@@ -97,16 +97,10 @@ func Map(container net.Addr, hostIP net.IP, hostPort int) (host net.Addr, err er

 		return nil, err

 	}

-	m.userlandProxy = proxy

-	currentMappings[key] = m

-

 	cleanup := func() error {

 		// need to undo the iptables rules before we return

-		forward(iptables.Delete, m.proto, hostIP, allocatedHostPort, containerIP.String(), containerPort)

 		proxy.Stop()

 		forward(iptables.Delete, m.proto, hostIP, allocatedHostPort, containerIP.String(), containerPort)

-		m.userlandProxy = nil

-		delete(currentMappings, key)

 		if err := portallocator.ReleasePort(hostIP, m.proto, allocatedHostPort); err != nil {

 			return err

 		}

@@ -118,12 +112,10 @@ func Map(container net.Addr, hostIP net.IP, hostPort int) (host net.Addr, err er

 		if err := cleanup(); err != nil {

 			return nil, fmt.Errorf("Error during port allocation cleanup: %v", err)

 		}

-

-		if err == ErrPortMappingFailure {

-			return nil, portallocator.NewErrPortAlreadyAllocated(hostIP.String(), allocatedHostPort)

-		}

+		return nil, err

 	}

-

+	m.userlandProxy = proxy

+	currentMappings[key] = m

 	return m.host, nil

 }

@@ -1,8 +1,9 @@

 package portmapper

 import (

-	"errors"

 	"flag"

+	"fmt"

+	"io/ioutil"

 	"log"

 	"net"

 	"os"

@@ -16,8 +17,6 @@ import (

 	"github.com/docker/docker/reexec"

 )

-var ErrPortMappingFailure = errors.New("Failure Mapping Port")

-

 const userlandProxyCommandName = "docker-proxy"

 func init() {

@@ -42,12 +41,11 @@ func execProxy() {

 	p, err := proxy.NewProxy(host, container)

 	if err != nil {

 		os.Stdout.WriteString("1\n")

+		fmt.Fprint(os.Stderr, err)

 		os.Exit(1)

 	}

-

-	os.Stdout.WriteString("0\n")

-

 	go handleStopSignals(p)

+	os.Stdout.WriteString("0\n")

 	// Run will block until the proxy stops

 	p.Run()

@@ -117,40 +115,43 @@ func (p *proxyCommand) Start() error {

 	if err != nil {

 		return err

 	}

+	defer stdout.Close()

+	stderr, err := p.cmd.StderrPipe()

+	if err != nil {

+		return err

+	}

+	defer stderr.Close()

 	if err := p.cmd.Start(); err != nil {

 		return err

 	}

-	errchan := make(chan error)

-	after := time.After(1 * time.Second)

+	errchan := make(chan error, 1)

 	go func() {

 		buf := make([]byte, 2)

 		stdout.Read(buf)

 		if string(buf) != "0\n" {

-			errchan <- ErrPortMappingFailure

-		} else {

-			errchan <- nil

+			errStr, _ := ioutil.ReadAll(stderr)

+			errchan <- fmt.Errorf("Error starting userland proxy: %s", errStr)

+			return

 		}

+		errchan <- nil

 	}()

-	var readErr error

-

 	select {

-	case readErr = <-errchan:

-	case <-after:

-		readErr = ErrPortMappingFailure

+	case err := <-errchan:

+		return err

+	case <-time.After(1 * time.Second):

+		return fmt.Errorf("Timed out proxy starting the userland proxy")

 	}

-

-	return readErr

 }

 func (p *proxyCommand) Stop() error {

 	if p.cmd.Process != nil {

-		err := p.cmd.Process.Signal(os.Interrupt)

-		p.cmd.Wait()

-		return err

+		if err := p.cmd.Process.Signal(os.Interrupt); err != nil {

+			return err

+		}

+		return p.cmd.Wait()

 	}

-

 	return nil

 }

@@ -1887,10 +1887,12 @@ func TestRunPortInUse(t *testing.T) {

 	cmd := exec.Command(dockerBinary, "run", "-p", port+":80", "busybox", "true")

 	out, _, err := runCommandWithOutput(cmd)

 	if err == nil {

-		t.Fatalf("Host port %s already in use, has been allocated by docker: %q", port, out)

+		t.Fatalf("Binding on used port must fail")

+	}

+	if !strings.Contains(out, "address already in use") {

+		t.Fatalf("Out must be about \"address already in use\", got %s", out)

 	}

 	deleteAllContainers()

-

-	logDone("run - port in use")

+	logDone("run - fail if port already in use")

 }