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I have configured a Cisco 1921 router to act as a DNS Caching Server by using the following configuration snippet:

ip dns view default
ip dns server
ip domain retry 1
ip domain timeout 1
ip domain name mgmt.local
ip name-server 10.110.176.12
ip name-server 8.8.4.4
ip name-server 8.8.8.8

The idea is that if my local server (10.110.176.12) becomes unresponsive, I can still get results from Google's public DNS.

However, despite my configuration, it appears that the timeout is still 3 seconds, and the retry is 3. Clients which use this caching server will fail beucase that delay is too long for them, and therefore they will not be able to connect to Internet resources if the primary DNS server is unresponsive.

Here is the debug (with cache operations removed) :

122849: May 18 12:33:57.125: DNS: Incoming UDP query (id#2)
122850: May 18 12:33:57.125: DNS: Type 1 DNS query (id#2) for host 'maps.google.co.uk' from 192.168.110.56(53806)
122851: May 18 12:33:57.125: DNS: Servicing request using view default
122873: May 18 12:33:57.125: DNS: Re-sending DNS query (type 1, id#51028) to 10.110.176.12
122874: May 18 12:34:01.089: DNS: Resending query id #2
122875: May 18 12:34:01.089: DNS: Re-sending DNS query (type 1, id#51028) to 10.110.176.12
122876: May 18 12:34:04.089: DNS: Resending query id #2
122877: May 18 12:34:04.089: DNS: Re-sending DNS query (type 1, id#51028) to 10.110.176.12
122878: May 18 12:34:07.089: DNS: Resending query id #2
122879: May 18 12:34:07.089: DNS: Re-sending DNS query (type 1, id#51028) to 8.8.4.4
122880: May 18 12:34:07.117: DNS: Incoming UDP query (id#51028)
122881: May 18 12:34:07.117: DNS: Type 1 response (id#51028) for host <maps.google.co.uk> from 8.8.4.4(53)
122906: May 18 12:34:07.121: DNS: Forwarding back A response - no director required
122907: May 18 12:34:07.121: DNS: Finished processing query (id#2) in 9.996 secs
122908: May 18 12:34:07.121: DNS: Forwarding back reply to 192.168.110.56/53806

Local name resolution works correctly - it times out after 1 second and moves on to the next server. It's only the DNS Server portion of IOS which is ignoring these values.

Question - which values should I use for the DNS Server if it doesn't use the settings shown above? I have checked some online guides including this one: http://www.itsyourip.com/cisco/configure-cisco-router-as-cachingforwarding-dns-server/ and I've not been able to figure it out.

Cisco1921/K9 running Cisco Version 15.0(1r)M9, RELEASE SOFTWARE (fc1)

Update 1:

I've found some more settings that I need to configure:

LESNOSKIRT01#show ip dns view
DNS View default parameters:
Logging is off
DNS Resolver settings:
  Domain lookup is enabled
  Default domain name: mgmt.local
  Domain search list:
  Lookup timeout: 1 seconds
  Lookup retries: 1
  Domain name-servers:
    10.110.176.12
    8.8.4.4
    8.8.8.8
DNS Server settings:
  Forwarding of queries is enabled
  Forwarder timeout: 3 seconds
  Forwarder retries: 2
  Forwarder addresses:

So it looks like i need to try to find some settings to change the "Forwarder timeout" and "Forwarder retries"

  • Just a stupid question: Can you look up hosts on the router? I mean is the local configuration correct? Default gateway or default route, so that the router can actually access the DNS servers. – user36472 May 18 '17 at 11:17
  • Local resolution works fine, I've edited my question to reflect this. It's only the DNS server portion of IOS which seems to ignore the retry and timeout values. – KeyszerS May 18 '17 at 12:02
2

I figured it out:

DNS server on IOS does not use the standard lookup timeout or retries - you have to configure forwarding lookups and retries. Here is an example:

ip dns view default
dns forwarding timeout 1
dns forwarding retry 1
  • I was writing that as you posted. Nice work. – user36472 May 18 '17 at 12:19
  • 2
    Remember to mark your own answer as correct answer to help others in the future. :-) – user36472 May 18 '17 at 12:20

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