How do I set a return URL during login on Codeigniter / Tank_Auth?

Question

The issue here is that when my user logs into my app, they always are redirected to the default controller.

I would like the user to be redirected to the page they were on before logging in.

So for example, if the user is reading forum post #12 (reading does not require login) and then decides to post an answer (answering requires login), once they login they should go back to post #12.

I am using PHP/Codeigniter 2.0.2 and the Tank_Auth library, and have in several of my controllers

function __construct()
{
    parent::__construct();

    if (!$this->tank_auth->is_logged_in()) {
        redirect('/auth/login/');
    } else {

        //load stuff 
     }

My question is

What is the best way to set a return URL (Cookie? GET?), and how would that be implemented?

If you're familiar with Tank_Auth, in which files should I make these changes?

Any roadmaps are welcome, even if you don't use Tank_Auth.