Use sysfs to set hairpin mode

Set the hairpin mode using the sysfs interface which
looks like it is working all the way to the oldest
of RHEL6.6 kernels.

Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>

libnetwork/drivers/bridge/bridge.go

@@ -3,10 +3,13 @@ package bridge
 import (
 	"errors"
 	"fmt"
+	"io/ioutil"
 	"net"
 	"os/exec"
+	"path/filepath"
 	"strconv"
 	"sync"
+	"syscall"
 
 	"github.com/Sirupsen/logrus"
 	"github.com/docker/libnetwork/driverapi"
@@ -772,6 +775,37 @@ func addToBridge(ifaceName, bridgeName string) error {
 	return ioctlAddToBridge(iface, master)
 }
 
+func setHairpinMode(link netlink.Link, enable bool) error {
+	err := netlink.LinkSetHairpin(link, enable)
+	if err != nil && err != syscall.EINVAL {
+		// If error is not EINVAL something else went wrong, bail out right away
+		return fmt.Errorf("unable to set hairpin mode on %s via netlink: %v",
+			link.Attrs().Name, err)
+	}
+
+	// Hairpin mode successfully set up
+	if err == nil {
+		return nil
+	}
+
+	// The netlink method failed with EINVAL which is probably because of an older
+	// kernel. Try one more time via the sysfs method.
+	path := filepath.Join("/sys/class/net", link.Attrs().Name, "brport/hairpin_mode")
+
+	var val []byte
+	if enable {
+		val = []byte{'1', '\n'}
+	} else {
+		val = []byte{'0', '\n'}
+	}
+
+	if err := ioutil.WriteFile(path, val, 0644); err != nil {
+		return fmt.Errorf("unable to set hairpin mode on %s via sysfs: %v", link.Attrs().Name, err)
+	}
+
+	return nil
+}
+
 func (d *driver) CreateEndpoint(nid, eid types.UUID, epInfo driverapi.EndpointInfo, epOptions map[string]interface{}) error {
 	var (
 		ipv6Addr *net.IPNet
@@ -902,7 +936,7 @@ func (d *driver) CreateEndpoint(nid, eid types.UUID, epInfo driverapi.EndpointIn
 	}
 
 	if !config.EnableUserlandProxy {
-		err = netlink.LinkSetHairpin(host, true)
+		err = setHairpinMode(host, true)
 		if err != nil {
 			return err
 		}