Putting this here for safekeeping so my future self can find it. Mod_rewrite is one of my favorite tools, but it’s easy to spend 30 minutes crafting a 2-line directive that actually does what you want. I put this in a .htaccess file in the DocumentRoot of the server, put a “We’re down” message in maintenance.html (or whatever), and all requests will get a 302 redirect to /maintenance.html, except requests for /maintenance.html (for obvious reasons). It appends the original request in case you want to do something with it but that’s not really important. It also doesn’t do the redirect for images/js/css so those can actually be used in the maintenance message.
RewriteEngine on
RewriteRule ^maintenance.html - [PT,L]
RewriteCond %{REQUEST_URI} !(gif|jpg|css|js)$
RewriteRule (.*) /maintenance.html?redir=true&originalRequest=%{REQUEST_URI} [R=302]
Evan Hoffman's Blog 