Errorf("got an error closing the probe listener, error: %s", err)
-				}
-				log.Infof("successfully found dns binding candidate at %s", addrString)
-				return ip, port, nil
-			}
-			log.Debugf("binding dns on %s is not available, error: %s", addrString, err)
+		ips = []string{s.wgInterface.Address().IP.String(), defaultIP, customIP}
+	} else {
+		ips = []string{defaultIP, customIP}
+	}
+
+	for _, ip := range ips {
+		if !s.tryToBind(ip, port) {
+			continue
 		}
+
+		return ip, true
 	}
-	return "", 0, fmt.Errorf("unable to find an unused ip and port combination. IPs tested: %v and ports %v", ips, ports)
+	return "", false
 }
 
-func (s *serviceViaListener) evalListenAddress() (string, int, error) {
-	if s.customAddr != nil {
-		return s.customAddr.Addr().String(), int(s.customAddr.Port()), nil
+func (s *serviceViaListener) tryToBind(ip string, port int) bool {
+	addrString := fmt.Sprintf("%s:%d", ip, port)
+	udpAddr := net.UDPAddrFromAddrPort(netip.MustParseAddrPort(addrString))
+	probeListener, err := net.ListenUDP("udp", udpAddr)
+	if err != nil {
+		log.Warnf("binding dns on %s is not available, error: %s", addrString, err)
+		return false
 	}
 
-	return s.getFirstDNSListenerAvailable()
+	err = probeListener.Close()
+	if err != nil {
+		log.Errorf("got an error closing the probe listener, error: %s", err)
+	}
+	return true
 }
 
-// shouldApplyPortFwd decides whether to apply eBPF program to capture DNS traffic on port 53.
-// This is needed because on some operating systems if we start a DNS server not on a default port 53, the domain name
-// resolution won't work.
-// So, in case we are running on Linux and picked a non-default port (53) we should fall back to the eBPF solution that will capture
-// traffic on port 53 and forward it to a local DNS server running on 5053.
-func (s *serviceViaListener) shouldApplyPortFwd() bool {
+// tryToUseeBPF decides whether to apply eBPF program to capture DNS traffic on port 53.
+// This is needed because on some operating systems if we start a DNS server not on a default port 53,
+// the domain name  resolution won't work. So, in case we are running on Linux and picked a free
+// port we should fall back to the eBPF solution that will capture traffic on port 53 and forward
+// it to a local DNS server running on the chosen port.
+func (s *serviceViaListener) tryToUseeBPF() (ebpfMgr.Manager, uint16, bool) {
 	if runtime.GOOS != "linux" {
-		return false
+		return nil, 0, false
 	}
 
-	if s.customAddr != nil {
-		return false
+	port, err := s.generateFreePort() //nolint:staticcheck,unused
+	if err != nil {
+		log.Warnf("failed to generate a free port for eBPF DNS forwarder server: %s", err)
+		return nil, 0, false
 	}
 
-	if s.listenPort == defaultPort {
-		return false
+	ebpfSrv := ebpf.GetEbpfManagerInstance()
+	err = ebpfSrv.LoadDNSFwd(s.wgInterface.Address().IP.String(), int(port))
+	if err != nil {
+		log.Warnf("failed to load DNS forwarder eBPF program, error: %s", err)
+		return nil, 0, false
 	}
-	return true
+
+	return ebpfSrv, port, true
+}
+
+func (s *serviceViaListener) generateFreePort() (uint16, error) {
+	ok := s.tryToBind(s.wgInterface.Address().IP.String(), customPort)
+	if ok {
+		return customPort, nil
+	}
+
+	udpAddr := net.UDPAddrFromAddrPort(netip.MustParseAddrPort("0.0.0.0:0"))
+	probeListener, err := net.ListenUDP("udp", udpAddr)
+	if err != nil {
+		log.Debugf("failed to bind random port for DNS: %s", err)
+		return 0, err
+	}
+
+	addrPort := netip.MustParseAddrPort(probeListener.LocalAddr().String()) // might panic if address is incorrect
+	err = probeListener.Close()
+	if err != nil {
+		log.Debugf("failed to free up DNS port: %s", err)
+		return 0, err
+	}
+	return addrPort.Port(), nil
 }

client/internal/ebpf/ebpf/dns_fwd_linux.go

@@ -13,7 +13,7 @@ const (
 )
 
 func (tf *GeneralManager) LoadDNSFwd(ip string, dnsPort int) error {
-	log.Debugf("load ebpf DNS forwarder: address: %s:%d", ip, dnsPort)
+	log.Debugf("load eBPF DNS forwarder, watching addr: %s:53, redirect to port: %d", ip, dnsPort)
 	tf.lock.Lock()
 	defer tf.lock.Unlock()
 

client/internal/ebpf/ebpf/src/readme.md

@@ -1,5 +1,10 @@
-# Debug
+# DNS forwarder
+
+The agent attach the XDP program to the lo device. We can not use fake address in eBPF because the
+traffic does not appear in the eBPF program. The program capture the traffic on wg_ip:53 and 
+overwrite in it the destination port to 5053.
 
+# Debug
 
 The CONFIG_BPF_EVENTS kernel module is required for bpf_printk.
 Apply this code to use bpf_printk