apache redirect from non www to www

Using the rewrite engine is a pretty heavyweight way to solve this problem. Here is a simpler solution:

<VirtualHost *:80>
    ServerName example.com
    Redirect permanent / http://www.example.com/
</VirtualHost>

<VirtualHost *:80>
    ServerName www.example.com
    # real server configuration
</VirtualHost>

And then you’ll have another <VirtualHost> section with ServerName www.example.com for your real server configuration. Apache automatically preserves anything after the / when using the Redirect directive, which is a common misconception about why this method won’t work (when in fact it does).