@@ -1,17 +1,21 @@

 package build // import "github.com/docker/docker/api/server/router/build"

-import "github.com/docker/docker/api/server/router"

+import (

+	"github.com/docker/docker/api/server/router"

+	"github.com/docker/docker/api/types"

+)

 // buildRouter is a router to talk with the build controller

 type buildRouter struct {

-	backend Backend

-	daemon  experimentalProvider

-	routes  []router.Route

+	backend        Backend

+	daemon         experimentalProvider

+	routes         []router.Route

+	builderVersion types.BuilderVersion

 }

 // NewRouter initializes a new build router

-func NewRouter(b Backend, d experimentalProvider) router.Router {

-	r := &buildRouter{backend: b, daemon: d}

+func NewRouter(b Backend, d experimentalProvider, bv types.BuilderVersion) router.Router {

+	r := &buildRouter{backend: b, daemon: d, builderVersion: bv}

 	r.initRoutes()

 	return r

 }

@@ -230,8 +230,10 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *

 		return errdefs.InvalidParameter(errors.New("squash is only supported with experimental mode"))

 	}

-	if buildOptions.Version == types.BuilderBuildKit && !br.daemon.HasExperimental() {

-		return errdefs.InvalidParameter(errors.New("buildkit is only supported with experimental mode"))

+	// check if the builder feature has been enabled from daemon as well.

+	if buildOptions.Version == types.BuilderBuildKit &&

+		(br.builderVersion != types.BuilderBuildKit || !br.daemon.HasExperimental()) {

+		return errdefs.InvalidParameter(errors.New("buildkit is not enabled on daemon"))

 	}

 	out := io.Writer(output)

@@ -2,6 +2,7 @@ package system // import "github.com/docker/docker/api/server/router/system"

 import (

 	"github.com/docker/docker/api/server/router"

+	"github.com/docker/docker/api/types"

 	buildkit "github.com/docker/docker/builder/builder-next"

 	"github.com/docker/docker/builder/fscache"

 )

@@ -9,25 +10,27 @@ import (

 // systemRouter provides information about the Docker system overall.

 // It gathers information about host, daemon and container events.

 type systemRouter struct {

-	backend Backend

-	cluster ClusterBackend

-	routes  []router.Route

-	fscache *fscache.FSCache // legacy

-	builder *buildkit.Builder

+	backend        Backend

+	cluster        ClusterBackend

+	routes         []router.Route

+	fscache        *fscache.FSCache // legacy

+	builder        *buildkit.Builder

+	builderVersion types.BuilderVersion

 }

 // NewRouter initializes a new system router

-func NewRouter(b Backend, c ClusterBackend, fscache *fscache.FSCache, builder *buildkit.Builder) router.Router {

+func NewRouter(b Backend, c ClusterBackend, fscache *fscache.FSCache, builder *buildkit.Builder, bv types.BuilderVersion) router.Router {

 	r := &systemRouter{

-		backend: b,

-		cluster: c,

-		fscache: fscache,

-		builder: builder,

+		backend:        b,

+		cluster:        c,

+		fscache:        fscache,

+		builder:        builder,

+		builderVersion: bv,

 	}

 	r.routes = []router.Route{

 		router.NewOptionsRoute("/{anyroute:.*}", optionsHandler),

-		router.NewGetRoute("/_ping", pingHandler),

+		router.NewGetRoute("/_ping", r.pingHandler),

 		router.NewGetRoute("/events", r.getEvents, router.WithCancel),

 		router.NewGetRoute("/info", r.getInfo),

 		router.NewGetRoute("/version", r.getVersion),

@@ -25,7 +25,10 @@ func optionsHandler(ctx context.Context, w http.ResponseWriter, r *http.Request,

 	return nil

 }

-func pingHandler(ctx context.Context, w http.ResponseWriter, r *http.Request, vars map[string]string) error {

+func (s *systemRouter) pingHandler(ctx context.Context, w http.ResponseWriter, r *http.Request, vars map[string]string) error {

+	if bv := s.builderVersion; bv != "" {

+		w.Header().Set("Builder-Version", string(bv))

+	}

 	_, err := w.Write([]byte{'O', 'K'})

 	return err

 }

@@ -6972,6 +6972,9 @@ paths:

             API-Version:

               type: "string"

               description: "Max API Version the server supports"

+            BuildKit-Version:

+              type: "string"

+              description: "Default version of docker image builder"

             Docker-Experimental:

               type: "boolean"

               description: "If the server is running with experimental mode enabled"

@@ -102,9 +102,10 @@ type ContainerStats struct {

 // Ping contains response of Engine API:

 // GET "/_ping"

 type Ping struct {

-	APIVersion   string

-	OSType       string

-	Experimental bool

+	APIVersion     string

+	OSType         string

+	Experimental   bool

+	BuilderVersion BuilderVersion

 }

 // ComponentVersion describes the version information for a specific component.

