/ BLOG / Outlook, Exchange and calendars asking for authentication

This one really had us at work and really threw us massively. Imagine a customer who has recently had a number of non trivial modifications to their network. Now imagine several users, whom have Outlook over HTTP RPC (Outlook Anywhere) configured and enabled for slow networks only, and get asked for authentication when accessing a handful calendars. On the same site as the servers (i.e. on the fast network). Disabling Outlook over HTTP RPC completely, or enabling it for slow and fast networks, and the problem would not occur. With no useful logs what so ever.

The main problem was that so much stuff had changed it was difficult to know where to start, and even harder with nothing useful being logged. It wasn’t hitting the proxy server, which had recently had authentication forcefully enabled, it wasn’t the IIS and Exchange box.

Turns out that the affected calendars that people were trying to view had recently had their mailbox moved from one storage group, to another, and we were obsessed with it being something else to the point that we’d dismissed this without looking into it. Without HTTP RPC enabled redirection happened without trouble, but with it on it required authentication. But the weird thing was that you could actually see the calendar if you selected Open other user’s mailbox, rather than using the “shortcut” (i.e. tickbox) that had already been added in the calendar view.

Simple solution - remove and readd the affected calendars. Our best guess is that this is stored with various pointers or references to not only the mailbox, but also the storage group. After all, it would make sense from a performance point of view if you didn’t have to look it up each time.

The moral for me personally, since I’d taken quite a lot of this problem on, is to never forget Occam’s Razor. I might actually get a representation of that tattoo'ed on my body, whenever I get around to that part of life again.