@@ -166,22 +166,7 @@ func (container *Container) readHostConfig() error {

 		return err

 	}

-	// Make sure the dns fields are never nil.

-	// New containers don't ever have those fields nil,

-	// but pre created containers can still have those nil values.

-	// See https://github.com/docker/docker/pull/17779

-	// for a more detailed explanation on why we don't want that.

-	if container.hostConfig.DNS == nil {

-		container.hostConfig.DNS = make([]string, 0)

-	}

-

-	if container.hostConfig.DNSSearch == nil {

-		container.hostConfig.DNSSearch = make([]string, 0)

-	}

-

-	if container.hostConfig.DNSOptions == nil {

-		container.hostConfig.DNSOptions = make([]string, 0)

-	}

+	initDNSHostConfig(container)

 	return nil

 }

@@ -543,3 +528,25 @@ func (container *Container) stopSignal() int {

 	}

 	return int(stopSignal)

 }

+

+// initDNSHostConfig ensures that the dns fields are never nil.

+// New containers don't ever have those fields nil,

+// but pre created containers can still have those nil values.

+// The non-recommended host configuration in the start api can

+// make these fields nil again, this corrects that issue until

+// we remove that behavior for good.

+// See https://github.com/docker/docker/pull/17779

+// for a more detailed explanation on why we don't want that.

+func initDNSHostConfig(container *Container) {

+	if container.hostConfig.DNS == nil {

+		container.hostConfig.DNS = make([]string, 0)

+	}

+

+	if container.hostConfig.DNSSearch == nil {

+		container.hostConfig.DNSSearch = make([]string, 0)

+	}

+

+	if container.hostConfig.DNSOptions == nil {

+		container.hostConfig.DNSOptions = make([]string, 0)

+	}

+}

@@ -29,6 +29,7 @@ func (daemon *Daemon) ContainerStart(name string, hostConfig *runconfig.HostConf

 		// This is kept for backward compatibility - hostconfig should be passed when

 		// creating a container, not during start.

 		if hostConfig != nil {

+			logrus.Warn("DEPRECATED: Setting host configuration options when the container starts is deprecated and will be removed in Docker 1.12")

 			container.Lock()

 			if err := parseSecurityOpt(container, hostConfig); err != nil {

 				container.Unlock()

@@ -38,6 +39,7 @@ func (daemon *Daemon) ContainerStart(name string, hostConfig *runconfig.HostConf

 			if err := daemon.setHostConfig(container, hostConfig); err != nil {

 				return err

 			}

+			initDNSHostConfig(container)

 		}

 	} else {

 		if hostConfig != nil {

@@ -1371,3 +1371,20 @@ func (s *DockerSuite) TestPostContainersCreateWithWrongCpusetValues(c *check.C)

 	expected = "Invalid value 42-3,1-- for cpuset mems.\n"

 	c.Assert(string(body), checker.Equals, expected)

 }

+

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

+	testRequires(c, DaemonIsLinux)

+	out, _ := dockerCmd(c, "create", "busybox")

+	containerID := strings.TrimSpace(out)

+

+	config := `{"HostConfig": {"Dns": null}}`

+

+	res, b, err := sockRequestRaw("POST", "/containers/"+containerID+"/start", strings.NewReader(config), "application/json")

+	c.Assert(err, checker.IsNil)

+	c.Assert(res.StatusCode, checker.Equals, http.StatusNoContent)

+	b.Close()

+

+	dns, err := inspectFieldJSON(containerID, "HostConfig.Dns")

+	c.Assert(err, checker.IsNil)

+	c.Assert(dns, checker.Equals, "[]")

+}