don't try to use default route MTU as bridge MTU

Signed-off-by: Patrick Hemmer <patrick.hemmer@gmail.com>

daemon/container_operations_windows.go

@@ -162,10 +162,6 @@ func detachMounted(path string) error {
 	return nil
 }
 
-func getDefaultRouteMtu() (int, error) {
-	return -1, errSystemNotSupported
-}
-
 func killProcessDirectly(container *container.Container) error {
 	return nil
 }

daemon/daemon.go

@@ -1382,13 +1382,8 @@ func setDefaultMtu(config *Config) {
 		return
 	}
 	config.Mtu = defaultNetworkMtu
-	if routeMtu, err := getDefaultRouteMtu(); err == nil {
-		config.Mtu = routeMtu
-	}
 }
 
-var errNoDefaultRoute = errors.New("no default route was found")
-
 // verifyContainerSettings performs validation of the hostconfig and config
 // structures.
 func (daemon *Daemon) verifyContainerSettings(hostConfig *runconfig.HostConfig, config *runconfig.Config) ([]string, error) {

daemon/daemon_unix.go

@@ -33,7 +33,6 @@ import (
 	"github.com/docker/libnetwork/types"
 	blkiodev "github.com/opencontainers/runc/libcontainer/configs"
 	"github.com/opencontainers/runc/libcontainer/label"
-	"github.com/vishvananda/netlink"
 )
 
 const (
@@ -670,25 +669,6 @@ func (daemon *Daemon) conditionalUnmountOnCleanup(container *container.Container
 	daemon.Unmount(container)
 }
 
-// getDefaultRouteMtu returns the MTU for the default route's interface.
-func getDefaultRouteMtu() (int, error) {
-	routes, err := netlink.RouteList(nil, 0)
-	if err != nil {
-		return 0, err
-	}
-	for _, r := range routes {
-		// a nil Dst means that this is the default route.
-		if r.Dst == nil {
-			i, err := net.InterfaceByIndex(r.LinkIndex)
-			if err != nil {
-				continue
-			}
-			return i.MTU, nil
-		}
-	}
-	return 0, errNoDefaultRoute
-}
-
 func restoreCustomImage(driver graphdriver.Driver, is image.Store, ls layer.Store, ts tag.Store) error {
 	// Unix has no custom images to register
 	return nil

docs/userguide/networking/default_network/custom-docker0.md

@@ -16,7 +16,7 @@ The information in this section explains how to customize the Docker default bri
 
 By default, the Docker server creates and configures the host system's `docker0` interface as an _Ethernet bridge_ inside the Linux kernel that can pass packets back and forth between other physical or virtual network interfaces so that they behave as a single Ethernet network.
 
-Docker configures `docker0` with an IP address, netmask and IP allocation range. The host machine can both receive and send packets to containers connected to the bridge, and gives it an MTU -- the _maximum transmission unit_ or largest packet length that the interface will allow -- of either 1,500 bytes or else a more specific value copied from the Docker host's interface that supports its default route.  These options are configurable at server startup:
+Docker configures `docker0` with an IP address, netmask and IP allocation range. The host machine can both receive and send packets to containers connected to the bridge, and gives it an MTU -- the _maximum transmission unit_ or largest packet length that the interface will allow -- of 1,500 bytes. These options are configurable at server startup:
 - `--bip=CIDR` -- supply a specific IP address and netmask for the `docker0` bridge, using standard CIDR notation like `192.168.1.5/24`.
 
 - `--fixed-cidr=CIDR` -- restrict the IP range from the `docker0` subnet, using the standard CIDR notation like `172.167.1.0/28`. This range must be an IPv4 range for fixed IPs (ex: 10.20.0.0/16) and must be a subset of the bridge IP range (`docker0` or set using `--bridge`). For example with `--fixed-cidr=192.168.1.0/25`, IPs for your containers will be chosen from the first half of `192.168.1.0/24` subnet.