[client] Use native windows sock opts to avoid routing loops (#4314)

- Move `util/grpc` and `util/net` to `client` so `internal` packages can be accessed
 - Add methods to return the next best interface after the NetBird interface.
- Use `IP_UNICAST_IF` sock opt to force the outgoing interface for the NetBird `net.Dialer` and `net.ListenerConfig` to avoid routing loops. The interface is picked by the new route lookup method.
- Some refactoring to avoid import cycles
- Old behavior is available through `NB_USE_LEGACY_ROUTING=true` env var

client/grpc/dialer.go

@@ -0,0 +1,95 @@
+package grpc
+
+import (
+	"context"
+	"crypto/tls"
+	"crypto/x509"
+	"fmt"
+	"net"
+	"os/user"
+	"runtime"
+	"time"
+
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+
+	"github.com/cenkalti/backoff/v4"
+	log "github.com/sirupsen/logrus"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/keepalive"
+
+	nbnet "github.com/netbirdio/netbird/client/net"
+
+	"github.com/netbirdio/netbird/util/embeddedroots"
+)
+
+func WithCustomDialer() grpc.DialOption {
+	return grpc.WithContextDialer(func(ctx context.Context, addr string) (net.Conn, error) {
+		if runtime.GOOS == "linux" {
+			currentUser, err := user.Current()
+			if err != nil {
+				return nil, status.Errorf(codes.FailedPrecondition, "failed to get current user: %v", err)
+			}
+
+			// the custom dialer requires root permissions which are not required for use cases run as non-root
+			if currentUser.Uid != "0" {
+				log.Debug("Not running as root, using standard dialer")
+				dialer := &net.Dialer{}
+				return dialer.DialContext(ctx, "tcp", addr)
+			}
+		}
+
+		conn, err := nbnet.NewDialer().DialContext(ctx, "tcp", addr)
+		if err != nil {
+			log.Errorf("Failed to dial: %s", err)
+			return nil, fmt.Errorf("nbnet.NewDialer().DialContext: %w", err)
+		}
+		return conn, nil
+	})
+}
+
+// grpcDialBackoff is the backoff mechanism for the grpc calls
+func Backoff(ctx context.Context) backoff.BackOff {
+	b := backoff.NewExponentialBackOff()
+	b.MaxElapsedTime = 10 * time.Second
+	b.Clock = backoff.SystemClock
+	return backoff.WithContext(b, ctx)
+}
+
+func CreateConnection(addr string, tlsEnabled bool) (*grpc.ClientConn, error) {
+	transportOption := grpc.WithTransportCredentials(insecure.NewCredentials())
+	if tlsEnabled {
+		certPool, err := x509.SystemCertPool()
+		if err != nil || certPool == nil {
+			log.Debugf("System cert pool not available; falling back to embedded cert, error: %v", err)
+			certPool = embeddedroots.Get()
+		}
+
+		transportOption = grpc.WithTransportCredentials(credentials.NewTLS(&tls.Config{
+			RootCAs: certPool,
+		}))
+	}
+
+	connCtx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	conn, err := grpc.DialContext(
+		connCtx,
+		addr,
+		transportOption,
+		WithCustomDialer(),
+		grpc.WithBlock(),
+		grpc.WithKeepaliveParams(keepalive.ClientParameters{
+			Time:    30 * time.Second,
+			Timeout: 10 * time.Second,
+		}),
+	)
+	if err != nil {
+		log.Printf("DialContext error: %v", err)
+		return nil, err
+	}
+
+	return conn, nil
+}