Internal Server Error 500 when Empty Host Header Sent

示されている config.ru の設定と空の Host ヘッダーを使ってリクエストを再現し、その後、WEBrick の httpserver.rb とスタックトレースで示されている rackup ハンドラーを通してリクエストを追跡します。現在、形式が不正なリクエストが SERVER_NAME の欠落した状態で Rack に到達していること、そして最終的な動作が 500 ではなく HTTP 400 レスポンスになることを確認します。

Overview

When sent a GET request with an empty host header, WEBrick responds with error code 500, but it should respond with Error Code 400, for an invalid request.

Reproducing the Bug

1. Clone, build, and install WEBrick. (Tested on ea124002ee5ff43b6cf846931b2cb898b3f47819)
2. Set up a config.ru file with the following contents:

class Server
  def call(env)
    [200, {}, []]
  end
end

run Server.new

3. Run rackup.
4. Send a GET request with an empty host header, and observe WEBrick's response as follows:

printf 'GET / HTTP/1.1\r\nHost: \r\n\r\n' | ncat localhost 9292

Rack::Lint::LintError: env missing required key SERVER_NAME (Rack::Lint::LintError)
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:283:in `block in check_environment'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:282:in `each'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:282:in `check_environment'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:63:in `response'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/lint.rb:41:in `call'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/show_exceptions.rb:31:in `call'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/common_logger.rb:43:in `call'
	/usr/share/rubygems-integration/all/gems/rack-3.1.16/lib/rack/content_length.rb:20:in `call'
	/usr/share/rubygems-integration/all/gems/rackup-2.1.0/lib/rackup/handler/webrick.rb:111:in `service'
	/var/lib/gems/3.3.0/gems/webrick-1.9.1/lib/webrick/httpserver.rb:140:in `service'
	/var/lib/gems/3.3.0/gems/webrick-1.9.1/lib/webrick/httpserver.rb:96:in `run'
	/var/lib/gems/3.3.0/gems/webrick-1.9.1/lib/webrick/server.rb:310:in `block in start_thread'
::1 - - [27/Aug/2025:18:40:52 +0000] "GET / HTTP/1.1" 500 1183 0.0002