RackHeaderRenamer middleware for Rack; copy a request header to a different name before Rails etc sees it. e.g. config.middleware.insert(0, RackHeaderRenamer, "X-Varnish-Proto" => "X-Forwarded-Proto")

rack_header_renamer.rb

class RackHeaderRenamer

  # mapping example, sets/overwrites second header with first:
  # {"X-Varnish-Proto" => "X-Forwarded-Proto"}
  def initialize(app, mapping)
    @app = app
    @mapping = mapping
  end

  def call(env)
    @mapping.each_pair do |src, dst|
      Command.new(env, src, dst).rename
    end
    @app.call(env)
  end

  private

  class Command

    def initialize(env, src, dst)
      @env = env
      @src = src
      @dst = dst
    end

    def rename
      if @env.key?(src_key)
        @env[dst_key] = @env[src_key]
      end
    end

    private

    def src_key
      header_to_key(@src)
    end

    def dst_key
      header_to_key(@dst)
    end

    def header_to_key(header_name)
      "HTTP_" << header_name.upcase.gsub("-", "_")
    end

  end

end

rack_header_renamer_spec.rb

RSpec.describe RackHeaderRenamer do

  subject(:renamer) do
    RackHeaderRenamer.new(app, "X-Source-Header" => "X-Destination-Header")
  end

  let(:app) { double("app", call: [200, {}, []]) }

  let(:env) do
    {"HTTP_X_HERRING" => "red"}
  end

  context "with only source header present" do
    before do
      env["HTTP_X_SOURCE_HEADER"] = "only source"
    end
    it "creates new destination header" do
      renamer.call(env)
      expect(env["HTTP_X_DESTINATION_HEADER"]).to eq("only source")
    end
  end

  context "with only destination header present" do
    before do
      env["HTTP_X_DESTINATION_HEADER"] = "only destination"
    end
    it "does not overwrite destination header" do
      renamer.call(env)
      expect(env["HTTP_X_DESTINATION_HEADER"]).to eq("only destination")
    end
  end

  context "with neither header present" do
    it "does not touch env" do
      expect { renamer.call(env) }.to_not change { env }
    end
  end

  context "with both headers present" do
    before do
      env["HTTP_X_SOURCE_HEADER"] = "source"
      env["HTTP_X_DESTINATION_HEADER"] = "destination"
    end
    it "overwrites destination header" do
      renamer.call(env)
      expect(env["HTTP_X_DESTINATION_HEADER"]).to eq("source")
    end
  end

end