/dev/rob0 rob0 at gmx.co.uk
Mon Mar 9 14:37:23 UTC 2015

On Mon, Mar 09, 2015 at 10:07:55AM +0300, Mohammed Ejaz wrote:
> This lately we have been receiving complain from our customer that 
> they are unable to open the websites when they use our DNS server 
> (ns1.cyberia.net.sa) , one of the Eg. Is www.twitter.com, it works 
> only If I removed my internal cache it works for some time. And 
> after some time it won't.
> Our DNS sometimes resolves the domain and sometimes are not

This is not enough information to be able to diagnose the problem.
Other posters have already guessed that you might not have had 
recursion enabled for your clients, but that's not consistent with 
the SERVFAIL result you showed below.

> Default Server:  ns1.cyberia.net.sa
> Address:
> > www.twitter.com
> Server:  ns1.cyberia.net.sa
> Address:
> *** ns1.cyberia.net.sa can't find www.twitter.com: Server failed

Two suggestions: first, get rid of nslookup.  Use dig and share the 
dig query and result with the list.

Second, check your logs for the exact time of these SERVFAIL 
responses you're seeing.  Sometimes these will be logged.
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