How to authenticate to grails from R using RCurl

The trick is to open the login page before actually trying to post the login data. I assume something is stored in the cookie, but I only know this is how it works. See also my question on stackoverflow: http://stackoverflow.com/questions/20355874/how-to-use-rcurl-to-authenticate-to-spring-security-grails-app/20370222#20370222