What is the A-Record entry of a domain in the nameserver

The address directory for domains on the internet consists of a decentralized network of Domain Name Servers (DNS) that map a website to a globally unique address in Internet Protocol Version 4 (IPv4). Information about a specific domain is stored in a predefined format known as an A-Record, while a separate AAAA-Record stores the IPv6 address for Internet Protocol Version 6 (IPv6). When a website is accessed or an email is delivered, the services used such as browsers or mail servers contact a Nameserver responsible for the connection. Upon request, the Nameserver sends the A-Record registered for the respective domain, thus resolving addresses into a format readable by computers.

What entries does a DNS Nameserver store?

The IP address stored in the A record has the task of uniquely identifying the server responsible for a registered domain. To achieve this, DNS stores all relevant information in a series of records - referred to by the English terms A, C, or MX-Record - each consisting of one or more entries. In total, when queried, DNS nameservers provide a client with up to eight different entries:

• A-Record: contains the IPv4 addresses belonging to a domain and its subdomains
• AAAA-Record: points to the IPv6 addresses of domains and subdomains
• CNAME-Record: links the name of a domain to another (sub)domain
• MX-Record: includes information for a mail server about responsible domains
• PTR Resource Record: allows mapping an IP to a domain (Reverse Lookup)
• TXT Resource Record: stores a user-defined text for a name
• SOA Resource Record: data about query parameters and the zone of the nameserver
• NS Resource Record: configuration, status, and linkage of the DNS

For setting up a website or mail server, only the first four records have a relevant significance, while the following four have a more descriptive or supplementary nature. Users also have no direct influence on some records containing technical parameters - for example, the NS Resource Record. The settings listed therein, which are globally valid on the nameserver, are determined by the operator. The records are solely data. Network protocols used or security measures like encryption via TLS, SSL, or StartTLS are not related to an A record. Entries can be made independently by customers with a good domain provider.

What is the A-Record syntax for entries?

The A record, like all entries in a DNS, consists of simple, non-binary encoded plaintext. Each line describes a separate entry referring to a domain or subdomain. To prevent erroneous entries, the nameserver or web interface subjects each change to a formal A record check, verifying the content and permission to manipulate entries.

A typical A record consists of various parameters, not all of which are mandatory. These include:

• Public name of the domain or subdomain
• (Optional) Time to Live (TTL) is the maximum time an A record is valid in the cache
• (Optional) Class of the entry, default is IN for Internet
• Type of entry, here the letter A for A record
• (Optional) Length of the entry in bytes
• IP address of the dedicated or virtual server hosting this domain

If details for TTL, class, and length are missing, the nameserver will use default values, either globally defined or locally set in the first lines of the A record. Each A record follows the order given above, separated by one or more spaces, resulting in the following A record syntax (items in parentheses can be omitted):

() () ()

A typical A record: Examples with explanation

The following examples show an A record for example.org and two more for subdomains, one for an online shop on an external server and the other for an independent mail server:

example.com. 3600 IN A 0.0.0.0
shop.example.com. 3600 IN A 1.1.1.1
mail.example.com. 7200 IN A 2.2.2.2

The formal convention dictates that there should be an additional dot (.) after each name. The first two entries define a 3600-second or two-hour deadline for a client to store the A record in the cache. After this period - in the case of the mail server, only after four hours - a new query is made.

What alternatives to an A record exist?

Addresses in the IPv6 format are stored in an AAAA record, which has a similar structure to the A record. In other cases, it may be more appropriate to use a CNAME record instead, which points from one name to an A record. Some commonly used examples are:

www.example.com. IN CNAME example.com.
ftp.example.com. IN CNAME example.com.
example.org IN CNAME example.com.

Here, all three names refer to the domain example.com, so that their server accepts any request addressed to one of the subdomains.

How to enter or modify an A record?

Every major domain provider operates its own nameservers, which its customers use after registering an internet address. Access is through the web interface in the customer area, where they can create a new record and manage existing entries. An integrated A record check prevents incorrect configurations and abuse.

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