在没有root权限的情况下，Android系统默认不允许我们修改iptables。但是，我们可以通过自定义Android系统来实现iptables的修改。具体步骤如下：

1. 下载Android源码，按照官方教程编译出自己的Android系统镜像。

2. 修改Android系统的源码，将iptables相关的源码编译进我们自己的系统镜像中。具体修改可以参考下面的代码：

在文件frameworks/base/cmds/ip/IpController.java中，将如下代码：

private static final String[] SYSTEM_IPTABLES_HOOKS = { "system/etc/iptables/iptables_hooks", // system first "vendor/etc/iptables/iptables_hooks", // vendor overlay next };

修改为：

private static final String[] SYSTEM_IPTABLES_HOOKS = { "system/etc/iptables/iptables_hooks", // system first "vendor/etc/iptables/iptables_hooks", // vendor overlay next "/data/iptables_rules.sh", // add our own iptables rules };

在同一个文件中，添加以下代码：

private static final String IPTABLES_FLUSH = "iptables -F\n"; private static final String IPTABLES_FORWARD_ALLOW_LOCAL = "iptables -A FORWARD -i wlan0 -o rmnet0 -j ACCEPT\n"; private static final String IPTABLES_FORWARD_ALLOW_WAN = "iptables -A FORWARD -i rmnet0 -o wlan0 -m state --state ESTABLISHED,RELATED -j ACCEPT\n"; private static final String IPTABLES_FORWARD_DROP = "iptables -A FORWARD -j DROP\n";

private void loadIptablesRules() { try { // flush any existing rules Runtime.getRuntime().exec(IPTABLES_FLUSH);

    // allow local traffic to be forwarded to WAN
    Runtime.getRuntime().exec(IPTABLES_FORWARD_ALLOW_LOCAL);

    // allow WAN traffic that is already established or related to an established connection
    Runtime.getRuntime().exec(IPTABLES_FORWARD_ALLOW_WAN);

    // drop all other traffic
    Runtime.getRuntime().exec(IPTABLES_FORWARD_DROP);
} catch (IOException e) {
    Log.e(TAG, "Error executing iptables rules: " + e.getMessage());
}

}

3. 部署我们自己的Android系统镜像。

4. 在应用中调用loadIptablesRules方法即可。

示例代码如下：

public class MainActivity extends Activity { private static final String TAG = "IPTABLES";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // load our custom iptables rules
    IpController ipController = IpController.getInstance();
    ipController.loadIptablesRules();
}

}

这样，我们就可以在没有root的情况下修改Android系统的iptables了。