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    Weird DNS resolution issue

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    dns dashrender
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    • DashrenderD
      Dashrender
      last edited by

      I ran into this yesterday.

      Client has cable modem to TP Link router, TP Link is DHCP for network.
      DHCP provides DNS addresses pointing to Comodo Secure DNS (8.26.56.26 and 8/20.247.20)

      Some computers are able to get DNS resolution - others cannot.

      All computers can ping 8.8.8.8 - so internet works.
      All computers can ping the Comodo DNS servers.

      Yet some computers just won't get DNS resolution.

      I fixed the issue by changing the DHCP to hand out 8.8.8.8 and 8.8.4.4, rebooted the clients - problem gone.

      Anyone heard of an issue like this? where a DNS provider seemingly blocks some requests from a given IP, but not others?
      Was there something else I should have tried?

      1 scottalanmillerS 4 Replies Last reply Reply Quote 0
      • 1
        1337 @Dashrender
        last edited by 1337

        @Dashrender said in Weird DNS resolution issue:

        I ran into this yesterday.

        Client has cable modem to TP Link router, TP Link is DHCP for network.
        DHCP provides DNS addresses pointing to Comodo Secure DNS (8.26.56.26 and 8/20.247.20)

        Some computers are able to get DNS resolution - others cannot.

        All computers can ping 8.8.8.8 - so internet works.
        All computers can ping the Comodo DNS servers.

        Yet some computers just won't get DNS resolution.

        I fixed the issue by changing the DHCP to hand out 8.8.8.8 and 8.8.4.4, rebooted the clients - problem gone.

        Anyone heard of an issue like this? where a DNS provider seemingly blocks some requests from a given IP, but not others?
        Was there something else I should have tried?

        Sounds like it might have been the reboot that actually solved the problem.

        All requests from the LAN will originate from the routers WAN IP address so it will be the same IP from the DNS servers point of view. In other words, it's unlikely it's some problem on the DNS servers.

        DashrenderD scottalanmillerS 3 Replies Last reply Reply Quote 0
        • 1
          1337 @Dashrender
          last edited by 1337

          @Dashrender said in Weird DNS resolution issue:

          I fixed the issue by changing the DHCP to hand out 8.8.8.8 and 8.8.4.4, rebooted the clients - problem gone.

          I usually let the router act as a DNS forwarder or resolver and cache.

          So in this case the computers would all get the IP of the router as DNS from the DHCP. And the router would forward DNS requests to whatever IPs you want.

          Not that it makes much difference in your case. But I think it's a cleaner and faster architecture.

          JaredBuschJ 1 Reply Last reply Reply Quote 0
          • 1
            1337 @Dashrender
            last edited by 1337

            @Dashrender said in Weird DNS resolution issue:

            Was there something else I should have tried?

            When troubleshooting you can make DNS queries to specific DNS servers that doesn't use the clients DHCP originated default DNS servers.

            For example:

            nslookup mangolassi.it 8.8.8.8

            or

            nslookup mangolassi.it 8.8.4.4

            It would be better than just pinging.

            You also have ipconfig as a tool on Windows.
            To clear the clients DNS cache

            ipconfig /flushdns

            to force DHCP renewal

            ipconfig /renew

            or to check what DNS server it has been given.

            ipconfig /all

            And look for "DNS Servers"

            DashrenderD 1 Reply Last reply Reply Quote 1
            • DashrenderD
              Dashrender @1337
              last edited by

              @Pete-S said in Weird DNS resolution issue:

              @Dashrender said in Weird DNS resolution issue:

              I ran into this yesterday.

              Client has cable modem to TP Link router, TP Link is DHCP for network.
              DHCP provides DNS addresses pointing to Comodo Secure DNS (8.26.56.26 and 8/20.247.20)

              Some computers are able to get DNS resolution - others cannot.

              All computers can ping 8.8.8.8 - so internet works.
              All computers can ping the Comodo DNS servers.

              Yet some computers just won't get DNS resolution.

              I fixed the issue by changing the DHCP to hand out 8.8.8.8 and 8.8.4.4, rebooted the clients - problem gone.

              Anyone heard of an issue like this? where a DNS provider seemingly blocks some requests from a given IP, but not others?
              Was there something else I should have tried?

              Sounds like it might have been the reboot that actually solved the problem.

              All requests from the LAN will originate from the routers WAN IP address so it will be the same IP from the DNS servers point of view. In other words, it's unlikely it's some problem on the DNS servers.

              nah - I rebooted the computers myself - still no go.

              I didn't personally reboot the router, but they replaced the cable modem and the router - and things still didn't work - so they put the old router back - all before I arrived.

              1 Reply Last reply Reply Quote 1
              • DashrenderD
                Dashrender @1337
                last edited by

                @Pete-S said in Weird DNS resolution issue:

                All requests from the LAN will originate from the routers WAN IP address so it will be the same IP from the DNS servers point of view. In other words, it's unlikely it's some problem on the DNS servers.

