DNS Records

I needed to understand the building blocks of domains, some providers call the same thing by different names. Many more record types exist and you can use tooling like https://www.nslookup.io to query these values. DNS stands for Domain Name System, its most basic job is to resolve Domain names to IP address. DNS has a hierarchy of 3 main servers Root Server -> Top Level Domain Server (TLD) -> Authoritative Name Server

Example request to example.com

  • Root Server, looks at the request and forwards the request to the correct TLD, example.com needs to go to the .com TLD
  • Top Level Domain Server, looks at the request and forwards the request to the correct Authoritative Name Server as it only knows information about top level domains like .com .org ect
  • Authoritative Name Server, knows everything about a domain, example IP address, so it resolves example.com to 12.34.56.78 using its zone file

Understanding the stucture, computers read the domain from right to left

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www.example.com.
               Root domain, its a hidden `.` at the end
            Top level domain, its the `com`
    2nd level domain, so the `example` bit
Subdomain, so the `www` bit

All records have a time to live (TTL) which is how long they should be cached for on the machine making the request. Example 7200 which is seconds, so 2 hours. (7200 / 60 minutes/hour = 120 minutes, 120 minutes / 60 minutes/hour = 2 hours).

A Record (Address)

This is the most common record, this is an IPv4 address (32 bit numeric address) that is resolved from the apex / root domain. Note an AAAA is the same thing but for IPv6 (128 bit alphanumeric address).

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Type  | Value               | Resolves to                      | Notes                                                                      |
------|---------------------|----------------------------------| ---------------------------------------------------------------------------|
A     | example.com         | 12.34.56.78                      | Commonly denoted with @, the address is IPv4                               |
AAAA  | example.com         | 2501:0:53b::3330:c2f4            | IPv6                                                                       |
CNAME | www.example.com     | example.com                      |                                                                            |
CNAME | ftp.example.com     | example.com                      | The webserver then directs the traffic to the FTP service based on the URL |
TXT   | _dnsauth            | _dx0vfgiwo433rjgh39zcv3ox8fxnlje |                                                                            |

CNAME Record (Canonical)

Typically used to resolve a subdomain such as www or mail to the domain hosting that subdomain’s content.

The root domain it resolves to however doesnt have to be the same, example www.foo.com can have a CNAME of bar.com

This is basically and ALIAS and you can also create an A record to point www.example.com -> example.com

CNAME Flattening

This is not a standard CNAME and is as a workaround, allowing you to effectively proxy your apex domain.

Note that CNAME Flattening, DNS chasing, ANAME and ALIAS are all the same thing and allow you to get the root ip addresses for a domain from a different domain. So that would allow the provider to maintain all the clients host records from a domain they control.

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client1.com                          -> CNAME for client1.myagencyrecords.net is -> A record for server1.myagencyrecords.net
CNAME is client1.myagencyrecords.net    server1.myagencyrecords.net                 is 135.148.47.71

TXT Record

Miscellaneouse domain information, can be used to verify domain ownership, ensure email security, and prevent spam and phishing. Also used to handle outgoing email.

MX Record

Mail exchanger record used for email. When you send an email to spam@example.com the mail transfer agent (MTA) will query the MX records for example.com looking for the email server(s). The DNS could respond back with mail1.example.com which is where it should send the email to.

There are normally two entries, primary and secondary. This is denoted based on the priority field, lower is primary.

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Type  | Priority | Name        | Host                     |
------|----------|-------------|--------------------------|
MX    | 10       | example.com | mail1.example.com        |
MX    | 20       | example.com | mail2.example.com        |

SOA Record

Start of authority, stores administrative information about a DNS zone. A DNS zone is a section of a domain name space that a certain administrator has been delegated control over. DNS zones allow a domain namespace to be devided into different sections. Examples shop.example.com, blog.example.com and support.example.com

You could have DNS ZONE 1 for shop.example.com and blog.example.com if together they only have a few computers and have one administrator to manage it. Then have DNS ZONE 2 for support.example.com if it has several times more computers and another administrator to manage it.

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Type  | MName (Primary name server)  | RName (email of the administrator, the left dot is actually an @) | Serial # (version in the zone) | Retry |
------|----------------------------- |-------------------------------------------------------------------|--------------------------------|-------|
SOA   | ns1.example.com              | admin.example.com                                                 | 510025                         | 60    |

NS Record

Name Server record provides the name of the authoritative name sever within a domain.

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Type  | Value            | Name        | Notes      |
------|--------------------------------|------------|
NS    | ns1.example.com  | example.com | Primary    |
NS    | ns2.example.com  | example.com | Secondary  |

SRV Record

Service record, points to a server including a port number, this is useful for Voice Over IP (VOIP), instant messaging, printers ect

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Type  | Priority | Service             | Port | Name        | Weight |
------|----------|---------------------|------|-------------|--------| 
SRV   | 10       | service.example.com | 999  | example.com | 0      |

PTR Record

Pointer record, this is basically the reverse of an A or AAAA record, so you give the IP 12.34.56.78 and get the DNS example.com. They are attached to email and are used to prevent email spam.

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Type  | IP Address  | Name        |
------|-------------|-------------| 
PTR   | 12.34.56.78 | example.com |

Examples, so an email sent from gmail.com should include 209.85.220.41, if it doesnt it will be flagged as spam.

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IP            | Email domain |
--------------|--------------| 
98.137.68.31  | yahoo.com    |
209.85.220.41 | gmail.com    |
12.101.27.50  | aol.com      |
25.145.71.56  | hotmain.com  |

References