Resource Records

Updated Jan 16, 2018 ·

Overview

Resource records are the building blocks of DNS zones. They store all the necessary information for a domain or IP.

• Each record stores specific data about a domain or IP
• Examples include A records for IPv4 and AAAA records for IPv6

DNS uses different types of resource records (RRs) to store information about domains and IP addresses. Each type serves a specific purpose in resolving names or providing additional data.

Record Type	Purpose	Example
A	Maps a domain to an IPv4 address	example.com → 93.184.216.34
AAAA	Maps a domain to an IPv6 address	example.com → 2606:2800:220:1:248:1893:25c8:1946
CNAME	Aliases one domain to another	www.example.com → example.com
MX	Mail exchange server for email	example.com → mail.example.com
NS	Authoritative name server for the domain	example.com → ns1.example.net
PTR	Maps an IP address back to a domain (reverse lookup)	93.184.216.34 → example.com
TXT	Stores arbitrary text data	SPF record for email verification

SOA Record (Start of Authority)

The SOA record marks the start of a DNS zone and defines the primary information about that zone.

• Only one SOA record per zone
• It must be the first record in the zone file
• It defines the primary server and administrative contact

The SOA record ensures that DNS servers know which server is authoritative for a zone and who to contact for updates.

SOA Record Fields

These fields define key information for managing a DNS zone, handling updates, and controlling caching.

Field	Description
Domain Name	The domain this SOA record belongs to
TTL	Time the record is cached
Class	Usually IN for Internet
Type	SOA
Primary Name Server	The main authoritative server for the zone
Admin Email	Contact email of the zone administrator (without @, e.g., info.example.com means info@example.com)
Serial Number	Version of the zone, incremented with each change
Refresh, Retry, Expire	Controls synchronization between primary and secondary servers
Minimum	TTL for negative caching

Querying the SOA Record

You can check the SOA record using common tools:

• On Windows with nslookup:

  nslookup -type=soa example.com

  Expected output includes primary server, admin email, serial number, and TTL values.

  Server:  dlinkrouter.local
  Address:  192.168.0.1
  
  Non-authoritative answer:
  example.com
          primary name server = ns.icann.org       
          responsible mail addr = noc.dns.icann.org
          serial  = 2017011748
          refresh = 7200 (2 hours)
          retry   = 3600 (1 hour)
          expire  = 1209600 (14 days)
          default TTL = 3600 (1 hour) 

• On Linux with dig:

  dig -t soa example.com +short

  Or for cleaner output:

  dig -t soa example.com +short | tr " " '\n'

  Sample output:

  ns.icann.org.
  noc.dns.icann.org.
  2017011748
  7200
  3600
  1209600
  3600

NS Records

NS records identify the authoritative name servers for a domain, which makes it accessible on the internet.

• Every zone should at least two NS records for redundancy
• NS records point to main servers holding the DNS information
• Name servers are usually on separate networks for better reliability

If a domain (such as example.com) doesn't have any DNS records configured in its zone:

1. There will be no references to its name servers
2. The .com TLD server cannot return a list of name servers for example.com
3. DNS queries will fail to find the servers
4. Users will be unable to access the domain

This is why every zone should at least have to NS records for redundancy, with each one pointing to a different name server. This ensures that if one name server goes down, DNS queries can still be resolved by another.

When you register a domain, most DNS-as-a-service providers automatically assign three to four name servers, which are typically listed in a format like this:

ns1.digitalocean.com
ns2.digitalocean.com
ns3.digitalocean.com

NS Record Syntax

The format is simple:

example.com. 3600 IN NS ns1.example.com.
example.com. 3600 IN NS ns2.example.com. 

Where:

• example.com. is the domain
• 3600 is the TTL (time to live)
• IN is the class (Internet)
• NS is the record type
• ns1.example.com. is the hostname of the name server

Querying NS Records

• On Windows:

  nslookup -type=ns securityzone.com

  Expected output:

  Server:  resolver.local
  Address:  192.168.1.1
  
  Non-authoritative answer:
  securityzone.com  nameserver = ns1.securityzone.com
  securityzone.com  nameserver = ns2.securityzone.com

• On Linux:

  dig -t ns securityzone.com

  Expected output:

  ;; ANSWER SECTION:
  securityzone.com.  3600  IN  NS  ns1.securityzone.com.
  securityzone.com.  3600  IN  NS  ns2.securityzone.com.

