How to change DNS in android 4.x+ using internal linux commands within app?

Question

I have been working on this small project in college about changing the default DNS of wifi network to a custom DNS like Google, OpenDNS, Metacert, etc.
I know I have to write a shellscript inside the app's code that would edit the hosts file in the filesystem.
The problem is I have no idea where to start from. I have researched on google for some time and I couldn't figure anything.
If anyone knows about it, please guide me. Please tell me the name of the file to be edited, its location, what commands are required and how to run those commands' combination as a shellscript on a click of a button on the UI of app. EDIT : I'm stuck only at this. Any help will be greatly appreciated.

Answer 1

I'm not sure about which files you would have to edit but this should give you the tools you need to do that.

The first thing you need to do is root the phone if you haven't already. If it's not rooted, you'll run into an issue like: Working Directory : null environment when running Process.Builder on android

There are a lot of guides available for that online. Install SuperSU as well. In order to run shell commands or scripts you should look at the ProcessBuilder class in Android: http://developer.android.com/reference/java/lang/ProcessBuilder.html

I've given some sample code below to help you along the way. You could execute this in an OnClick() for a button.

   /**
    * Runs the shell command.
    *
    * @param command an array of Strings. command[0] contains the name of the
    *                shell command. command[1]... contains parameters.
    *
    * @return        the text outputted by the command to stderr or stdout
    */
   String runCmd(String[] command, boolean readOutput,
         boolean waitForExit) {
        ProcessBuilder probuilder = new ProcessBuilder()
           .command(command)
           .redirectErrorStream(true);

        String output = "";
        Process process;

//        Log.d("MyShellCommand", "Executing " + command[0]);
        try {
           process = probuilder.start();
        } catch(IOException e) {
           return e.getMessage();
        }

        if (readOutput) {
           InputStream is = process.getInputStream();
           InputStreamReader isr = new InputStreamReader(is);
           BufferedReader br = new BufferedReader(isr);
           String line;

           try {
              while ((line = br.readLine()) != null) {
   //              Log.d("MyShellCommand", "Read a line: " + line);
                  output += line + "\n";
              }
           } catch(IOException e) {
              output = e.getMessage();
           }
        }