Types of DNS Records

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DNS records are basically mapping files that tell the DNS server which IP address each domain is associated with, and how to handle requests sent to each domain. When someone visits a web site, a request is sent to the DNS server and then forwarded to the web server provided by a web hosting company, which contain the data contained on the site.

Various strings of letters are used as commands that dictate the actions of the DNS server, and these strings of commands are called DNS syntax. Some DNS records syntax that are commonly used in nearly all DNS record configurations are A, AAAA, CNAME, MX, PTR, NS, SOA, SRV, TXT, and NAPTR. The following paragraph details the meaning and usage of each of these syntax.

A and AAAA

An A record matches up a domain (or subdomain) to an IP address. In other words, it points your domain name to your servers IP address, which allows web traffic to reach your Server. This is the core functionality of DNS. A typical A record looks like the following:

example.com     A       12.34.56.78

You can also make A records for subdomains you want to direct to your server:

hello.example.com       A       12.34.56.78

Note: You can point different subdomains to different IP addresses.

If you want to point every subdomain of example.com to your IP, you can use an asterisk (***) as your subdomain:

*.example.com   A       12.34.56.78

An AAAA record is just like an A record, but for IPv6 IP addresses. A typical AAAA record looks like the following:

example.com     AAAA        0123:4567:89ab:cdef:0123:4567:89ab:cdef

AXFR

An AXFR record is a type of DNS record used for DNS replication, although there are also more modern ways to do DNS replication. AXFR records are not used in ordinary zone files. Rather, they are used on a slave DNS server to replicate the zone file from a master DNS server. For an example of how to configure Servers nameservers as slave DNS servers using AXFR.

CAA

DNS Certification Authority Authorization uses DNS to allow the holder of a domain to specify which certificate authorities are allowed to issue certificates for that domain.

CNAME

A CNAME record or Canonical Name record matches up a domain (or subdomain) to a different domain. With a CNAME record, DNS lookups use the target domains DNS resolution as the alias's resolution. Here's an example:

alias.com       CNAME   example.com.
example.com     A       12.34.56.78

With this setup, when alias.com is requested, the initial DNS lookup will find the CNAME entry with the target of example.com. A new DNS lookup will be started for example.com, which will find the IP address 12.34.56.78. Finally, visitors to alias.com will be directed to 12.34.56.78.

CNAME records exist so that domains can have aliases. You should not use a CNAME record for a domain that gets email, because some mail servers handle mail oddly for domains with CNAME records. Likewise, MX records cannot reference CNAME-defined hostnames. Also, the target domain for a CNAME record should have a normal A-record resolution. Chaining or looping CNAME records is not recommended.

Note: In some cases, a CNAME record can be an effective way to redirect traffic from one domain to another while keeping the same URL. However, keep in mind that a CNAME record does not function the same way as a URL redirect. A CNAME record directs web traffic for a particular domain to the target domains IP address. Once the visitor reaches that IP address, the local Apache (or other web server) configuration will determine how the domain is handled. If that domain is not configured on the server, the server will simply display its default web page (if any). This may or may not be the web page for the target domain in the CNAME record, depending on how the server is configured.

DKIM

A DKIM record or domain keys identified mail record displays the public key for authenticating messages that have been signed with the DKIM protocol. This practice increases the capability to check mail authenticity. A typical DKIM record looks like the following:

selector1._domainkey.example.com        TXT     k=rsa;p=J8eTBu224i086iK

DKIM records are implemented as text records. The record must be created for a subdomain, which has a unique selector for that key, then a period (.), and then _domainkey.example.com. The type is TXT, and the value includes the type of key, followed by the actual key.

MX

An MX record or mail exchange record sets the mail delivery destination for a domain (or subdomain). A typical MX record looks like the following:

example.com         MX      10  mail.example.com.
mail.example.com    A           12.34.56.78

The above records direct mail for example.com to the mail.example.com server. The target domain (mail.example.com above) needs to have its own A record that resolves to your Server. Ideally, an MX record should point to a domain that is also the hostname for its server.

Your MX records don't necessarily have to point to your server. If you're using a third-party mail service, like <en, you should use the MX records they provide.

Priority is another component of MX records. This is the number written between the record type and the target server (10 in the example above). Priority allows you to designate a fallback server (or servers) for mail for a particular domain. Lower numbers have a higher priority. Here's an example of a domain that has two fallback mail servers:

example.com         MX      10  mail_1.example.com
example.com         MX      20  mail_2.example.com
example.com         MX      30  mail_3.example.com

In this example, if mail_1.example.com is down, mail will be delivered to mail_2.example.com. If mail_2.example.com is also down, mail will be delivered to mail_3.example.com.

NS

NS records or name server records set the nameservers for a domain (or subdomain). The primary nameserver records for your domain are set both at your registrar and in your zone file. Typical nameserver records (you need at least two) look like this:

example.com     NS      ns1.example.com.
example.com     NS      ns2.example.com.
example.com     NS      ns3.example.com.
example.com     NS      ns4.example.com.
example.com     NS      ns5.example.com.

The nameservers you designate at your registrar then carry the zone file for your domain.

