What are "Microsoft Office Protocol Discovery" and "OfficeLiveConnector" and why do they access invalid URLs?

Microsoft has a kb article (link currently broken, Internet Archive snapshot) that covers Protocol Discovery in fine detail. Essentially, Office is trying to determine if your server supports WebDAV (or something like it) so that changes the user makes to the Office document can be pushed back directly to the server.

On servers I have to maintain, this seems to occur due to html e-mail using external images hosted on our servers.

It looks like Microsoft Office Outlook clients, which uses Microsoft Word for editing e-mail (and for viewing them since 2007 edition), trigger those "Microsoft Office Protocol Discovery" requests.

In my case, web sites without any kind of online contribution, I see that as annoying noise. If your site is some kind of sharing site with documents editing capabilities, you may not consider those requests as annoying noise, depending on your site implementation.

This worked for me:

# Intercept Microsoft Office Protocol Discovery
RewriteCond %{REQUEST_METHOD} ^(OPTIONS|PROPFIND)$ [NC]
RewriteCond %{HTTP_USER_AGENT} ^Microsoft\ Office\ Protocol\ Discovery [OR]
RewriteCond %{HTTP_USER_AGENT} ^Microsoft\ Office\ Existence\ Discovery [OR]
RewriteCond %{HTTP_USER_AGENT} ^Microsoft\-WebDAV\-MiniRedir.*$
RewriteRule .* - [R=501,L]