title: Failed to build gem native extension (mkmf (LoadError)) - Ubuntu 12.04
body: <p>Hey I'm trying to install rails on a new ubuntu machine. I have ruby and rvm installed but I'm getting a "failed to build gem native extension" error. What does this mean?</p>
  
  <pre><code>$ sudo gem install rails -v 3.2.9
  </code></pre>
  
  <p>(without the sudo says I don't have permissions)</p>
  
  <p>Then it outputs a lot of 'fetching' commands and ultimately this error:</p>
  
  <pre><code>    Building native extensions.  This could take a while...
      ERROR:  Error installing rails:
          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 `&lt;main&gt;'
  
  
      Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.8.0 for inspection.
      Results logged to /var/lib/gems/1.9.1/gems/json-1.8.0/ext/json/ext/generator/gem_make.out
  </code></pre>
  
  <p>When I type</p>
  
  <pre><code>$rails server
  </code></pre>
  
  <p>It says rails is not currently installed</p>