About DNS

FQDN -- Fully Qualified Domain Name

TTL -- Time To Live 

TLD -- Top Level Domain 

gTLD -- Generic Top Level Domains

CCTLD -- Country Code Top Level Domains

RTT -- Round Trip Time

 #> dig +trace domain_name

DNS Queries:

1. Recursive query
2. Iterative query OR Nonrecursive query
3. Inverse queries

PS: Root server's are always iterative servers.

DNS port 53 (基於 UDP）

1. Zone files some special keywords:

1. A - A record  (IP 地址）
2. NS - Name Server 
3. MX - Mail for eXchange
4. CNAME - Canonical Name （別名）
5. PTR - Pointer （Used for Reverse Lookup)
6. SOA - Start of Authority

2. MX record should nerver point to a CNAME