Symfony2 Form Login: Rename fields name atributte's

form_login.php

changing your firewall to match this:
    security:
    firewalls:
        my_firewall:
            pattern:    ^/
            form_login:
                username_parameter: "form_name[username_field_name]"
                password_parameter: "form_name[password_field_name]"
                csrf_parameter: "form_name[_token]"
                csrf_provider: form.csrf_provider
                intention: authentication

in your form type:
    $resolver->setDefaults(array(
        'intention' => 'authentication'
    ));
   public function getName()
   {
       return 'form_name';
   }