Discover the IP address to based on default gateway

Without this check, Linux bridge docker0 or Winwdows DockerNAT could be
selected as "host IP" but won't allow to route to container
Same issue probalby exists on Linux with docker0 bridge
Here we discover the default gateway, and search for the interface
configured on the same network.

Signed-off-by: Nicolas De Loof <nicolas.deloof@gmail.com>

Author: ndeloof

Gopkg.lock

@@ -121,6 +121,12 @@ required = ["github.com/wadey/gocovmerge"]
   name = "github.com/wadey/gocovmerge"
   branch = "master"
 
+[[constraint]]
+  # see https://github.com/jackpal/gateway/pull/21
+  source = "github.com/ndeloof/gateway"
+  name = "github.com/jackpal/gateway"
+  revision = "10816db7ab2f4b71e71dba9420bfa67d5be27b33"
+
 [prune]
   non-go = true
   unused-packages = true

Gopkg.toml

@@ -3,16 +3,17 @@ package e2e
 import (
 	"fmt"
 	"io/ioutil"
+	"net"
 	"strconv"
 	"strings"
 	"testing"
 	"time"
 
 	"github.com/docker/app/internal"
+	"github.com/jackpal/gateway"
 	"gotest.tools/assert"
 	"gotest.tools/fs"
 	"gotest.tools/icmd"
-	net2 "k8s.io/apimachinery/pkg/util/net"
 )
 
 // readFile returns the content of the file at the designated path normalizing
@@ -157,9 +158,23 @@ func (c *Container) getIP(t *testing.T) string {
 	if host != "" {
 		return host
 	}
-	ip, err := net2.ChooseHostInterface()
+	// Discover default gateway
+	gw, err := gateway.DiscoverGateway()
 	assert.NilError(t, err)
-	host = ip.String()
+
+	// Search for the interface configured on the same network as the gateway
+	addrs, err := net.InterfaceAddrs()
+	assert.NilError(t, err)
+	for _, a := range addrs {
+		if ipnet, ok := a.(*net.IPNet); ok && !ipnet.IP.IsLoopback() {
+			net1 := ipnet.IP.Mask(ipnet.Mask).String()
+			net2 := gw.Mask(ipnet.Mask).String()
+			if net1 == net2 {
+				host = ipnet.IP.String()
+				break
+			}
+		}
+	}
 	return host
 }