#Symptom
# gem install rest-client
Fetching: unf_ext-0.0.7.1.gem (100%)
Building native extensions. This could take a while...
ERROR: Error installing rest-client:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from extconf.rb:1:in `<main>'
#Resolution
apt-get install ruby-dev