Adds rel nofollow to external links

Gemfile

@@ -24,3 +24,4 @@ group :jekyll_plugins do
 end
 
 gem 'sinatra', '~> 1.4.2'
+gem 'nokogiri'

Gemfile.lock

@@ -42,6 +42,9 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
     mercenary (0.3.6)
     method_source (0.8.2)
+    mini_portile2 (2.3.0)
+    nokogiri (1.8.2)
+      mini_portile2 (~> 2.3.0)
     octopress (3.0.11)
       jekyll (>= 2.0)
       mercenary (~> 0.3.2)
@@ -100,6 +103,7 @@ DEPENDENCIES
   jekyll-redirect-from
   jekyll-sitemap
   jekyll-time-to-read
+  nokogiri
   octopress (~> 3.0)
   octopress-include-tag
   pry
@@ -113,4 +117,4 @@ RUBY VERSION
    ruby 2.4.1p111
 
 BUNDLED WITH
-   1.15.4
+   1.16.1

plugins/no_follow.rb

@@ -0,0 +1,33 @@
+# Jekyll Auto Nofollow Plugin
+# Automatically adds rel='external nofollow' to outgoing links.
+
+require 'jekyll'
+require 'nokogiri'
+
+module Jekyll
+    module NoFollow
+        def nofollow(content)
+            dom = Nokogiri::HTML.fragment(content)
+
+            # Find all links
+            dom.css('a').each do |link|
+                rel = ['external', 'nofollow']
+
+                # All external links start with 'http', skip when this one does not
+                next unless link.get_attribute('href') =~ /\Ahttp/i
+
+                # Play nice with our own links
+                next if link.get_attribute('href') =~ /\Ahttps?:\/\/\w*.?home-assistant.io/i
+
+                # Play nice with links that already have a rel attribute set
+                rel.unshift(link.get_attribute('rel'))
+
+                # Add rel attribute to link
+                link.set_attribute('rel', rel.join(' ').strip)
+            end
+            dom.to_s
+        end
+    end
+end
+
+Liquid::Template.register_filter(Jekyll::NoFollow)

source/_layouts/default.html

@@ -24,7 +24,7 @@
         <div class="grid__item two-thirds lap-one-whole palm-one-whole">
         {% endif %}
 
-          {{ content }}
+          {{ content | nofollow }}
 
         </div>