Stefano

When a post is saved, the plugin adds the rel=”nofollow” attributes to post external links. The plugin also provides an apt checkbox to exclude a post from plugin’s action.
Links which already have a rel attribute are just ignored, so you can set some dofollows are well.
By default, the plugin will only act on posts, no pages or other custom post types. To include (some of) them, use a code like the following, for ex. by putting it in your theme’s functions.php:
<pre><code>add_filter( 'rnf_post_types', function( $post_types ) {
 return array( 'post', 'page' ); #specify all desired CPTs, comma-separated
});
</code></pre>

Rel Nofollow

Adds rel="nofollow" to posts external links unless specified otherwise.

Viktor Androshuck

This lightweight plugin automatically scans the content of your posts and pages and adds <code>rel="nofollow noopener noreferrer"</code> to all external links. 
Internal links are untouched. Improves SEO and security with zero configuration.
<ul>
<li>No admin panel needed</li>
<li>Safe and fast</li>
<li>Does not affect internal links</li>
<li>Helps comply with SEO best practices</li>
</ul>

Auto External Link Nofollow


Does exactly what it says on the tin! No fluff, just "no follow" external links across your site 👍


Great Plugin

Automatically adds rel="nofollow noopener noreferrer" to all external links in post and page content.

Rel Nofollow vs Auto External Link Nofollow