You have at least three options:

1. The curb gem
2. The multipart-post Net:HTTP extension
3. Calling curl from Ruby with, for example, Open3.

      1  Open3.popen3('curl  <and your parameters>') do |input, output, error|
      2      # do something
      3      end
   

Cookies are stored and retrieved from cookies.txt. Post data is set using the data switch:

   1  curl --cookie cookies.txt --cookie-jar cookies.txt --user-agent Mozilla/4.0 --data "user=xxxxx&password=xxxxx" http://www.com/login -v
   2  curl --cookie cookies.txt --user-agent Mozilla/4.0 http://www.com/protected/resource