Failed to build gem native extension (mkmf (LoadError)) - Ubuntu 12.04 [duplicate]

Answer 1

Possible duplicate of:

Unable to install gem - Failed to build gem native extension - cannot load such file -- mkmf (LoadError)

Depending on your ruby version, run:

sudo apt-get install ruby1.9.1-dev

Answer 2

If you are using RVM, you shouldn't ever be using 'sudo'. Using sudo calls libs and such out of the rvm path. Try just a flat:

gem install rails -v3.2.9

and see if that helps.