Rails middleware ensures the Rails stack obtains the correct IP

cloudflare.rb

# config/initializers/cloudflare_real_ip.rb

# frozen_string_literal: true

# CloudFlare masks the true IP
# This middleware ensures the Rails stack obtains the correct IP
# when using request.remote_ip
# See https://support.cloudflare.com/hc/en-us/articles/200170786
class CloudflareRealIp
  def initialize(app, opts={}, &block)
    @app = app
  end

  def call(env)
    if env['HTTP_CF_CONNECTING_IP']
      env['HTTP_REMOTE_ADDR_BEFORE_CF'] = env['REMOTE_ADDR']
      env['HTTP_X_FORWARDED_FOR_BEFORE_CF'] = env['HTTP_X_FORWARDED_FOR']
      env['REMOTE_ADDR'] = env['HTTP_CF_CONNECTING_IP']
      env['HTTP_X_FORWARDED_FOR'] = env['HTTP_CF_CONNECTING_IP']
    end
    @app.call(env)
  end
end

Rails.application.config.middleware.insert_before 0, CloudflareRealIp