lets-encrypt-middleware.rb

class LetsEncryptResponse
  ACME_ENDPOINT_REGEX = /\A\/\.well-known\/acme-challenge\/(?<challenge>.*)\z/
  def initialize(app)
    @app = app
  end

  def call(env)
    request = Rack::Request.new env
    if request.path.start_with?("/.well-known/acme-challenge/")
      match = request.path.match ACME_ENDPOINT_REGEX
    challenge = match[:challenge]
      if ENV["ACME_CHALLENGE"] && ENV["ACME_RESPONSE"] && challenge == ENV["ACME_CHALLENGE"]
        [200, { "Content-Type" => "text/plain" }, [ENV["ACME_RESPONSE"]]]
      else
        [400, {}, ["This is a horrible error message."]]
      end
    else
      @app.call env
    end
  end
end

Rails.config.middleware.insert_before ActionDispatch::SSL, LetsEncryptResponse