pkg/netutil: get default interface for tc commands

Fix https://github.com/coreos/etcd/issues/6841.

pkg/netutil/routes_linux_test.go

@@ -0,0 +1,27 @@
+// Copyright 2016 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build linux
+
+package netutil
+
+import "testing"
+
+func TestGetDefaultInterface(t *testing.T) {
+	ifc, err := GetDefaultInterface()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("default network interface: %q\n", ifc)
+}