How does outlook handle a blank Autoconfig.xml file?

We had an issue last weekend with our web provider freaking out due to an unusual high number of requests for the autoconfig.xml file. Our exchange box was down for extended maintenance, so apparently all of the outlook clients started hitting our public facing web host to retrieve a new autoconfig.xml in order to verify settings, etc.  The host's firewall blocked our access for requesting a file that doesnt exist too many times.

as a "favor", our web host created a blank autoconfig.xml file to allow the server to serve up the requested file and not bog down the server with 404 errors. They stated the problem was solved because they added that file, but I suspect its more likely that since exchange is functional, the workstations arent requesting the file so from their end they THINK they fixed it.

 At this time its not causing any issues because the exchange server is up, but there is scheduled maintenance this weekend and I'm concerned that as soon as exchange drops, workstations will start referring to that blank file and I have absolutely no idea what the repercussions will be.  

What happens when outlook hits an absolutely blank (as in 0 byte, no contents whatsoever, not even header/formatting data, etc) autoconfig.xml file? Will this cause problems for my workstations, or will they ignore the absolute lack of data and carry on with the original config?
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If you have autodiscover record in your domain it should point to your exchange server

for example for Office365 looks like this

Type       Priority     Host name       Points to address                  TTL
CNAME   -                     autodiscover      1 Hour

Set the appropriate record for your exchange server. When it's down, your outlook clients will just fail to connect.


cameramonkeyAuthor Commented:
Nope. Our autodiscover record points to the exchange server, not the public web server.

I did just notice one of our domains does not have the autodiscover record listed at all (50 or so mailboxes) Would that cause it to fail over to the root domain address as a fallback, this creating this problem?
Simon Butler (Sembee)ConsultantCommented:
Autodiscover tries a number of URLs by default. One of those URLs is the root of the domain the email address is in.
Internally, if the machine is a member of the domain then it doesn't do a DNS lookup, as the client gets its information from the domain on where to look. It will only start using DNS queries if that isn't available.
Therefore as long as you don't have anyone using Outlook externally, and the Exchange server is available, the root of the domain wouldn't be used.

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cameramonkeyAuthor Commented:
Simon, thats the problem. When the exchange server is down for extended after hours maintenance, we see the issues.

Our external web host naturally doesnt have the xml file, so they decided to publish a zero byte blank file to make their servers happy and allow them to serve the requested (all be it incorrect and theoretically harmless) content instead of producing a 404 error.

My concern is what outlook does with that blank file? Worst case I could see outlook wiping settings and taking that mail client down to the point of IT intervention. (IT staff of 4+ 650 workstations= bad news!)
Simon Butler (Sembee)ConsultantCommented:
Extended after hours maintainance? What are you doing to your Exchange servers that need that? The longest I have had an Exchange server down for scheduled maintainance is 90 minutes, and that is for a service pack, the rest of the time it is down for 45 minutes at most.

A blank file isn't going to do anything because the format isn't recognised. If the file was formatted correctly, with the relevant fields blanked out, then I would be worried.

I don't see what the issue with 404s is. One of the sites I look after gets 15,000 an hour.


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cameramonkeyAuthor Commented:
your time estimate is correct for a normal outage involving a patch, reboot, etc. We have had issues with patches that took the server down, as well as fiber disruptions that lasted several hours. Thats when we see these problems.

If you are talking extended downtime then what you are getting is exactly what you should get.

It is the way the system works.

IMHO I would leave well alone.

it may be a good excuse to have a tested DR plan with a backup machine that can run Exchange if required. Not sure how big the environment is, but that will give you the peace of mind if you have problems with updates. You can do the test on the offline DR copy first, and then go on to the production machine later.
A Virtual environment is great for this stop the server copy the Virtual disk(s) and restart. If the patch fails you have an instant restore.
cameramonkeyAuthor Commented:
The consensus across several experts is that as long as the file is blank, we are safe.

(the hosting provider is rejecting requests to that file now and is OK with the periodic spurts in access now that they understand what is going on)
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