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Multiple Nameserver in Conditional Forwarding Setup


we start wondering how in detail MWG is using Nameserver for name resolution if Conditional DNS Forwarding is active and multiple (lets assume 4) different nameservers are configured.

We configured NS like following
external ns1, external ns2, internal ns1, internal ns2

for domain "pure.internal" use internal ns1 and internal ns2.

The problem we are facing is domain "split.domain" for our customer which is having internal and external hosts configured.

Of course we can each host list as individual entry in conditional forwarding but thats not really our intention.

We noticed that "hostA.split.domain" is sometimes resolvable and sometimes not.which indicates that ALL of the listed 4 Nameservers are used for lookup in some kind of round robin.

Please confirm or explain how this works.

we were under the impression, that 2nd NS is only used if 1st NS is not replying at all but here the 3rd or 4th NS seems to resolve sometimes "hostA.split.domain" when 1st is replying with not resolvable or by using round robin NS from list of multiple configured NS.



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Re: Multiple Nameserver in Conditional Forwarding Setup

Hello @ChrLu 

bind uses forwarder selection algorythm based on the response time:


>> 2nd NS is only used if 1st NS is not replying at all 

this is the "stub resolver" logic that used if you use default settings with up to three dns servers,

When you enable conditional dns forwarding MWG switches from using fixed dns servers to using a local dns server ( that provides beforementioned forwarder selection algorythm based on SRTT (smooth round trip time) - i.e. response time of forwarders.

You can check srtt of configured forwarders with "rndc dumpdb" command and check last lines in  /var/​cache/bind/named_dump.db file



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