DNS (Domain Name System) is just like address book of internet. Each domain has unique + static IPaddress in internet. It eliminates humans to memorize IP address of server (it's easy to memorize omniguru.in rather than IP address like 192.168.1.234).
DNS consists many domain name servers and provides name-to-IPAddress mapping for registered servers.
URL structure: https://telugu.OmniGuru.in
DNS Server types
DNS Server performs forward NS lookup & reverse NS lookup. Forward DNS Lookup is resolving a host name to IP address. Reverse DNS lookup is resolving IP address to host name.
 DNS Resolver
Acts as middleware between client & DNS nameserver.aka recursive resolver.
Get DNS query from client, if it has cache for that query, will send cache data.
Otherwise  <=> ,  <=> , <=> .
Then save data in cache and send it to client.
 Root Nameservers
It responds by pointing the DNS resolver to a TLD NS.
*racks of books
 TLD Nameservers
It responds by pointing to the authoritative NS.
E.g. .com, .in
omniguru.in will be in .in TLD NS.
 Authoritative NS
Provides IP address of domain from DNS records.
*dictionary of rack
DNS records also known as Zone files
Common DNS record types
contains IP address of a domain
Canonical name forwards one domain or subdomain to another domain, No IP address.
Delegate name record creates a domain alias, like CNAME, but it will redirect all subdomains also.
Mail exchanger directs mail to an email server
Name server record stores the name server for a DNS entry.
Admin can store text notes in the record.
Start of authority record stores admin information about a domain.
Service record specifies a port for specific services.
Pinter record provides a domain name in reverse-lookups.
DNS database contains name server records, host-name-to-address records, host-name-aliases records.
The 3 strategies for updating DNS servers are dynamic, read-write, and read-only
The entire hostname has a maximum of 255 characters
Basic function of NS lookup program is to probe the DNS database for various types of information.
/etc/resolv.conf is the name of DNS client configuration file and does the nslookup program to locate a name server.
/etc/hosts file resolves hostname to IP addresses locally.
A record is most common used record type out of all DNS records.