                Right - which is why it was so weird why some machines got DNS responses and some did not.

                scottalanmillerS 1 Reply Last reply Reply Quote 1
                • DashrenderD
                  Dashrender @1337
                  last edited by

                  @Pete-S said in Weird DNS resolution issue:

                  @Dashrender said in Weird DNS resolution issue:

                  Was there something else I should have tried?

                  When troubleshooting you can make DNS queries to specific DNS servers that doesn't use the clients DHCP originated default DNS servers.

                  For example:

                  nslookup mangolassi.it 8.8.8.8

                  or

                  nslookup mangolassi.it 8.8.4.4

                  It would be better than just pinging.

                  Nice - thanks didn't know that.

                  I did all the ipconfig stuff - no change... at least not until I change the DNS in the DHCP server.

                  1 1 Reply Last reply Reply Quote 1
                  • 1
                    1337 @Dashrender
                    last edited by

                    @Dashrender said in Weird DNS resolution issue:

                    @Pete-S said in Weird DNS resolution issue:

                    @Dashrender said in Weird DNS resolution issue:

                    Was there something else I should have tried?

                    When troubleshooting you can make DNS queries to specific DNS servers that doesn't use the clients DHCP originated default DNS servers.

                    For example:

                    nslookup mangolassi.it 8.8.8.8

                    or

                    nslookup mangolassi.it 8.8.4.4

                    It would be better than just pinging.

                    Nice - thanks didn't know that.

                    Well, I don't do it often enough so I always forget how to do it and then I have to look it up...everytime....

                    1 Reply Last reply Reply Quote 0
                    • scottalanmillerS
                      scottalanmiller @Dashrender
                      last edited by

                      @Dashrender said in Weird DNS resolution issue:

                      Anyone heard of an issue like this? where a DNS provider seemingly blocks some requests from a given IP, but not others?

                      It happens. Can be based on a lot of factors. Can also happen from your router.

                      1 Reply Last reply Reply Quote 0
                      • scottalanmillerS
                        scottalanmiller @1337
                        last edited by scottalanmiller

                        @Pete-S said in Weird DNS resolution issue:

                        All requests from the LAN will originate from the routers WAN IP address so it will be the same IP from the DNS servers point of view. In other words, it's unlikely it's some problem on the DNS servers.

                        Right, it's far more likely to be UDP flooding on a per client basis.

                        1 Reply Last reply Reply Quote 0
                        • scottalanmillerS
                          scottalanmiller @Dashrender
                          last edited by

                          @Dashrender said in Weird DNS resolution issue:

                          @Pete-S said in Weird DNS resolution issue:

                          All requests from the LAN will originate from the routers WAN IP address so it will be the same IP from the DNS servers point of view. In other words, it's unlikely it's some problem on the DNS servers.

                          Right - which is why it was so weird why some machines got DNS responses and some did not.

                          Unlikely, but still possible. Keep in mind that each machine on your network gets its own session. And each session can be on a different server on the DNS provider's end. So it might have been different servers that they were reaching.

                          1 Reply Last reply Reply Quote 0
                          • JaredBuschJ
                            JaredBusch @1337
                            last edited by

                            @Pete-S said in Weird DNS resolution issue:

                            I usually let the router act as a DNS forwarder or resolver and cache.

                            Always this.

                            DashrenderD 1 Reply Last reply Reply Quote 0
                            • DashrenderD
                              Dashrender @JaredBusch
                              last edited by

                              @JaredBusch said in Weird DNS resolution issue:

                              @Pete-S said in Weird DNS resolution issue:

                              I usually let the router act as a DNS forwarder or resolver and cache.

                              Always this.

                              meh - 6 of one!

                              I suppose it's possible that would have resolved this specific issue as the router would have been the only device making connections to the external DNS... but then again - it could have caused all machines to go without DNS when the upstream server stopped responding...

                              I didn't dig into the acient TP link router they had - they didn't want me to replace it with the newer Netgear they had once I got it working.

                              scottalanmillerS 1 Reply Last reply Reply Quote 0
                              • scottalanmillerS
                                scottalanmiller @Dashrender
                                last edited by

                                @Dashrender said in Weird DNS resolution issue:

                                I suppose it's possible that would have resolved this specific issue as the router would have been the only device making connections to the external DNS... but then again - it could have caused all machines to go without DNS when the upstream server stopped responding...

                                Not very likely. Plausible, but not likely enough to avoid it.

                                DashrenderD 1 Reply Last reply Reply Quote 0
                                • DashrenderD
                                  Dashrender @scottalanmiller
                                  last edited by

                                  @scottalanmiller said in Weird DNS resolution issue:

                                  @Dashrender said in Weird DNS resolution issue:

                                  I suppose it's possible that would have resolved this specific issue as the router would have been the only device making connections to the external DNS... but then again - it could have caused all machines to go without DNS when the upstream server stopped responding...

                                  Not very likely. Plausible, but not likely enough to avoid it.

                                  sure - but then again, I've never seen this situation before either - so I would have previously called it unlikely.

                                  1 Reply Last reply Reply Quote 0
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