@@ -103,7 +103,12 @@ func NewDockerCli(in io.ReadCloser, out, err io.Writer, clientFlags *cli.ClientF

 		}

 		customHeaders["User-Agent"] = "Docker-Client/" + dockerversion.Version + " (" + runtime.GOOS + ")"

-		client, err := lib.NewClient(host, string(api.Version), clientFlags.Common.TLSOptions, customHeaders)

+		verStr := string(api.DefaultVersion)

+		if tmpStr := os.Getenv("DOCKER_API_VERSION"); tmpStr != "" {

+			verStr = tmpStr

+		}

+

+		client, err := lib.NewClient(host, verStr, clientFlags.Common.TLSOptions, customHeaders)

 		if err != nil {

 			return err

 		}

@@ -17,6 +17,7 @@ import (

 // apiClient is an interface that clients that talk with a docker server must implement.

 type apiClient interface {

+	ClientVersion() string

 	ContainerAttach(options types.ContainerAttachOptions) (types.HijackedResponse, error)

 	ContainerCommit(options types.ContainerCommitOptions) (types.ContainerCommitResponse, error)

 	ContainerCreate(config *runconfig.ContainerConfigWrapper, containerName string) (types.ContainerCreateResponse, error)

@@ -96,3 +96,10 @@ func (cli *Client) getAPIPath(p string, query url.Values) string {

 	}

 	return apiPath

 }

+

+// ClientVersion returns the version string associated with this

+// instance of the Client. Note that this value can be changed

+// via the DOCKER_API_VERSION env var.

+func (cli *Client) ClientVersion() string {

+	return cli.version

+}

@@ -5,11 +5,11 @@ import (

 	"text/template"

 	"time"

-	"github.com/docker/docker/api"

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

 	Cli "github.com/docker/docker/cli"

 	"github.com/docker/docker/dockerversion"

 	flag "github.com/docker/docker/pkg/mflag"

+	"github.com/docker/docker/pkg/version"

 	"github.com/docker/docker/utils"

 )

@@ -57,7 +57,7 @@ func (cli *DockerCli) CmdVersion(args ...string) (err error) {

 	vd := types.VersionResponse{

 		Client: &types.Version{

 			Version:      dockerversion.Version,

-			APIVersion:   api.Version,

+			APIVersion:   version.Version(cli.client.ClientVersion()),

 			GoVersion:    runtime.Version(),

 			GitCommit:    dockerversion.GitCommit,

 			BuildTime:    dockerversion.BuildTime,

@@ -18,7 +18,7 @@ import (

 // Common constants for daemon and client.

 const (

 	// Version of Current REST API

-	Version version.Version = "1.22"

+	DefaultVersion version.Version = "1.22"

 	// MinVersion represents Minimum REST API version supported

 	MinVersion version.Version = "1.12"

@@ -121,14 +121,14 @@ func versionMiddleware(handler httputils.APIFunc) httputils.APIFunc {

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

 		apiVersion := version.Version(vars["version"])

 		if apiVersion == "" {

-			apiVersion = api.Version

+			apiVersion = api.DefaultVersion

 		}

-		if apiVersion.GreaterThan(api.Version) {

-			return errors.ErrorCodeNewerClientVersion.WithArgs(apiVersion, api.Version)

+		if apiVersion.GreaterThan(api.DefaultVersion) {

+			return errors.ErrorCodeNewerClientVersion.WithArgs(apiVersion, api.DefaultVersion)

 		}

 		if apiVersion.LessThan(api.MinVersion) {

-			return errors.ErrorCodeOldClientVersion.WithArgs(apiVersion, api.Version)

+			return errors.ErrorCodeOldClientVersion.WithArgs(apiVersion, api.DefaultVersion)

 		}

 		w.Header().Set("Server", "Docker/"+dockerversion.Version+" ("+runtime.GOOS+")")

@@ -38,7 +38,7 @@ func (s *systemRouter) getInfo(ctx context.Context, w http.ResponseWriter, r *ht

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

 	info := s.backend.SystemVersion()

-	info.APIVersion = api.Version

+	info.APIVersion = api.DefaultVersion

 	return httputils.WriteJSON(w, http.StatusOK, info)

 }

@@ -38,6 +38,7 @@ the [installation](../../installation/index.md) instructions for your operating

 For easy reference, the following list of environment variables are supported

 by the `docker` command line:

+* `DOCKER_API_VERSION` The API version to use (e.g. `1.19`)

 * `DOCKER_CONFIG` The location of your client configuration files.

 * `DOCKER_CERT_PATH` The location of your authentication keys.

 * `DOCKER_DRIVER` The graph driver to use.

@@ -2,7 +2,10 @@ package main

 import (

 	"net/http"

+	"net/http/httptest"

 	"net/http/httputil"

+	"os"

+	"os/exec"

 	"strconv"

 	"strings"

 	"time"

@@ -46,7 +49,7 @@ func (s *DockerSuite) TestApiVersionStatusCode(c *check.C) {

 }

 func (s *DockerSuite) TestApiClientVersionNewerThanServer(c *check.C) {

-	v := strings.Split(string(api.Version), ".")

+	v := strings.Split(string(api.DefaultVersion), ".")

 	vMinInt, err := strconv.Atoi(v[1])

 	c.Assert(err, checker.IsNil)

 	vMinInt++

@@ -72,3 +75,25 @@ func (s *DockerSuite) TestApiClientVersionOldNotSupported(c *check.C) {

 	c.Assert(status, checker.Equals, http.StatusBadRequest)

 	c.Assert(len(string(body)), checker.Not(check.Equals), 0) // Expected not empty body

 }

+

+func (s *DockerSuite) TestApiDockerApiVersion(c *check.C) {

+	var svrVersion string

+

+	server := httptest.NewServer(http.HandlerFunc(

+		func(w http.ResponseWriter, r *http.Request) {

+			url := r.URL.Path

+			svrVersion = url

+		}))

+	defer server.Close()

+

+	// Test using the env var first

+	cmd := exec.Command(dockerBinary, "-H="+server.URL[7:], "version")

+	cmd.Env = append([]string{"DOCKER_API_VERSION=xxx"}, os.Environ()...)

+	out, _, _ := runCommandWithOutput(cmd)

+

+	c.Assert(svrVersion, check.Equals, "/vxxx/version")

+

+	if !strings.Contains(out, "API version:  xxx") {

+		c.Fatalf("Out didn't have 'xxx' for the API version, had:\n%s", out)

+	}

+}