Address Records

Address records connect domain names to their IP addresses so users can reach the correct server.

• A records link domains to IPv4 addresses
• AAAA records link domains to IPv6 addresses

It is possible for both an A record and an AAAA record to point to the same domasin in cases where dual stack is required.

A Record

An A record holds a domain name and its IPv4 address.

example.com. 3600 IN A 192.0.2.10

AAAA Record

An AAAA record holds a domain name and its IPv6 address, which is larger (128 bits vs 32 bits).

example.com. 3600 IN AAAA 2001:db8::10

The reason why its represented as AAAA record is to signify that the value stored in it is four times as big as the one stored in the A record.

Querying Address Records

• On Windows:

  nslookup -type=A example.com
  nslookup -type=AAAA example.com

  Expected output (example):

  Name:    example.com
  Address: 192.0.2.10
  
  Name:    example.com
  Address: 2001:db8::10

• On Linux:

  dig -t A example.com +short
  dig -t AAAA example.com +short

  Expected output (example):

  192.0.2.10
  2001:db8::10

Pointer Records

Pointer records map an IP address back to its domain name, allowing reverse lookups.

• Used for reverse DNS lookups
• Works with both IPv4 and IPv6 addresses
• Found in reverse lookup zones

PTR Records Format

A PTR record contains the following in this order:

• IP address in reverse format
• A special reverse DNS domain
• Record class
• Record type (PTR)
• Fully qualified domain name it points to.

For IPv4, the reverse domain ends with .in-addr.arpa:

8.1.1.10.in-addr.arpa. IN PTR server.example.net.

For IPv6, it ends with .ip6.arpa.

0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. IN PTR server.example.net.

Querying PTR Records

• On Windows:

  nslookup -type=PTR 10.1.1.8
  nslookup 10.1.1.8

  Expected output:

  Name:    server.example.net
  Address: 10.1.1.8

  To query IPv6 records:

  nslookup -type=ptr 2600:: 
  nslookup 2600:: 

• On Linux:

  dig -x 10.1.1.8 +short

  Expected output:

  server.example.net

  To query IPv6 records:

  dig -x 2600:: 
  dig -x 2600:: +short +answer

CNAME Records

CNAME records, or Canonical Name records, map one domain name (alias) to another domain name (the real or canonical name).

• Used to point an alias to another domain
• Common for subdomains, redirects, and domain verification
• Must point to another domain, never to an IP address

A CNAME record contains:

• Alias
• Record class (usually IN)
• Record type (CNAME)
• TTL value,
• Canonical name it points to.

They are useful for making one domain behave exactly like another.

Querying CNAME Records

• On Windows:

  nslookup -type=CNAME ns1.example.net

  Expected output:

  ns1.example.net   canonical name = nameserver.example.net

• On Linux:

  dig -t CNAME ns1.example.net +short

  Expected output:

  nameserver.example.net

Common Uses

In addition to proving domain ownership during service setup, CNAMEs has other use cases:

• Pointing subdomains to main domains

  `www.example.com` → `example.com`

• Map subdomains to their main domain

  `blog.example.net` → `example.net`

• Redirect multiple TLDs to one main domain

  `mydomain.com` and `mydomain.co` → `mydomain.net`

Restrictions

CNAME records make domain management flexible, but they work best when kept simple and direct.

• Cannot point to an IP address
• Cannot point to NS or MX records
• Cannot coexist with another record of the same name

Chaining CNAMEs (one CNAME pointing to another) is possible but discouraged due to slower lookups.

TXT Records

TXT records store text information associated with a domain. They are flexible and can hold both human-readable notes and machine-readable data.

• Can store any text related to the domain
• Multiple TXT records can exist for the same domain
• Used for verification, notes, or email security

A TXT record includes:

• Domain name
• Record class (usually IN)
• Record type (TXT)
• TTL value
• Rext content.

Querying TXT Records

• On Windows:

  nslookup -type=TXT example.net

  Expected output:

  example.net text = "v=spf1 include:_spf.example.net ~all"

• On Linux:

  dig -t TXT example.net +short

  Expected output:

  "v=spf1 include:_spf.example.net ~all"

Common Uses

TXT records are easy to set up and provide a flexible way to attach important text or security data to a domain.

• Store human-readable notes about a domain
• Verify domain ownership for services
• Improve email security with SPF, DKIM, or DMARC records