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  • Status: Solved
  • Priority: Medium
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DNS no longer allowing us to get to external websites

We came in this morning and noticed that there was a DNS problem where we could not get to external sites.
Internal DNS settings appear to be working, but we cannot get to any external websites without typing in the IP address.

I'm not aware of any updates that were installed or changes that were made.

We are in a windows environment and have split DNS setup so (under normal circumstances) we can resolve to internal and external ip's.

Where should I start troubleshooting?
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MBisch
Asked:
MBisch
4 Solutions
 
R. Toby RichardsNetwork AdministratorCommented:
I'm curious what you mean by "split DNS setup". Each of your internal DNS servers should have a forwarder to "All other DNS domains" with at least one external DNS server. I have found that forwarding "All other DNS domains" to 8.8.8.8 (Google's public DNS server) and 4.4.4.1 (Level-3's public DNS server) (in that order) is faster than using my ISP's DNS servers.
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OriNetworksCommented:
If this only suddenly broke you obviously have it configured in a working condition to begin with so lets address things that may change. I assume you haven't changed any firewall rules, you have tried a simple restart, and this effect all clients and not just one or two. Since you mention split DNS setup I assume you have active directory configured and you use your DC to host DNS services to your internal clients through DNS forwarders.

1. ping a known working ip address. e.g. ping several 8.8.8.8, 8.8.4.4, 4.4.4.1 If this fails, you have a routing/internet problem and probably should call your ISP
2. one of your DNS forwarders may be unavailable. For example, most ISPs have dns servers that they hand out and may be temporarily offline. Check your forwarders from the DNS console on the domain controller and as the previous poster suggested, add 8.8.8.8 and 4.4.4.1 to the forwarders list.
3. Test DNS again, from your client machine try to ping google.com to make sure it can resolve to an ip address. It doesn't matter so much if it can successfully ping but it should receive reply. The important this here is that you see the ip address it is resolving for google.com
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MBischAuthor Commented:
Thanks for your input.
I had it setup to use other internal DNS Servers as forwarders then to use root hints if there are no forwarders.
I have gone in and removed the other internal DNS servers from the forwarders and have added 8.8.8.8 and 4.4.4.1.
I haven't changed any config and it worked yesterday.
It appears that something happened overnight for this to stop working.
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MBischAuthor Commented:
OriNetworks,
I can successfully ping 8.8.8.8 and 4.4.4.1, but can't get external domain names to resolve.
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R. Toby RichardsNetwork AdministratorCommented:
What happens if you do "nslookup google.com"?
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MBischAuthor Commented:
When I do an nslookup for google.com it comes back with an IP address for a local DNS server

server: internalDNSserver.comr
address: IP address of internal DNS server

name: a.different.internalDNSserver.com
addresses: IP address of internal DNS server
     IP address of internal DNS server
Aliases: google.com.mydomain.com
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R. Toby RichardsNetwork AdministratorCommented:
So the first thing to investigate is why it's appending ".mydomain.com" to "google.com"
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R. Toby RichardsNetwork AdministratorCommented:
It looks to me like maybe you have a wildcard DNS zone or record. In your DNS records on your internal DNS server look for *.mydomain.com.

Explanation of DNS Wildcards
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MaheshArchitectCommented:
ensure that you are able to telnet ISP DNS servers on TCP port 53 from domain controller

test name resolution from all servers to identify which server fails:
nslookup
set debug
google.com
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MBischAuthor Commented:
there was a wildcard in the domain and I have since removed it. Now the nslookup times out on the primary DNS server after 2 seconds.

It appears that the DNS server service is running on the server.
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R. Toby RichardsNetwork AdministratorCommented:
And the DNS server that nslookup reports using has forwarders to an ISP or Google or Level-3 DNS server?
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R. Toby RichardsNetwork AdministratorCommented:
Oh, and on the Advanced tab for the DNS server properties, make sure that "Disable recursion" is NOT checked. Also make sure on the forwarders tab under "All other DNS domains" "Do not use recursion for this domain" is NOT checked.
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MBischAuthor Commented:
I originally had the forwarders of 8.8.8.8 and 4.4.4.1 and they was not resolving correctly.
I added 8.8.4.4 and it started resolving correctly.
AND THEN...
I received a call from our Internet Service Provider, Windstream.
It turns out that there was a network based firewall that was having memory issues that caused DNS resolution to fail.

So. the problem all along was caused by our firewall.
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