What is a DNS CNAME record?

There are several types of records - or Resource Records as they are called - in the Domain Name System (DNS). This page explains what the CNAME record is and how it's used.

The purpose of a DNS CNAME record

The CNAME record is a special kind of DNS record. If can be looked at as an alias or - in Linux terms - a symlink to another record.

A CNAME isn't just an alias for a single record type, but for the entire host that it applies to. Let's explain that when diving into the structure of a CNAME record next.

The structure of a DNS CNAME record

This is an example of a CNAME record:

www.ohdear.app.		60	IN	CNAME	ohdear.app.

With this, we have instructed that every DNS lookup for www.ohdear.app should be forwarded to ohdear.app instead. As a result, when a client (ie: your webbrowser) wants to resolve www.ohdear.app, it will make another DNS query to ohdear.app and use that response as the canonical value (= CNAME) of the www.-subdomain.

The structure is similar to A or CNAME records:

<host>          <TTL>   IN  CNAME   <domain-name>

A CNAME must point to another domain name, never to an IP address.

Special notes on DNS CNAME records

This kind of record can be a little confusing. There's one important note on CNAME records, and that is that once you define a CNAME record for a host record, no other record can be present.

For instance, this isn't allowed:

www.ohdear.app.		60	IN	CNAME	ohdear.app.
www.ohdear.app.		60	IN	MX	    10 mailserver.ohdear.app.
www.ohdear.app.		60	IN	TXT	    "google-site-verification=123".

Because a CNAME was issued for www.ohdear.app, you cannot define a MX or TXT record.

Instead, the CNAME instructs every other resource record to be found at the target (in this case: ohdear.app) instead.

If you were to request the MX record for www.ohdear.app in the above example, you would get back the MX record defined at the CNAME target instead: ohdear.app. This is why there can be no overlap with CNAME records.

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