Add more detail about what hosts are allowed

actionpack/lib/action_dispatch/middleware/templates/rescues/blocked_host.html.erb

@@ -2,6 +2,6 @@
   <h1>Blocked host: <%= @host %></h1>
 </header>
 <main role="main" id="container">
-  <h2>To allow requests to <%= @host %>, add the following to your environment configuration:</h2>
+  <h2>To allow requests to <%= @host %> make sure it is a valid hostname (containing only numbers, letters, dashes and dots), then add the following to your environment configuration:</h2>
   <pre>config.hosts &lt;&lt; "<%= @host %>"</pre>
 </main>

actionpack/lib/action_dispatch/middleware/templates/rescues/blocked_host.text.erb

@@ -1,5 +1,5 @@
 Blocked host: <%= @host %>
 
-To allow requests to <%= @host %>, add the following to your environment configuration:
+To allow requests to <%= @host %> make sure it is a valid hostname (containing only numbers, letters, dashes and dots), then add the following to your environment configuration:
 
   config.hosts << "<%= @host %>"