conflicts between OpenDNS and Sonicwall

I have been experiencing periodic disconnection issues on my network.  My Win 2012 domain controller has DNS set to the OpenDNS service.  I am running a Sonicwall NSA220 with filtering also enabled and pointing to Comcast DNS servers.  I have 20 Mac users in an education program and they experience periodic problems with not being able to access their content.  (Curiously, accessing the same content from a PC does not have these issues)

I have found that if I change the DNS on my DC to the Comcast DNS server, my disconnection problem goes away.  

I don't think the issue is with OpenDNS service but I don't know quite else it could be.
Ken HerrIT DirectorAsked:
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Dan McFaddenSystems EngineerCommented:
How are clients getting IP Addresses & its configuration?  Do you have DHCP running someplace in the environment?

Are there any rules controlling access to port 53/udp (DNS Query traffic) on the Sonicwall?

Dan

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Ken HerrIT DirectorAuthor Commented:
Thank you Dan.  Upon further investigation the problem is ONLY happening with Macbook clients.  These clients have Avast for Business 2015 installed on them.  Unfortunately, these computers do not belong to us so we cannot manage the software settings.  I have found that by disabling OpenDNS on our network the problems with the Macbooks goes away.  Even with OpenDNS enabled we do not have any difficulty with intermittent connections on users with PCs.

So, in summary, the problem only exists on our Macbooks when OpenDNS is enabled, and only about 40% 0f the time.  The problem never occurs on a PC going to the same web sites
Rich BullockIT Systems EngineerCommented:
What symptoms/error messages do the clients get? Are they time out errors or something else?
Ken HerrIT DirectorAuthor Commented:
Pages simply do not load and eventually time out.  Without any intervention, the page will usually load.  Using Chrome, the page loading icon spins counter-clockwise, indicating that the page is trying to resolve.
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