The Domain Name System (DNS) is a worldwide directory that maps names to addresses and vice versa. It also provides many other types of information about computing resources. Although the DNS operates largely hidden from view, the global Internet would not work without it.

This guide explains what DNS is, how it works, and how to build your own authoritative name server.

This is the first in a series of four DNS guides. Future guides cover primary and secondary name server setup, securing DNS with DNSSEC, and other security measures to address user privacy concerns.

What Is DNS?

DNS is a global directory of computing resources. It can locate servers, PCs, phones, IoT devices, routers, switches, and essentially anything with an IP address. DNS also holds information linking computers to security keys, services such as instant messaging, GPS coordinates, and much more.

DNS design is hierarchical and distributed. The diagram below illustrates both points. It shows how DNS divides all resources into domains, and delegates different levels of name servers to provide information within each domain.

Learning about DNS records and system structure.

The Domain Name System (DNS) is the Internet’s address book. DNS directs web traffic to your server and email to your inbox by mapping memorable domain names like example.com to IP addresses like 12.34.56.78 or 0123:4567:89ab:cdef:0123:4567:89ab:cdef. This guide introduces basic DNS concepts and the different types of DNS records.

How DNS Works

Before adding any DNS records, you should learn the basics of DNS. You’ll start by dissecting a domain name, and then you’ll learn about the mechanics of DNS resolution, including name servers, zone files, and individual DNS records.

Domain Names

Domain names are best understood by reading from right to left. The broadest domain classification is on the right, and become more specific as you move to the left. In the examples below, the top-level domain, or TLD, is .com.

    example.com
    mail.hello.example.com

Every term to the left of the TLD and separated by a period is considered a more specific subdomain, although conventionally, first-level subdomains plus their TLDs (example.com) are referred to as “domains.” Moving to the left, hello and mail are the second- and third-level subdomains, respectively. Typically, subdomains are used to uniquely identify specific machines or services, but this is left up to the domain owner.

Name Servers

Choosing and specifying name servers is an essential part of domain ownership. If you don’t, the Internet won’t know where to find your DNS information, and your domain won’t resolve. Name servers host a domain’s DNS information in a text file called the zone file. They’re are also known as Servers of Authority (SOAs). You can host your DNS information on name servers in one of several locations:

• Your registrar
• Your own DNS server
• Third-party DNS hosting

However, you can look into the options offered by your registrar and third-party DNS hosts, or host your own DNS if you want to take control of as much of the DNS process as possible.

You’ll specify name servers on your domain registrar’s website. They’ll take care of publishing that information to the higher-level name servers. You’ll want to specify at least two name servers. That way, if one of them is down, the next one can continue to serve your DNS information.

DNS Records and Zone Files

The next aspect of DNS management is specifying DNS records, which actually match domain names to IP addresses. The DNS records are then automatically bundled up into a zone file, which is what allows the Internet to look up the correct IP address for your domain. If you decide to use cloudflare’s name servers, our DNS Manager will help you create a default zone file. It contains records similar to the following:

    ; example.com [448369]
    $TTL 86400
    @   IN  SOA ns1.cloudflare.com. admin.example.com. 2013062147 14400 14400 1209600 86400
    @       NS  ns1.cloudflare.com.
    @       NS  ns2.cloudflare.com.
    @       NS  ns3.cloudflare.com.
    @       NS  ns4.cloudflare.com.
    @       NS  ns5.cloudflare.com.
    @           MX  10  mail.example.com.
    @           A   12.34.56.78
    mail        A   12.34.56.78
    www         A   12.34.56.78

Every domain’s zone file contains the admin’s email address, the name servers, and the DNS records. Of course, you are not limited to these default entries. You can create a variety of DNS records for as many different subdomains as you wish.

DNS Resolution

So how does DNS actually work? First, the domain name needs to get translated into your server’s IP address. DNS matches human-friendly domain names like example.com to computer-friendly IP addresses like 12.34.56.78. This happens in a special text file called a zone file, which lists domains and their corresponding IP addresses (and a few other things). A zone file is a lot like a phone book that matches names with street addresses.

Here’s how the DNS lookup process works:

1. You type a domain name like example.com in to the address bar.
2. Your computer connects to the Internet through an Internet Service Provider (ISP).
3. The ISP’s DNS resolver queries a root nameserver for the proper TLD nameserver. In other words, it asks the root nameserver, “Where can I find the nameserver for .com domains?”
4. The root nameserver responds with the IP address for the .com nameserver.
5. The ISP’s DNS resolver visits the .com nameserver, using the IP address it got from the root nameserver. It asks the .com nameserver, “Where can I find the nameserver for example.com?”
6. The .com nameserver responds with the IP address for the example.com nameserver.
7. The ISP’s DNS resolver visits your domain’s nameserver and reads the zone file.
8. The zone file shows which IP address goes with the domain.
9. Now that the ISP has the IP address for example.com, it connects you to your cloudflare.
10. Apache handles everything after that, ensuring that the correct files and folders get displayed in your visitor’s browser.

The scenario described above is what happens if the ISP has no current information about the requested domain. In actuality, ISPs cache a lot of DNS information after they’ve looked it up the first time. This results in faster lookups and less strain on DNS servers. Usually caching is a good thing, but it can be a problem if you’ve recently made a change to your DNS information, like when you move to cloudflare from a different hosting provider. In those cases, you’ll want to pay attention to your zone file’s time to live (TTL) so that your DNS change happens as quickly as possible.

Types of DNS Records

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 server’s 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 cloudflare’s 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 cloudflare’s nameservers as slave DNS servers using AXFR.

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 domain’s 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 domain’s 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 cloudflare. 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 Google Apps, 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.cloudflare.com.
    example.com     NS      ns2.cloudflare.com.
    example.com     NS      ns3.cloudflare.com.
    example.com     NS      ns4.cloudflare.com.
    example.com     NS      ns5.cloudflare.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 cloudflare’s nameservers, you could configure separate NS records in your cloudflare 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 domain’s 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 domain’s zone file. This means that you’ll always set reverse DNS for your Linodes in the cloudflare Manager, even if your nameservers are elsewhere. Likewise, if you have servers somewhere else but are using cloudflare’s 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 cloudflare’s IPv4 address. If you want an IPv6 PTR record, point the domain to your cloudflare’s 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.cloudflare.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 domain’s 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 cloudflare. 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 cloudflare 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.

Références:

- '[DNS Manager](/docs/networking/dns/dns-manager)'
- '[Reverse DNS](/docs/networking/dns/setting-reverse-dns)'
- '[Common DNS Configurations](/docs/networking/dns/common-dns-configurations)'
- '[DNS for Rocket Scientists](http://zytrax.com/books/dns/)'