Resource records (RRs) are at the core of DNS. They are what DNS clients request. Most of the time, you ask for a record that maps a domain name to an IP address (an A or AAAA record), but there are many other record types. On DNS servers, resource records are typically defined in a zone file.
Basic Structure
For example, a resource record mapping wikipedia.org to its IP address looks like this (output from dig wikipedia.org):
wikipedia.org. 13 IN A 185.15.59.224
A resource record consists of the following fields, in order:
| Name | Meaning | Here |
|---|---|---|
| NAME | Domain name | wikipedia.org. |
| TTL | Time to live (in seconds); specifies how long a DNS client should cache this record before querying the server again. | 13 |
| CLASS | Group/Namespace of the record type; usually IN (Internet). Other classes exist but are rare. |
IN |
| TYPE | Record type (e.g., A for IPv4 address, AAAA for IPv6 address, MX for mail server) |
A |
| RDATA | Type-specific data (e.g., an IPv4 address for A, mail server name for MX). Can be more than one value. |
185.15.59.224 |
The record name wikipedia.org. ends with a dot (.), which represents the root domain (above .com, .net, etc.). See DNS resolving for details.
A name ending with a dot is absolute. Names without a trailing dot are relative and appear mainly in configuration files (such as zone files). On the wire, all names are absolute.
Record Types
The IN resource class defines many record types. In practice, you’ll most often encounter these:
| Record Type | Description |
|---|---|
A |
Maps a DNS name to an IPv4 address |
AAAA |
Maps a DNS name to an IPv6 address |
CNAME |
Maps a DNS name to another DNS name (alias) |
MX |
Specifies the SMTP server for a domain or subdomain |
TXT |
Stores free-form text for a DNS name; used for things like the ACME DNS01 challenge |
SOA |
“Start of authority”; contains administrative info about a DNS zone. Each zone must have one. |
NS |
Specifies an authoritative DNS server for a DNS zone. This is always a DNS name, never an IP address. Each zone should have at least one NS record, or it may be considered “broken”. |
You can query any record type with: dig <domain-name> <record-type>
Time to Live (TTL)
The TTL field specifies how many seconds a DNS client should cache this record before querying the DNS server again.
When you get a resource record directly from an authoritative DNS server, its TTL has the original value.
On the other hand, when you get a resource record from a cache (for example, from a recursive resolver or stub resolver), the TTL value will be reduced by the number of seconds the record has been in the cache.