DNS forwarder and common ebpf loader (#1083)

In case the 53 UDP port is not an option to bind then we hijack the DNS traffic with eBPF, and we forward the traffic to the listener on a custom port. With this implementation, we should be able to listen to DNS queries on any address and still set the local host system to send queries to the custom address on port 53.

Because we tried to attach multiple XDP programs to the same interface, I did a refactor in the WG traffic forward code also.

client/internal/dns/service_listener.go

@@ -11,6 +11,9 @@ import (
 
 	"github.com/miekg/dns"
 	log "github.com/sirupsen/logrus"
+
+	"github.com/netbirdio/netbird/client/internal/ebpf"
+	ebpfMgr "github.com/netbirdio/netbird/client/internal/ebpf/manager"
 )
 
 const (
@@ -24,10 +27,11 @@ type serviceViaListener struct {
 	dnsMux            *dns.ServeMux
 	customAddr        *netip.AddrPort
 	server            *dns.Server
-	runtimeIP         string
-	runtimePort       int
+	listenIP          string
+	listenPort        int
 	listenerIsRunning bool
 	listenerFlagLock  sync.Mutex
+	ebpfService       ebpfMgr.Manager
 }
 
 func newServiceViaListener(wgIface WGIface, customAddr *netip.AddrPort) *serviceViaListener {
@@ -43,6 +47,7 @@ func newServiceViaListener(wgIface WGIface, customAddr *netip.AddrPort) *service
 			UDPSize: 65535,
 		},
 	}
+
 	return s
 }
 
@@ -55,13 +60,21 @@ func (s *serviceViaListener) Listen() error {
 	}
 
 	var err error
-	s.runtimeIP, s.runtimePort, err = s.evalRuntimeAddress()
+	s.listenIP, s.listenPort, err = s.evalListenAddress()
 	if err != nil {
 		log.Errorf("failed to eval runtime address: %s", err)
 		return err
 	}
-	s.server.Addr = fmt.Sprintf("%s:%d", s.runtimeIP, s.runtimePort)
+	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)
@@ -69,9 +82,10 @@ func (s *serviceViaListener) Listen() error {
 
 		err := s.server.ListenAndServe()
 		if err != nil {
-			log.Errorf("dns server running with %d port returned an error: %v. Will not retry", s.runtimePort, err)
+			log.Errorf("dns server running with %d port returned an error: %v. Will not retry", s.listenPort, err)
 		}
 	}()
+
 	return nil
 }
 
@@ -90,6 +104,13 @@ func (s *serviceViaListener) Stop() {
 	if err != nil {
 		log.Errorf("stopping dns server listener returned an error: %v", err)
 	}
+
+	if s.ebpfService != nil {
+		err = s.ebpfService.FreeDNSFwd()
+		if err != nil {
+			log.Errorf("stopping traffic forwarder returned an error: %v", err)
+		}
+	}
 }
 
 func (s *serviceViaListener) RegisterMux(pattern string, handler dns.Handler) {
@@ -101,11 +122,18 @@ func (s *serviceViaListener) DeregisterMux(pattern string) {
 }
 
 func (s *serviceViaListener) RuntimePort() int {
-	return s.runtimePort
+	s.listenerFlagLock.Lock()
+	defer s.listenerFlagLock.Unlock()
+
+	if s.ebpfService != nil {
+		return defaultPort
+	} else {
+		return s.listenPort
+	}
 }
 
 func (s *serviceViaListener) RuntimeIP() string {
-	return s.runtimeIP
+	return s.listenIP
 }
 
 func (s *serviceViaListener) setListenerStatus(running bool) {
@@ -136,10 +164,30 @@ func (s *serviceViaListener) getFirstListenerAvailable() (string, int, error) {
 	return "", 0, fmt.Errorf("unable to find an unused ip and port combination. IPs tested: %v and ports %v", ips, ports)
 }
 
-func (s *serviceViaListener) evalRuntimeAddress() (string, int, error) {
+func (s *serviceViaListener) evalListenAddress() (string, int, error) {
 	if s.customAddr != nil {
 		return s.customAddr.Addr().String(), int(s.customAddr.Port()), nil
 	}
 
 	return s.getFirstListenerAvailable()
 }
+
+// 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 {
+	if runtime.GOOS != "linux" {
+		return false
+	}
+
+	if s.customAddr != nil {
+		return false
+	}
+
+	if s.listenPort == defaultPort {
+		return false
+	}
+	return true
+}