Difference between A Rec CNAME in Route53

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As Amazon Route 53 allows you to set an A record or a CNAME to a different sub-domain name using Alias, I am wondering what is the difference between A record and CNAME?

Oct 1, 2018 in AWS by Archana
• 4,170 points

1 answer to this question.

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 Route 53, an A record can give the appearance of referencing either a fully-qualified hostname from another domain, or a hostname or subdomain from its own domain, while in proper DNS, the only way to reference another hostname is with a CNAME, yet those can't be provisioned at the apex of a domain ("example.com") pointing to a subdomain ("www.example.com").

An alias, you're saying, seems to be functionally equivalent but more flexible than a CNAME so is there a difference?

The difference between a CNAME and an alias is that a CNAME is still a proper, standard, ordinary DNS CNAME, which returns a DNS query response "hi, you need to know that the real ("canonical") name of the server you are looking for is 'x'" and, depending on circumstances, may also include the A record for the target host or may not, requiring the client to go for a second lookup.

Aliases, on the other hand, are a clever hack created by Amazon which does not break the protocol yet allows Route 53 to hand out the A record directly from resource "X" when the A record for resource "Y" is requested. Aliases can also reference other record types, but the record type they reference must be the same and for the purposes of this discussion, I am talking about A record aliases.

answered Oct 1, 2018 by datageek
• 2,530 points

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