add nofollow inside posts

You’ll have to define what you mean. Do you want all your posts links to be nofollowed? Or only some of them? Or what?

A plugin is still probably the easiest way to do it, but if you just want all the post content links to have it, then this should work:
add_filter('the_content', 'wp_rel_nofollow');

Stick that in a theme’s functions.php file and there you go.

can someone name a plugin where I could select (or manually add) sites to add the nofollow tag?
I tried NoFollow Filter, which should work as I need, but it crashes whenever I try to add a new site to the list.
I use the latest version.

Or a way to add some exception to the code Otto42 suggested.