Add nofollow to all internal links.

app/helpers/application_helper.rb

@@ -233,16 +233,29 @@ module ApplicationHelper
   end
 
   def link_to(name = nil, options = nil, html_options = nil, &block)
+    begin
+      uri = URI(options)
+      host = uri.host
+      absolute_uri = uri.absolute?
+    rescue URI::InvalidURIError, ArgumentError
+      host = nil
+      absolute_uri = nil
+    end
+
+    # Add "nofollow" only to external links
+    if host && host != Gitlab.config.gitlab.host && absolute_uri
       if html_options
         if html_options[:rel]
-        html_options[:rel] << " noreferrer"
+          html_options[:rel] << " nofollow"
         else
-        html_options.merge(rel: "noreferrer")
+          html_options.merge!(rel: "nofollow")
         end
       else
         html_options = Hash.new
-      html_options[:rel] = "noreferrer"
+        html_options[:rel] = "nofollow"
       end
+    end
+
     super
   end
 end

spec/helpers/application_helper_spec.rb

@@ -195,4 +195,27 @@ describe ApplicationHelper do
       simple_sanitize(input).should == a_tag
     end
   end
+
+  describe "link_to" do
+
+    it "should not include rel=nofollow for internal links" do
+      expect(link_to("Home", root_path)).to eq("<a href=\"/\">Home</a>")
+    end
+
+    it "should include rel=nofollow for external links" do
+      expect(link_to("Example", "http://www.example.com")).to eq("<a href=\"http://www.example.com\" rel=\"nofollow\">Example</a>")
+    end
+
+    it "should include re=nofollow for external links and honor existing html_options" do
+      expect(
+        link_to("Example", "http://www.example.com", class: "toggle", data: {toggle: "dropdown"})
+      ).to eq("<a class=\"toggle\" data-toggle=\"dropdown\" href=\"http://www.example.com\" rel=\"nofollow\">Example</a>")
+    end
+
+    it "should include rel=nofollow for external links and preserver other rel values" do
+      expect(
+        link_to("Example", "http://www.example.com", rel: "noreferrer")
+      ).to eq("<a href=\"http://www.example.com\" rel=\"noreferrer nofollow\">Example</a>")
+    end
+  end
 end