docker: detect defaultNetworkMtu from default route

Docker-DCO-1.1-Signed-off-by: Johan Euphrosine <proppy@google.com> (github: google)

config.go

@@ -1,12 +1,14 @@
 package docker
 
 import (
-	"github.com/dotcloud/docker/engine"
 	"net"
+
+	"github.com/dotcloud/docker/engine"
+	"github.com/dotcloud/docker/networkdriver"
 )
 
 const (
-	DefaultNetworkMtu    = 1500
+	defaultNetworkMtu    = 1500
 	DisableNetworkBridge = "none"
 )
 
@@ -53,3 +55,10 @@ func DaemonConfigFromJob(job *engine.Job) *DaemonConfig {
 
 	return config
 }
+
+func GetDefaultNetworkMtu() int {
+	if iface, err := networkdriver.GetDefaultRouteIface(); err == nil {
+		return iface.MTU
+	}
+	return defaultNetworkMtu
+}

docker/docker.go

@@ -2,15 +2,16 @@ package main
 
 import (
 	"fmt"
+	"log"
+	"os"
+	"strings"
+
 	"github.com/dotcloud/docker"
 	"github.com/dotcloud/docker/api"
 	"github.com/dotcloud/docker/engine"
 	flag "github.com/dotcloud/docker/pkg/mflag"
 	"github.com/dotcloud/docker/sysinit"
 	"github.com/dotcloud/docker/utils"
-	"log"
-	"os"
-	"strings"
 )
 
 var (
@@ -42,7 +43,7 @@ func main() {
 		flInterContainerComm = flag.Bool([]string{"#icc", "-icc"}, true, "Enable inter-container communication")
 		flGraphDriver        = flag.String([]string{"s", "-storage-driver"}, "", "Force the docker runtime to use a specific storage driver")
 		flHosts              = docker.NewListOpts(docker.ValidateHost)
-		flMtu                = flag.Int([]string{"#mtu", "-mtu"}, docker.DefaultNetworkMtu, "Set the containers network mtu")
+		flMtu                = flag.Int([]string{"#mtu", "-mtu"}, docker.GetDefaultNetworkMtu(), "Set the containers network mtu")
 	)
 	flag.Var(&flDns, []string{"#dns", "-dns"}, "Force docker to use specific DNS servers")
 	flag.Var(&flHosts, []string{"H", "-host"}, "tcp://host:port, unix://path/to/socket, fd://* or fd://socketfd to use in daemon mode. Multiple sockets can be specified")

networkdriver/utils.go

@@ -3,8 +3,9 @@ package networkdriver
 import (
 	"encoding/binary"
 	"fmt"
-	"github.com/dotcloud/docker/pkg/netlink"
 	"net"
+
+	"github.com/dotcloud/docker/pkg/netlink"
 )
 
 var (
@@ -100,3 +101,16 @@ func GetIfaceAddr(name string) (net.Addr, error) {
 	}
 	return addrs4[0], nil
 }
+
+func GetDefaultRouteIface() (*net.Interface, error) {
+	rs, err := netlink.NetworkGetRoutes()
+	if err != nil {
+		return nil, fmt.Errorf("unable to get routes: %v", err)
+	}
+	for _, r := range rs {
+		if r.Default {
+			return r.Iface, nil
+		}
+	}
+	return nil, fmt.Errorf("no default route")
+}