Also, CNAMEs won’t work for this, because cannot create a CNAME on a bare domain name because a CNAME must be the only record for a given name. Bare domain names always have an SOA record, which rules out the use of CNAME.

It’s probably possible to do what you describe entirely in mod_rewrite, but there’s some kind of special psychological term for people who hurt themselves on purpose.

So I did it using a combo of mod_rewrite and PHP:

RewriteEngine on
RewriteRule ^.*$ /index.php

$strURL = "http://www.{$_SERVER['HTTP_HOST']}${_SERVER['REQUEST_URI']}";
header("Location: {$strURL}");
print "Redirecting to <a href=\"{$strURL}\">{$strURL}</a>";

You can use this functionality yourself, but I also liked the idea that we could just provide it since it’s so common. So, we’ve added an “Add Bare Domain Forward” option to our DNS management panel. It’ll appear if the bare domain name (e.g. example.com) is not in use and the www version (e.g. http://www.example.com) is.

I don’t see us doing the reverse, as it would eat another IP address and it’s far less common. But it’s still easy to do on your own, using any of the techniques described here.

Thanks for the feedback!

A suitable expression would be:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www\.(.+\..+)$ [NC]
RewriteRule ^.*$ http://www.%1%{REQUEST_URI} [R=301,L]

Given that this can be applied to all of your sites, one wonders if NFSN could make two sites that we could all CNAME to, one that adds “www” if required, and one that removes “www” if present, to match the most common of these cases. Then the really simple thing to suggest people do is point the alias that they want changing at the appropriate NFSN-wide site.