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Offline Backup DNS causes internet connection speeds to slow to 10%

    Question

  • Primary DNS: 2008 Foundation Server R2

    Backup DNS: 2003 Server R2

    Why the 90% slow down in internet connection speeds for all network machines when we shut down the backup DNS?

    Please advise.

    Thursday, July 21, 2011 9:10 PM

Answers

  •   I would say that there is a problem with the  setup on the primary server. Even if there was a delay in DNS lookup the slowdown in traffic would be very slight indeed. You normally only do a few DNS lookups when you connect to a remote site. Once you have resolved the URLs to IP addresses, DNS takes no further part in the download (as [JM] pointed out).

      It is more likely that, without the backup DNS server, your traffic is somehow being directed through a different path which is slower

    .


    Bill
    • Marked as answer by Bruce-Liu Friday, July 29, 2011 5:40 AM
    Friday, July 22, 2011 5:56 AM
  • Primary DNS: 2008 Foundation Server R2

    Backup DNS: 2003 Server R2

    Why the 90% slow down in internet connection speeds for all network machines when we shut down the backup DNS?

    Please advise.

    I suspect the DNS resolution from the primary DNS may be always slow, to the point that the clients resolvers get some timeouts and switch to the secondary DNS; then, if you shutdown the backup, the clients are forced to use the primary and this slows things down

    I think that the solution may be checking the config of your primary DNS to find out the reasons for such a slowness, solving it will probably solve your issue; start by ensuring that your firewall is allowing DNS resolution traffic to correctly flow out from the primary DNS, that is, ensure that outbound traffic toward port 53/UDP and 53/TCP is allowed, also, and since you're at it, if your firewall allows to configure traffic shaping or if you have a QoS policy, ensure that DNS traffic has the highest priority; then, ensure that EDNS is enabled and that your DNS servers and firewall are supporting it (read here for details and infos); by the way the above assumes that your DNS is correctly configured (no forwarders, updated root hints) and that you don't have any other networking issues (e.g. packet loss, network congestions...)



    • Marked as answer by Bruce-Liu Friday, July 29, 2011 5:40 AM
    Saturday, July 23, 2011 7:41 AM

All replies

  • DNS name resolution is a very small part of the "internet connection" experience.  If you are seeing slowness when the DNS server is taken offline, you need to review the application flow for DNS in your organization.  For instance, do you have one server forwarding to this backup server (a long chain of fowarders)?  Check your forwarding configs...make sure the settings are correct.  Have you checked to ensure that your clients have more than one DNS server configured in their settings?

     


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    Thursday, July 21, 2011 10:56 PM
  •   I would say that there is a problem with the  setup on the primary server. Even if there was a delay in DNS lookup the slowdown in traffic would be very slight indeed. You normally only do a few DNS lookups when you connect to a remote site. Once you have resolved the URLs to IP addresses, DNS takes no further part in the download (as [JM] pointed out).

      It is more likely that, without the backup DNS server, your traffic is somehow being directed through a different path which is slower

    .


    Bill
    • Marked as answer by Bruce-Liu Friday, July 29, 2011 5:40 AM
    Friday, July 22, 2011 5:56 AM
  • Primary DNS: 2008 Foundation Server R2

    Backup DNS: 2003 Server R2

    Why the 90% slow down in internet connection speeds for all network machines when we shut down the backup DNS?

    Please advise.

    I suspect the DNS resolution from the primary DNS may be always slow, to the point that the clients resolvers get some timeouts and switch to the secondary DNS; then, if you shutdown the backup, the clients are forced to use the primary and this slows things down

    I think that the solution may be checking the config of your primary DNS to find out the reasons for such a slowness, solving it will probably solve your issue; start by ensuring that your firewall is allowing DNS resolution traffic to correctly flow out from the primary DNS, that is, ensure that outbound traffic toward port 53/UDP and 53/TCP is allowed, also, and since you're at it, if your firewall allows to configure traffic shaping or if you have a QoS policy, ensure that DNS traffic has the highest priority; then, ensure that EDNS is enabled and that your DNS servers and firewall are supporting it (read here for details and infos); by the way the above assumes that your DNS is correctly configured (no forwarders, updated root hints) and that you don't have any other networking issues (e.g. packet loss, network congestions...)



    • Marked as answer by Bruce-Liu Friday, July 29, 2011 5:40 AM
    Saturday, July 23, 2011 7:41 AM