How do nameservers “failover”

binddns-serverdomain-name-systemnameserver

How do nameservers do failover? What decides the order in which nameservers are queried when the primary nameserver is not reachable?

To illustrate, here's part of my zonefile:

$TTL 3m;
site.com.       IN      SOA     ns1.sitese.com. admin.site.com. (
                   2007010403           ; Serial
                         1800           ; Refresh [1h=3600] 1800=30m
                          600           ; Retry   [10m]
                        86400           ; Expire  [2weeks] 86400=1day
                          180 )         ; Negative Cache TTL [1h]
;
site.com.      IN     NS      ns1.sitese.com.
site.com.      IN     NS      ns2.sitese.com.
site.com.      IN     NS      ns3.sitese.com.   
ns1.sitese.com.  IN     A       199.168.35.23
ns2.sitese.com.  IN     A       38.124.113.106
ns3.sitese.com.  IN     A       38.128.98.213

Obviously the primary nameserver is ns1.sitese.com as it is listed in the SOA record. But which one becomes the secondary nameserver if the primary is unreachable? Is it ns2, or ns3? What decides the order in which secondary nameservers are queried? Is it the order in which they appear in the zonefile? Is it something else?

Best Answer

AFAIK the order in which NSs are queried is up to the client. Even if your primary is up, a client can elect to query the secondary, if it feels so inclined.

From a client standpoint, all RRs returned by either the secondary or primary NSs are equivalent as all of them are marked as authoritative.

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