DNS problem: works on public dns but not on internal dns server

We have two DNS servers on our domain that handle everything going to internet for clients with dynamic DNS in their settings and they work fine for most sites.  However, some sites are not accessible until we put in a public DNS address manually for the client.  Example: grainger.com just says it can't find the site.  If i put in a public dns (such as manually in the client settings, it goes to it. So why won't it go there with our dns servers?
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

Check both DNS servers, what are the forwarding DNS entries? After you find out, check what grainger.com resolves to on those DNS servers.

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
MikeIT ManagerCommented:
Either your DNS servers are not forwarding to an outside DNS provider properly, or whomever service you are using for external DNS resolution is having issues.

I would try adding a new DNS Forwarder from a different service (I use OpenDNS and Google) to your DNS Servers and clear the cache and flush the DNS on the workstations and try again.
That's because your internals DNS knows somehow that he it is responable for that domain and not forwarding the request, but try to resolve himself.

With your internal DNS registered at client type in a CMD:

nslookup grainger.com
ping grainger.com

and see what you receive as response

Best regards!
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today

From novice to tech pro — start learning today.