actionpack/lib/action_dispatch/middleware/templates/rescues/blocked_host.html.erb
<header>
<h1>Blocked hosts: <%= @hosts.join(", ") %></h1>
</header>
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<h2>To allow requests to these hosts, make sure they are valid hostnames (containing only numbers, letters, dashes and dots), then add the following to your environment configuration:</h2>
<pre>
<% @hosts.each do |host| %>
config.hosts << "<%= host %>"
<% end %>
</pre>
<p>For more details view: <a href="https://guides.rubyonrails.org/configuring.html#actiondispatch-hostauthorization">the Host Authorization guide</a></p>
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