An ALIAS record maps one DNS record to another record of the same type. DNS Check can monitor ALIAS records, and verify that they're working correctly. For example, in the screenshot below we're checking if the widgets.com A record maps to the same IP address(es) as the www.widgets.com A record:
This differs from a CNAME record in the following ways:
- ALIAS records can be used for a domain's APEX
CNAME records are incredibly handy, but have the limitation of not being usable for the apex (or root) of a domain. This is also sometimes called the "naked domain". In the above screenshot, widgets.com is the apex domain.
ALIAS records do not share this limitation. In fact, this is one of the primary reasons that ALIAS records were created.
- ALIAS records require a single DNS lookup. CNAME records might require multiple lookups.
ALIAS records transparently alias one domain name to another. In other words, if you do a DNS lookup on an ALIAS record, the response will contain the aliased record's values, without any reference to the fact that an ALIAS is in place.
By contrast, if you do a lookup on a DNS record that is actually a CNAME, the DNS resolver will sometimes need to do two or more lookups. For example, if you lookup the track.dnscheck.co A record, what gets returned is a CNAME record pointing to mandrillapp.com. Your DNS resolver will then need to do a lookup on the mandrillapp.com A record, unless an upstream resolver already did this for you.
- ALIAS records apply to a single DNS record type
If you create an ALIAS record which maps the www.widgets.com A record to widgets.com, then this mapping only applies to A records.
CNAME records, by contrast apply to all record types.
If you create a CNAME record which aliases www.example.com to example.com, then all example.com DNS records get aliased to www.example.com. This could include A, AAAA, MX and TXT records, for example.
- ALIAS records are non-standard. CNAME records are a widely supported standard
ALIAS records are non-standard. They aren't specced in any RFCs, and are not supported by most DNS servers or hosting providers.
ALIAS records also don't have a standard name at this point. Other names, like "ANAME" are sometimes used for DNS records which offer similar functionality.
Those DNS hosting providers which do support ALIAS records differ in their implementations. For example, some DNS providers support ALIAS record functionality for A and AAAA records only, while others support more record types.
Here are some DNS hosting providers that offer what we refer to as ALIAS records:
- Amazon Route 53
- DNS Made Easy refers to these DNS records as "ANAME records"
- easyDNS refers to these DNS records as "ANAME records"
Most DNS providers support CNAME records.
How to Check ALIAS Records
When entering a new ALIAS record into DNS Check, the following fields are present:
|Name||The fully qualified domain name of the record that's being aliased.||widgets.com|
|Type||The DNS record type being aliased.||A|
|Value||The fully qualified domain name that the Name aliases.||www.widgets.com|
|Require exact match|
With this option On, the records returned when looking up the Name must exactly match the records returned when looking up the Value.
With this option Off, the records returned when looking up the Name must be a subset or exact match of the records returned when looking up the Value.