After updating the zone file
If you use firewall make sure you do not block port 53 for DNS tcp and udp requests.
By default dns lookups use UDP protocol while zone transfers and notifications use TCP protocol of port 53.
For proper mail delivery, the following anti-spam methos are very important to make sure the email is delivered to users inbox.
Understanding DNS and its working is one of the most difficult computer engineering subject and yet most experienced network administrators struggle in this topic when it comes to DNS zone file writing. (see RFC 1912 section 2.3) In the above example we are mapping each NS records to IP address (A record) thus binding nameservers to IP (that is glue them).
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.
To illustrate this, we have two nameservers running bind for domain
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'.
Search for after updating the zone file:
You can use dig command to lookup NS records at root server level. If there is a nameserver missing at root level, add the missing nameserver to your domain registrar.