SCOM 2007 R2

Archive for February, 2012

Website monitoring, another gotcha!

Posted by rob1974 on February 13, 2012

Recently I had an issue with website monitoring in a SCOM demo environment. I had configured a website test (through the template) every 2 minutes. I had created a DNS zone hosting the FQDN for this website. Then I paused the DNS zone and waited for the HTTP test to fail. I expected the HTTP test to fail and have an alert in SCOM within 2 minutes. However, after 10 minutes I had nothing… Not really what you want when you do a live demo.

So what was happening here? Some of you might already got it, it’s DNS caching of the client running the HTTP test. So how to stop this? Well there are 3 things you can do.


1. Default the DNS Client service will be started on a windows machine. Simply stopping the DNS Client service and the caching will stop (dns queries will still resolve).

2. Increase the frequency of the HTTP test. Anything more than 15 minutes will do…

3. Decrease the default cache time for the queries to something less than your test frequency.


As I was giving a demo, option 2 was not an option for me. But I would seriously thing about this when I do website tests in production. Obviously service level agreements should play a part in this, but a delay of max 30 minutes on a SLA of 8 hours would definitely be acceptable for me.

Option 1 was no option either. Beside caching the dns client service also registers domain joined hosts in dns, so not something I would recommend either. Besides caching helps performance wise, not sure if I ever wanted to disable this.

So left with option 3, but how to do this? In HKLM\SYSTEM\CurrentControlSet\services\Dnscache\parameters create the DWord MaxCacheTtl (and MaxNegativeCacheTtl if it’s not already on 0 with “NegativeCacheTime”) and give it a value of below the frequency of the website test. For a 2 minutes test I used a value of 90 (seconds).


Normally I think option 2 will be the best to go for. No use to run tests more than you would need to. However if you have a dedicated host for running website tests and you run those tests more often than every 15 minutes, consider reducing the max. cache time of the dns client.

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