I have two windows domain controllers. Primary ( win 2008 r2 ) Replica ( win 2012 r2 )

The second one is configured as a replica of the first.

About once per week, the primary DC will negatively cache most .io domains. This makes it so noone in the company can access sites like:


Strangely I can still access some .io pages, like the ones at github.io


The solution is to RDP into the DNS server and run dnscmd /clearcache. That fixes the problem for 7 to 10 days.

Further symptoms

  • Only affects the primary domain controller (the secondary, and other domain controllers can resolve these sites just fine)
  • google dns servers also work
  • Usually happens at about 11 am on wednesdays.

I'm not very familiar with windows, but here are the things I've tried

  • Look at logs, I only see the following lines that look interesting

The DNS server wrote version 4638 of zone 254.10.in-addr.arpa to file 254.10.in-addr.arpa.dns..in-addr.arpa to file 254.10.in-addr.arpa.dns.

A more recent version, version 4639 of zone 254.10.in-addr.arpa was found at the DNS server at Zone transfer is in progress.ic replication between domain controllers in a common domain or forest. By installing multiple domain controllers in a domain running DNS Server, you can ensure that DNS will continue to work when a domain co
  • Verify there are no forward or reverse lookup zones for the .io domain
  • Ensure there is nothing in the hosts file blocking the .io domain
  • Compare the output of ipconfig /displaydns on all domain controllers

Is there anything else I can investigate to find out why the dns cache keeps getting corrupt so predictably? Is there a windows dns setting that can forcibly flush the cache when doing zone transer

I've narrowed this down to the fact that I often switch from wired to wireless right before the Wednesday meeting. The wireless has 1 windows 2008 dns server and 1 windows 2012 dns server. When the 2008 server is selected as primary, the problem returns. The workaround is to run this dnscmd /clearcache. Since the 2008 server is going away, I'm sure this problem will fix itself.

  • That's awfully specific. It's like the nameserver data for the io TLD is getting clobbered, or an upstream network device that is not shared with the secondary DC is going haywire due to deep packet inspection policies. Make sure there are no zones on the primary DC that could interfere with upstream nameservers for that TLD. (., io, net, ac, uk, co.uk, ns13.net, nic.io, nic.ac, icb.co.uk, communitydns.net) Sounds silly, but people sometimes do very braindead things when attempting to use their DC as a DNS firewalling solution. – Andrew B Jul 17 '15 at 20:48
  • Another thing to do is check how your DNS servers do non-local lookups. For each of your DNS servers check the settings for Forwarders and Root Hints. If one is using a filtered forwarder and the other isn't that might be your issue. Alternately, if you have only one forwarder and and incomplete set of root hints you may have look up issues from that. – Mark Jul 17 '15 at 21:26
  • 1
    What else in your system happens at 11am on Wednesdays, that might cause that server to fail to look up those domains (and cache the failure)? – Calle Dybedahl Jul 18 '15 at 14:09
  • so the issue is on client, some *.io domains are not resolved at all until you clear cache? If you go on DNS console, does the console GUI show correct IPs for those names in cache? – strongline Oct 1 '15 at 14:53
  • Is your DNS server configured to use forwarders? – Mike Marseglia Dec 29 '15 at 15:29

Consider updating your root.hints file. Maybe it’s pointing to some old root name servers that (for some reason) aren’t returning .io domains.

Maybe you have a routing issue that prevents accessing them (ie: you’re black-holing the IP range they run on) which prevents looking up the domains within it. This one is my bet- maybe you have a firewall rule against a country or IP block. Use my results below to check your firewall or do a dig/nslookup for the .io TLD servers (you can download a binary for Windows from http://www.isc.org/downloads/

# dig +trace +identify git.io
io.                     172800  IN      NS      b0.nic.io.
io.                     172800  IN      NS      a0.nic.io.
io.                     172800  IN      NS      a2.nic.io.
io.                     172800  IN      NS      ns-a1.io.
io.                     172800  IN      NS      ns-a3.io.
io.                     172800  IN      NS      c0.nic.io.

Can you reach all of these DNS servers directly? Your DNS server may repeatedly use the first in the list for example. Keep in mind, this list is at a point in time (right now) and changes, but should give you an initial point to see if you can reach the .io root name servers.

# for i in b0.nic.io a0.nic.io a2.nic.io ns-a1.io ns-a3.io c0.nic.io; do host $i; done
b0.nic.io has address
b0.nic.io has IPv6 address 2a01:8840:9f::17
a0.nic.io has address
a0.nic.io has IPv6 address 2a01:8840:9e::17
a2.nic.io has address
a2.nic.io has IPv6 address 2a01:8840:a1::17
ns-a1.io has address
ns-a1.io has IPv6 address 2001:678:4::1
ns-a3.io has address
c0.nic.io has address
c0.nic.io has IPv6 address 2a01:8840:a0::17

If you’re using forwarders, test an nslookup to those forwarders directly. If it doesn’t return, contact the person who runs them (your ISP).

==== Update: Given your update, where you note that it happens when you change ISPs, I would guess one of your connections is using IPv6 and the other is only IPv4 capable? It could be that it’s caching IPv6 return address, but that isn’t reachable once you switch connections.

Your Answer

By clicking "Post Your Answer", you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.