Adding a cookie value on Socket.IO

Take a look at Authorization with Socket.io. This is where it handles the initial connection request, and where you can set a response header.

I'm not sure if the cookie(name,value) shorthand works, though you can try setting it manually:

    var date = new Date();
    date.setTime(date.getTime()+(days*24*60*60*1000)); // set day value to expiry
    var expires = "; expires="+date.toGMTString();

    handshake.headers.cookie = name+"="+value+expires+"; path=/";