@@ -7,7 +7,7 @@ import (

 	"github.com/docker/docker/api/types"

 )

-// Ping pings the server and returns the value of the "Docker-Experimental", "OS-Type" & "API-Version" headers

+// Ping pings the server and returns the value of the "Docker-Experimental", "Builder-Version", "OS-Type" & "API-Version" headers

 func (cli *Client) Ping(ctx context.Context) (types.Ping, error) {

 	var ping types.Ping

 	req, err := cli.buildRequest("GET", path.Join(cli.basePath, "/_ping"), nil, nil)

@@ -27,6 +27,9 @@ func (cli *Client) Ping(ctx context.Context) (types.Ping, error) {

 			ping.Experimental = true

 		}

 		ping.OSType = serverResp.header.Get("OSType")

+		if bv := serverResp.header.Get("Builder-Version"); bv != "" {

+			ping.BuilderVersion = types.BuilderVersion(bv)

+		}

 	}

 	return ping, cli.checkResponseErr(serverResp)

 }

@@ -65,7 +65,6 @@ func installCommonConfigFlags(conf *config.Config, flags *pflag.FlagSet) {

 	flags.StringVar(&conf.SwarmDefaultAdvertiseAddr, "swarm-default-advertise-addr", "", "Set default address or interface for swarm advertised address")

 	flags.BoolVar(&conf.Experimental, "experimental", false, "Enable experimental features")

-

 	flags.StringVar(&conf.MetricsAddress, "metrics-addr", "", "Set default address and port to serve the metrics api on")

 	flags.Var(opts.NewNamedListOptsRef("node-generic-resources", &conf.NodeGenericResources, opts.ValidateSingleGenericResource), "node-generic-resource", "Advertise user-defined resource")

@@ -27,6 +27,7 @@ import (

 	swarmrouter "github.com/docker/docker/api/server/router/swarm"

 	systemrouter "github.com/docker/docker/api/server/router/system"

 	"github.com/docker/docker/api/server/router/volume"

+	"github.com/docker/docker/api/types"

 	buildkit "github.com/docker/docker/builder/builder-next"

 	"github.com/docker/docker/builder/dockerfile"

 	"github.com/docker/docker/builder/fscache"

@@ -253,6 +254,7 @@ type routerOptions struct {

 	buildBackend   *buildbackend.Backend

 	buildCache     *fscache.FSCache // legacy

 	buildkit       *buildkit.Builder

+	builderVersion types.BuilderVersion

 	daemon         *daemon.Daemon

 	api            *apiserver.Server

 	cluster        *cluster.Cluster

@@ -283,8 +285,7 @@ func newRouterOptions(config *config.Config, daemon *daemon.Daemon) (routerOptio

 	if err != nil {

 		return opts, err

 	}

-

-	buildkit, err := buildkit.New(buildkit.Opt{

+	bk, err := buildkit.New(buildkit.Opt{

 		SessionManager: sm,

 		Root:           filepath.Join(config.Root, "buildkit"),

 		Dist:           daemon.DistributionServices(),

@@ -293,16 +294,24 @@ func newRouterOptions(config *config.Config, daemon *daemon.Daemon) (routerOptio

 		return opts, err

 	}

-	bb, err := buildbackend.NewBackend(daemon.ImageService(), manager, buildCache, buildkit)

+	bb, err := buildbackend.NewBackend(daemon.ImageService(), manager, buildCache, bk)

 	if err != nil {

 		return opts, errors.Wrap(err, "failed to create buildmanager")

 	}

-

+	var bv types.BuilderVersion

+	if v, ok := config.Features["buildkit"]; ok {

+		if v {

+			bv = types.BuilderBuildKit

+		} else {

+			bv = types.BuilderV1

+		}

+	}

 	return routerOptions{

 		sessionManager: sm,

 		buildBackend:   bb,

 		buildCache:     buildCache,

-		buildkit:       buildkit,

+		buildkit:       bk,

+		builderVersion: bv,

 		daemon:         daemon,

 	}, nil

 }

@@ -476,9 +485,9 @@ func initRouter(opts routerOptions) {

 		checkpointrouter.NewRouter(opts.daemon, decoder),

 		container.NewRouter(opts.daemon, decoder),

 		image.NewRouter(opts.daemon.ImageService()),

-		systemrouter.NewRouter(opts.daemon, opts.cluster, opts.buildCache, opts.buildkit),

+		systemrouter.NewRouter(opts.daemon, opts.cluster, opts.buildCache, opts.buildkit, opts.builderVersion),

 		volume.NewRouter(opts.daemon.VolumesService()),

-		build.NewRouter(opts.buildBackend, opts.daemon),

+		build.NewRouter(opts.buildBackend, opts.daemon, opts.builderVersion),

 		sessionrouter.NewRouter(opts.sessionManager),

 		swarmrouter.NewRouter(opts.cluster),

 		pluginrouter.NewRouter(opts.daemon.PluginManager()),

@@ -56,6 +56,13 @@ var flatOptions = map[string]bool{

 	"default-ulimits":    true,

 }

+// skipValidateOptions contains configuration keys

+// that will be skipped from findConfigurationConflicts

+// for unknown flag validation.

+var skipValidateOptions = map[string]bool{

+	"features": true,

+}

+

 // LogConfig represents the default log configuration.

 // It includes json tags to deserialize configuration from a file

 // using the same names that the flags in the command line use.

@@ -203,6 +210,10 @@ type CommonConfig struct {

 	// should be configured with the CRI plugin enabled. This allows using

 	// Docker's containerd instance directly with a Kubernetes kubelet.

 	CriContainerd bool `json:"cri-containerd,omitempty"`

+

+	// Features contains a list of feature key value pairs indicating what features are enabled or disabled.

+	// If a certain feature doesn't appear in this list then it's unset (i.e. neither true nor false).

+	Features map[string]bool `json:"features,omitempty"`

 }

 // IsValueSet returns true if a configuration value

@@ -444,7 +455,7 @@ func findConfigurationConflicts(config map[string]interface{}, flags *pflag.Flag

 	// 1. Search keys from the file that we don't recognize as flags.

 	unknownKeys := make(map[string]interface{})

 	for key, value := range config {

-		if flag := flags.Lookup(key); flag == nil {

+		if flag := flags.Lookup(key); flag == nil && !skipValidateOptions[key] {

 			unknownKeys[key] = value

 		}

 	}