Chapter 4

DNS Record Types

A, AAAA, CNAME, MX, TXT, NS with worked examples.

Learning objectives

  • Choose the correct record type for common tasks
  • Read a zone file snippet and explain each line
  • Avoid CNAME conflicts at the apex

Records are rows in the phone book

Each DNS record is a row: name, type, value, and TTL (time to live). Here is Workshop Co.’s starter zone:

; Workshop Co. — example zone (simplified)
@       3600  IN  NS   ns1.swifthost.ca.
@       3600  IN  NS   ns2.swifthost.ca.
@       3600  IN  A    203.0.113.10
www     3600  IN  CNAME workshopco.ca.
@       3600  IN  MX   10 mail.workshopco.ca.
mail    3600  IN  A    203.0.113.20
@       3600  IN  TXT  "v=spf1 include:_spf.google.com ~all"

@ means the apex domain (workshopco.ca itself).

Record types you must know

A

Hostname → IPv4 address

www → 203.0.113.10

AAAA

Hostname → IPv6 address

www → 2001:db8::10

CNAME

Alias → another hostname

www → workshopco.ca.

MX

Mail for domain → mail server hostname + priority

10 mail.workshopco.ca.

TXT

Free-form text — SPF, DKIM, verification

"v=spf1 ..."

NS

Which nameservers are authoritative

ns1.swifthost.ca.
CNAME rules
  • A CNAME cannot coexist with other records on the same name.
  • Many DNS hosts do not allow CNAME at the apex (@) — use A/AAAA or ALIAS/ANAME if offered.
  • MX targets must be hostnames with A/AAAA records, not raw IPs.

Worked example — point www to a host

Workshop Co. hosts their site at IP 203.0.113.10. Two valid approaches:

ApproachRecordsWhen to use
Direct A record www A 203.0.113.10 Simple, one IP, easy to understand
CNAME to apex www CNAME workshopco.ca.
@ A 203.0.113.10		 Change apex IP once; www follows

Try it yourself — build a mini zone

Workshop Co. adds a booking app at book.workshopco.ca on IP 203.0.113.15. Write the DNS records (name, type, value) you would add.

Answer 
book.workshopco.ca.  3600  IN  A  203.0.113.15

Or if the host gives you a hostname: book CNAME app-host.provider.net.

Spot the mistake

Someone added these records for shop.workshopco.ca:

shop  CNAME  shops.myplatform.com.
shop  TXT    "verified"

Why might this fail?

Answer

CNAME and TXT on the same name shop violate the CNAME exclusivity rule. Move verification to a different name (e.g. _verify.shop) or use A record instead of CNAME.

Quick quiz

  1. Which record type do you use so email to hello@workshopco.ca is delivered?
  2. Can an MX record point directly to 203.0.113.20?
Answers
  1. MX — pointing to a mail server hostname.
  2. No — MX values must be hostnames. Create mail A 203.0.113.20 and @ MX 10 mail.workshopco.ca.