You can also set up different nameservers for any of your subdomains. Subdomain NS records get configured in your primary domain's zone file. For example, if you're using Servers nameservers, you could configure separate NS records in your server zone file for the subdomain mail.example.com as shown below:

mail.example.com    NS      ns1.nameserver.com
mail.example.com    NS      ns2.nameserver.com

Primary nameservers get configured at your registrar; secondary subdomain nameservers get configured in the primary domains zone file. The order of NS records does not matter; DNS requests are sent randomly to the different servers, and if one host fails to respond, another one will be queried.

PTR

A PTR record or pointer record matches up an IP address to a domain (or subdomain), allowing reverse DNS queries to function. It performs the opposite service an A record does, in that it allows you to look up the domain associated with a particular IP address, instead of vice versa.

PTR records are usually set with your hosting provider. They are not part of your domains zone file. This means that you'll always set reverse DNS for your servers even if your nameservers are elsewhere. Likewise, if you have servers somewhere else but are using local nameservers, you will still have to set up your PTR records with your hosting provider.

As a prerequisite for adding a PTR record, you need to create a valid, live A or AAAA record that points the desired domain to that IP. If you want an IPv4 PTR record, point the domain (or subdomain) to your servers IPv4 address. If you want an IPv6 PTR record, point the domain to your servers IPv6 address. Beyond that, IPv4 and IPv6 PTR records work the same way.

Note: It's possible to have different IPs (including both IPv4 and IPv6 addresses) that have the same domain set for reverse DNS. To do this, you will have to configure multiple A or AAAA records for that domain that point to the various IPs.

SOA

An SOA record or Start of Authority record labels a zone file with the name of the host where it was originally created. Next, it lists the contact email address for the person responsible for the domain. There are also various numbers, which we'll get into in detail in a moment. First, here's a typical SOA record:

@   IN  SOA ns1.example.com. admin.example.com. 2013062147 14400 14400 1209600 86400

Note: The administrative email address is written with a period (.) instead of an at symbol (<@>).

Here's what the numbers mean:

  • Serial number: The revision number for this domains zone file. It changes when the file gets updated.
  • Refresh time: The amount of time (in seconds) a secondary DNS server will keep the zone file before it checks for changes.
  • Retry time: The amount of time a secondary DNS server will wait before retrying a failed zone file transfer.
  • Expire time: The amount of time a secondary DNS server will wait before expiring its current zone file copy if it cannot update itself.
  • Minimum TTL: The minimum amount of time other servers should keep data cached from this zone file.

The single nameserver mentioned in the SOA record is considered the primary master for the purposes of Dynamic DNS and is the server where zone file changes get made before they are propagated to all other nameservers.

SPF

An SPF record or Sender Policy Framework record lists the designated mail servers for a domain (or subdomain). It helps establish the legitimacy of your mail server and reduces the chances of spoofing, which occurs when someone fakes the headers on an email to make it look like it's coming from your domain, even though the message did not originate from your Server. Spammers sometimes try to do this to get around spam filters. An SPF record for your domain tells other receiving mail servers which outgoing server(s) are valid sources of email, so they can reject spoofed email from your domain that has originated from unauthorized servers. A very basic SPF record looks like the following:

example.com   TXT     "v=spf1 a ~all"

In your SPF record, you should list all the mail servers from which you send mail, and then exclude all the others. Your SPF record will have a domain or subdomain, type (which is TXT, or SPF if your name server supports it), and text (which starts with "v=spf1" and contains the SPF record settings).

If your Server is the only mail server you use, you should be able to use the example record above. With this SPF record, the receiving server will check the IP addresses of both the sending server and the IP address of example.com. If the IPs match, the check passes. If not, the check will "soft fail" (i.e., the message will be marked but will not automatically be rejected for failing the SPF check).

Note: Make sure your SPF records are not too strict. If you accidentally exclude a legitimate mail server, its messages could get marked as spam. We strongly recommend visiting openspf.org to learn how SPF records work and how to construct one that works for your setup. Their examples are also helpful.

SRV

An SRV record or service record matches up a specific service that runs on your domain (or subdomain) to a target domain. This allows you to direct traffic for specific services, like instant messaging, to another server. A typical SRV record looks like the following:

_service._protocol.example.com  SRV     10      0       5060    service.example.com

Here's a breakdown of the elements in an SRV record:

  • Service: The name of the service must be preceded by an underscore (_) and followed by a period (.). The service could be something like _xmpp.
  • Protocol: The name of the protocol must be proceeded by an underscore (_) and followed by a period (.). The protocol could be something like _tcp.
  • Domain: The name of the domain that will receive the original traffic for this service.
  • Priority: The first number (10 in the example above) allows you to set the priority for the target server. You can set different targets with different priorities, which allows you to have a fallback server (or servers) for that service. Lower numbers have a higher priority.
  • Weight: If two records have the same priority, weight is used instead.
  • Port: The TCP or UDP port on which the service runs.
  • Target: The target domain or subdomain. This domain must have an A or AAAA record that resolves to an IP address.

An example use of SRV records would be to set up Federated VoIP.

TXT

A TXT record or text record provides information about the domain in question to other resources on the Internet. It's a flexible type of DNS record that can serve many different purposes depending on the specific contents. One common use of the TXT record is to create an SPF record on nameservers that don't natively support SPF. Another use is to create a DKIM record for mail signing.