While I was configuring Ruby on Rails in a Red Hat (CentOS) with RVM I found a problem with the library libffi-devel because the package wasn't available via
yum install, after search for a couple of days I found this solution.
Add rpmforge as a repository
Create a repo file in /etc/yum.repos.d/rpmforge.repo, you need to be an admin
Copy the follow text.
#Name: RPMforge RPM Repository for Red Hat Enterprise 5 - dag