Before I proceed with this article, the following are the MOST IMPORTANT points you should remember as otherwise you wouldnt understand bit. An A Record must ALWAYS contain IP address (map host to IP) Whenever you specify A record it must contain IP address on the Right side. A nameserver which gives non-authoritative answer is usually called 'LAME'. An example of lame delegation is ns1.is configured to have zone information about domain but ns2.was not configured properly and does not have any information about the domain.
The A record is so important in DNS without which the meaning of mapping hostnames to IP would be absurd. Every domain must have atleast 2 nameservers and if i ask each of them, and if they have domain zone information, I will get authoritative answer. So ns1 will answer authoritatively wheras ns2 won't which will be 'lame' until it is set up properly.
If there is a nameserver missing at root level, add the missing nameserver to your domain registrar.
If the nameserver missing at domain level, add the nameserver to the zone file of the domain and update all your secondary nameservers.
Running the dns server 'open' is a big security risk since it answers recursive queries both from inside and outside your network.
We ask ns1.example to resolve outside domain and if we get IP address (A record) in the answer section, then it means it is an 'open dns server'.
The whole meaning of domain names exists today just because of DNS. This DOT is so important and if you forget this you will have nightmares with your dns configuration. simply because it tells to start query from root servers (denoted by dot) 5.
The simplest way of explaining DNS in one line is to map domain name to IP address. MX records (for mail servers) should contain hostnames NOT IPs.
If this check cannot be completed, new checks are started every Retry interval.
If the secondary finds it impossible to perform a serial check within the Expire interval, it discards the zone.