Get Mtu from default route

If no Mtu value is provided to the docker daemon, get the mtu from the
default route's interface.  If there is no default route, default to a
mtu of 1500.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
(cherry picked from commit ff4e58ff56)

daemon/daemon.go

@@ -1,6 +1,7 @@
 package daemon
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -48,6 +49,7 @@ import (
 	"github.com/docker/docker/utils"
 	volumedrivers "github.com/docker/docker/volume/drivers"
 	"github.com/docker/docker/volume/local"
+	"github.com/docker/libcontainer/netlink"
 )
 
 var (
@@ -672,6 +674,8 @@ func NewDaemon(config *Config, registryService *registry.Service) (daemon *Daemo
 	if config.Bridge.Iface != "" && config.Bridge.IP != "" {
 		return nil, fmt.Errorf("You specified -b & --bip, mutually exclusive options. Please specify only one.")
 	}
+	setDefaultMtu(config)
+
 	if !config.Bridge.EnableIPTables && !config.Bridge.InterContainerCommunication {
 		return nil, fmt.Errorf("You specified --iptables=false with --icc=false. ICC uses iptables to function. Please set --icc or --iptables to true.")
 	}
@@ -1262,3 +1266,30 @@ func (daemon *Daemon) newBaseContainer(id string) CommonContainer {
 		root:         daemon.containerRoot(id),
 	}
 }
+
+func setDefaultMtu(config *Config) {
+	// do nothing if the config does not have the default 0 value.
+	if config.Mtu != 0 {
+		return
+	}
+	config.Mtu = defaultNetworkMtu
+	if routeMtu, err := getDefaultRouteMtu(); err == nil {
+		config.Mtu = routeMtu
+	}
+}
+
+var errNoDefaultRoute = errors.New("no default route was found")
+
+// getDefaultRouteMtu returns the MTU for the default route's interface.
+func getDefaultRouteMtu() (int, error) {
+	routes, err := netlink.NetworkGetRoutes()
+	if err != nil {
+		return 0, err
+	}
+	for _, r := range routes {
+		if r.Default {
+			return r.Iface.MTU, nil
+		}
+	}
+	return 0, errNoDefaultRoute
+}