DNS

DNS Resource Records

A Resource Record (RR) contains a specific information about the domain. Some common ones are A record which contains the IP address of the domain, AAAA record which holds the IPv6 information, and MX record which has mail servers of a domain.

A

An A record is used as the primary method of connecting a hostname to an IP address. A records point to IPv4 addresses. An A record will look like this in a zone file.

@                    5m      IN      A       172.217.4.238
blog                 5m      IN      A       172.217.4.238
blog.example.com.    5m      IN      A       172.217.4.238

AAAA

An AAAA record is just like an A record but points to IPv6 addresses instead of IPv4 address.

@                    5m      IN      AAAA    2607:f8b0:4009:812::200e
blog                 5m      IN      AAAA    2607:f8b0:4009:812::200e
blog.example.com.    5m      IN      AAAA    2607:f8b0:4009:812::200e

CNAME

CNAME records are aliases to an A or AAAA record (or both). They point a hostname to an already defined record.

www                  5m      IN      CNAME   example.com.

SOA

The SOA (Start of Authority) records are used to determine how your zone propagates to the secondary nameservers.

       1                 2              3        4    5     6     7
|--------------| |----------------| |--------| |---| |--| |----| |-|
ns1.example.com. admin.example.com. 2017021100 86400 7200 604800 300

1. The primary DNS server (ns1.example.com.).
2. The email address (with the @ replaced with a .) of the person responsible for the domain (admin.example.com).
3. A timestamp that changes whenever you update your domain in the format YYYYMMDD## where ## is the number of times the zone has been updated today (2017021100).
4. The number of seconds before the zone should be refreshed (86400).
5. The number of seconds before a failed refresh should be retried (7200).
6. The upper limit in seconds before a zone is considered no longer authoritative (604800).
7. The negative result TTL (for example, how long a resolver should consider a negative result for a subdomain to be valid before retrying) (300).