/*

 * Copyright (c) 2011, 2013, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package java.net;

/**

 * Choose a network interface to be the default for

 * outgoing IPv6 traffic that does not specify a scope_id (and which needs one).

 * We choose the first interface that is up and is (in order of preference):

 * 1. neither loopback nor point to point

 * 2. point to point

 * 3. loopback

 * 4. none.

 * Platforms that do not require a default interface implement a dummy

 * that returns null.

 */

import java.util.Enumeration;

import java.io.IOException;

class DefaultInterface {

        chooseDefaultInterface();

    static NetworkInterface getDefault() {

        return defaultInterface;

    }

    /**

     * Choose a default interface. This method returns an interface that is

     * both "up" and supports multicast. This method choses an interface in

     * order of preference:

     * 1. neither loopback nor point to point

     * 2. point to point

     * 3. loopback

     *

     * @return  the chosen interface or {@code null} if there isn't a suitable

     *          default

     */

    private static NetworkInterface chooseDefaultInterface() {

        Enumeration<NetworkInterface> nifs;

        try {

           nifs = NetworkInterface.getNetworkInterfaces();

        } catch (IOException ignore) {

            // unable to enumate network interfaces

            return null;

        }

        NetworkInterface ppp = null;

        NetworkInterface loopback = null;

        while (nifs.hasMoreElements()) {

            NetworkInterface ni = nifs.nextElement();

            try {

                if (ni.isUp() && ni.supportsMulticast()) {

                    boolean isLoopback = ni.isLoopback();

                    boolean isPPP = ni.isPointToPoint();

                    if (!isLoopback && !isPPP) {

                        // found an interface that is not the loopback or a

                        // point-to-point interface

                        return ni;

                    }

                    if (ppp == null && isPPP)

                        ppp = ni;

                    if (loopback == null && isLoopback)

                        loopback = ni;

                }

            } catch (IOException skip) { }

        }

        return (ppp != null) ? ppp : loopback;

    }

}