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DNS - Domain Name System ​

DNS is a system that translates domain names to IP addresses. This is used to route traffic via an human readable name to an ip address.

  • DNS names are only used for human readability
  • Servers use IP addresses to communicate with each other

Zones ​

A DNS zone is a segment of the domain namespace managed by a specific organization or administrator. It consists of a collection of DNS records maintained by a DNS server. Zones enable different parts of a domain to be managed by separate DNS servers, providing flexibility and delegation in domain administration.

DNS Records ​

DNS records are used to store information about a domain in a DNS zone file. General format of a DNS record:

text
<name> <ttl> <class> <type> <rdata>
  • <name>: the domain or subdomain
  • <ttl>: time to live (how long the record is cached)
  • <class>: the class of the record (IN for internet)
  • <type>: the type of record (A for address)
  • <rdlength>: length of the rdata field (internaly used)
  • <rdata>: the IP address or data associated with the record

INFO

  • <ttl>, <class> and <type> are optional fields. If not specified, they will be set to default values.
  • <rdlength> is an internal field

A and AAAA Records ​

A record is used to point a domain or subdomain to an IP address. A records are used for IPv4 addresses and AAAA records are used for IPv6 addresses.

text
example.com 3600 IN A 200.0.0.0
example.com 3600 IN AAAA 2001:0db8:85a3:0000:0000:8a2e:0370:7334

CNAME Record ​

CNAME record is used to point a domain or subdomain to another domain. The authoritative DNS server will resolve the CNAME record to the IP address of the target domain.

text
www.example.com 3600 IN CNAME example.com

WARNING

CNAME records cannot be used for root domains (apex) per RFC 2181.

NS Record ​

NS record is used to indicate the authoritative DNS server for a domain. With NS records, multiple subdomains can be managed with different DNS zones.

text
example.com 3600 IN NS ns1.google.com
example.com 3600 IN NS ns2.google.com
example.com 3600 IN NS ns3.google.com
example.com 3600 IN NS ns4.google.com

Often there are multiple NS records for a domain to provide redundancy and load balancing.

MX Record ​

MX record is used to specify the mail servers that are responsible for receiving emails for a domain.

text
example.com 3600 IN MX 10 mail1.example.com
example.com 3600 IN MX 20 mail2.example.com

The number (10, 20) in the MX record is the priority of the mail server. Lower numbers have higher priority.

Other Records ​

  • SOA Record: Start of Authority record is used to specify the authoritative DNS server for a domain.
  • TXT Record: Text record is used to store arbitrary text data.
  • PTR Record: Pointer record is used for reverse DNS lookups.
  • CAA Record: Certificate Authority Authorization record is used to specify which certificate authorities are allowed to issue certificates for a domain.
    text
    example.com CAA 0 issue "letsencrypt.org"