版本信息：
android5.1.1
Linux version 3.10.49

需求：通过4g访问外网，同时通过ｗifi访问内网。
　android系统默认情况下，ｗifi连网时，４g数据链接会被强制断开，但实际有需要wifi和4g同时上网，为了满足这个要求，需要修改android的网络管理机制。
　首先实现ｗifi连网时，不让４g数据链接被强制断开：
　修改　　frameworks/base/services/core/java/com/android/server/ConnectivityService.java

private void teardownUnneededNetwork(NetworkAgentInfo nai) {
        for (int i = 0; i < nai.networkRequests.size(); i++) {
            NetworkRequest nr = nai.networkRequests.valueAt(i);
            // Ignore listening requests.
            if (!isRequest(nr)) continue;
            loge("Dead network still had at least " + nr);
            break;
        }
        //nai.asyncChannel.disconnect();
    }

把nai.asyncChannel.disconnect();　行mark掉。

　　这样ｗifi连网，４g数据链接不会被强制断开了。
　 $ netcfg
rmnet0 UP 0.0.0.0/0 0x00000041 00:00:00:00:00:00
r_rmnet_data8 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
r_rmnet_data7 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
r_rmnet_data5 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
r_rmnet_data6 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
r_rmnet_data4 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
r_rmnet_data3 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
r_rmnet_data1 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
r_rmnet_data2 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
r_rmnet_data0 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
rmnet_data4 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
rmnet_data2 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
rmnet_data3 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
rmnet_data1 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
rmnet_data0 UP 10.66.94.93/30 0x00000041 00:00:00:00:00:00
rmnet_data7 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
rmnet_data5 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
rmnet_data6 DOWN 0.0.0.0/0 0x00000000 00:00:00:00:00:00
wlan0 UP 192.168.10.10/24 0x00001043 34:87:3d:31:e4:77
sit0 DOWN 0.0.0.0/0 0x00000080 00:00:00:00:00:00
p2p0 UP 0.0.0.0/0 0x00001003 36:87:3d:31:e4:77
lo UP 127.0.0.1/8 0x00000049 00:00:00:00:00:00
dummy0 DOWN 0.0.0.0/0 0x00000082 d6:d1:38:38:ed:c8

　　但此时，网络访问默认还是走ｗifi通道，把ｗifi AP与外网断开，ping下192.168.10.1可以ping通，但www.baidu.com会发现ping不通，
　　指定network,再ping
　　ping -I rmnet_data0 www.baidu.com 　
　　会发现可以ping通，说明此时ｗifi和４g都已经连上网了。
　　ＡPP程序可以通过setProcessDefaultNetwork把进程和network绑定来访问特定网络。但我们的要求是一个ＡPP有的数据走ｗifi有的数据走4g,不希望APP和某个network绑定，这样就是要求默认走4g上网，访问内网ip则走wifi.即修改系统让４g的优先级优于ｗifi,修改route rule，使得访问内网ip时，则走wifi.　
　　
　　修改系统让４g的优先级优于ｗifi:
　　
　android5.1是依据网络评分来评定网络的优先级，默认情况下，ｗifi的评分比４g高。可以修改
　　frameworks/base/services/core/java/com/android/server/connectivity/NetworkAgentInfo.java
“`

private int getCurrentScore(boolean pretendValidated) {
    // TODO: We may want to refactor this into a NetworkScore class that takes a base score from
    // the NetworkAgent and signals from the NetworkAgent and uses those signals to modify the
    // score.  The NetworkScore class would provide a nice place to centralize score constants
    // so they are not scattered about the transports.

　　　　　 if(networkCapabilities.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR)) {
　　　　　return 150;
}

    int score = currentScore;
    if (isCneWqeEnabled) return score;

    if (!everValidated && !pretendValidated) score -= UNVALIDATED_SCORE_PENALTY;
    if (score < 0) score = 0;

    if (networkMisc.explicitlySelected) score = EXPLICITLY_SELECTED_NETWORK_SCORE;

    return score;
}

把4g网络评分强制改为150,这样可以确保总是大于wifi网络的评分。
更新系统,ｐing www.baidu.com可以ping通，说明此时系统优先走4g网络(wifi AP与外网断开)。 ping 192.168.10.1 不通。

修改route rule:

查看默认route rule:
ip rule
0: from all lookup local
10000: from all fwmark 0xc0000/0xd0000 lookup 99
13000: from all fwmark 0x10063/0x1ffff lookup 97
13000: from all fwmark 0x10064/0x1ffff lookup 1022
13000: from all fwmark 0x10065/0x1ffff lookup 1005
14000: from all oif wlan0 lookup 1022
14000: from all oif rmnet_data0 lookup 1005
15000: from all fwmark 0x0/0x10000 lookup 99
16000: from all fwmark 0x0/0x10000 lookup 98
17000: from all fwmark 0x0/0x10000 lookup 97
19000: from all fwmark 0x64/0x1ffff lookup 1022
19000: from all fwmark 0x65/0x1ffff lookup 1005
22000: from all fwmark 0x0/0xffff lookup 1005
23000: from all fwmark 0x0/0xffff uidrange 0-0 lookup main
32000: from all unreachable

尝试添加一条route rule:
ip rule add to 192.168.10.0/24 table wlan0 pref 100
修改后route rule:
0: from all lookup local
100: from all to 192.168.10.0/24 lookup 1022
10000: from all fwmark 0xc0000/0xd0000 lookup 99
13000: from all fwmark 0x10063/0x1ffff lookup 97
13000: from all fwmark 0x10064/0x1ffff lookup 1022
13000: from all fwmark 0x10065/0x1ffff lookup 1005
14000: from all oif wlan0 lookup 1022
14000: from all oif rmnet_data0 lookup 1005
15000: from all fwmark 0x0/0x10000 lookup 99
16000: from all fwmark 0x0/0x10000 lookup 98
17000: from all fwmark 0x0/0x10000 lookup 97
19000: from all fwmark 0x64/0x1ffff lookup 1022
19000: from all fwmark 0x65/0x1ffff lookup 1005
22000: from all fwmark 0x0/0xffff lookup 1005
23000: from all fwmark 0x0/0xffff uidrange 0-0 lookup main
32000: from all unreachable

这样就可以直接ping 通192.168.10.1了。
　
APP程序没有修改route rule的权限，我们想让系统在wifi连接ＡP后自动添加需要的一条route rule.
可以修改netd (system/netd/server/)，netd有权限修改route rule.