Use the meta
method of the virtual filehandle:
open('http://google.com'){|f| pp f.meta }
{"x-frame-options"=>"SAMEORIGIN",
"expires"=>"-1",
"p3p"=>
"CP=\"This is not a P3P policy! See http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=151657 for more info.\"",
"content-type"=>"text/html; charset=ISO-8859-1",
"date"=>"Mon, 17 Dec 2012 14:37:29 GMT",
"server"=>"gws",
"x-xss-protection"=>"1; mode=block",
"set-cookie"=>
"PREF=ID=d2fb8a93d369bcd2:FF=0:TM=1355755049:LM=1355755049:S=ONVSP6n2jtluFgll; expires=Wed, 17-Dec-2014 14:37:29 GMT; path=/; domain=.google.com, NID=67=OFEvvHCOa3C6wScQCUIKfu_89oL9MSmnFjwN-u5LX_foP8NLsX7G9dq48NLVrf4WUXhqOA1jb38s0e9qeRp1Iwx_LT_N8IuF0Qi6dXVtR2zdvA86INqtfg5uNrKvxJfJ; expires=Tue, 18-Jun-2013 14:37:29 GMT; path=/; domain=.google.com; HttpOnly",
"cache-control"=>"private, max-age=0",
"transfer-encoding"=>"chunked"}
http://www.ruby-doc.org/stdlib-1.9.3/libdoc/open-uri/rdoc/OpenURI/Meta.html