WEBrick

webrick.rb

irb> require 'webrick'
#=> true
irb> server = WEBrick::HTTPServer.start :Port => 4000
#=> #<WEBrick::HTTPServer:0x007fc349081ec0 ...>
irb> server.mount_proc("/foo") { |req, res| res.status = 400; res.body = "WHOOPS\n" }
#=> [#<WEBrick::HTTPServlet::ProcHandler:0x007fc34a1131f8 @proc=#<Proc:0x007fc34a113248@(irb):4>>, []]
irb> server.start
# [2014-05-14 10:51:28] INFO  WEBrick::HTTPServer#start: pid=77103 port=4000

# Meanwhile, in the shell...

$ curl -i localhost:4000/foo
HTTP/1.1 400 Bad Request
Server: WEBrick/1.3.1 (Ruby/2.0.0/2013-06-27)
Date: Wed, 14 May 2014 14:51:49 GMT
Content-Length: 7
Connection: Keep-Alive

WHOOPS
$