CNAME
records were originally created to allow multiple names that provide the same resource to be aliased to a single "canonical name" for the resource. With the advent of name based virtual hosting, it has instead become commonplace to use them as a generic form of IP address aliasing. Unfortunately, most people who come from a web hosting background expect CNAME
records to indicate equivalence in the DNS, which has never been the intent. The apex contains record types which are clearly not used in the identification of a canonical host resource (NS
, SOA
), which cannot be aliased without breaking the standard at a fundamental level. (particularly in regards to zone cuts)
Unfortunately, the original DNS standard was written before the standards governing bodies realized that explicit verbiage was necessary to define consistent behavior (RFC 2119). It was necessary to create RFC 2181 to clarify several corner cases due to vague wording, and the updated verbiage makes it clearer that a CNAME
cannot be used to achieve apex aliasing without breaking the standard.
6.1. Zone authority
The authoritative servers for a zone are enumerated in the NS records
for the origin of the zone, which, along with a Start of Authority
(SOA) record are the mandatory records in every zone. Such a server
is authoritative for all resource records in a zone that are not in
another zone. The NS records that indicate a zone cut are the
property of the child zone created, as are any other records for the
origin of that child zone, or any sub-domains of it. A server for a
zone should not return authoritative answers for queries related to
names in another zone, which includes the NS, and perhaps A, records
at a zone cut, unless it also happens to be a server for the other
zone.
This establishes that SOA
and NS
records are mandatory, but it says nothing about A
or other types appearing here. It may seem superfluous that I quote this then, but it will become more relevant in a moment.
RFC 1034 was somewhat vague about the problems that can arise when a CNAME
exists alongside other record types. RFC 2181 removes the ambiguity and explicitly states the record types that are allowed to exist alongside them:
10.1. CNAME resource records
The DNS CNAME ("canonical name") record exists to provide the
canonical name associated with an alias name. There may be only one
such canonical name for any one alias. That name should generally be
a name that exists elsewhere in the DNS, though there are some rare
applications for aliases with the accompanying canonical name
undefined in the DNS. An alias name (label of a CNAME record) may,
if DNSSEC is in use, have SIG, NXT, and KEY RRs, but may have no
other data. That is, for any label in the DNS (any domain name)
exactly one of the following is true:
- one CNAME record exists, optionally accompanied by SIG, NXT, and
KEY RRs,
- one or more records exist, none being CNAME records,
- the name exists, but has no associated RRs of any type,
- the name does not exist at all.
"alias name" in this context is referring to the left hand side of the CNAME
record. The bulleted list makes it explicitly clear that a SOA
, NS
, and A
records cannot be seen at a node where a CNAME
also appears. When we combine this with section 6.1, it is impossible for a CNAME
to exist at the apex as it would have to live alongside mandatory SOA
and NS
records.
(This seems to do the job, but if someone has a shorter path to proof please give a crack at it.)
Update:
It seems that the more recent confusion is coming from Cloudflare's recent decision to allow an illegal CNAME record to be defined at the apex of domains, for which they will synthesize A records. "RFC compliant" as described by the linked article refers to the fact that the records synthesized by Cloudflare will play nicely with DNS. This does not change the fact that it is a completely custom behavior.
In my opinion this is a disservice to the larger DNS community: it is not in fact a CNAME record, and it misleads people into believing that other software is deficient for not allowing it. (as my question demonstrates)
There are some services that offer a special functionality of aliasing in the sense of "we'll publish an regular record (probably A
/AAAA
in your case) that we regularly look up from a name behind the scenes" that can be used in this kind of situation instead of a CNAME
record.
This is not that kind of service, in this case it appears that they are just violating standards by publishing a CNAME
record at the zone apex.
This leads to a situation where you have a conflicting set of records at the zone apex.
You have a CNAME
record, claiming that this entire name is an alias of whatever name you specified but then you also have some other records (which isn't possible), such as a SOA
record. (NS
records also appear to be missing, which seems problematic in itself.)
I would say that all bets are off, it's probably largely implementation dependent what will happen to work and what will break but if your goal includes having a reliably available service this is not a good approach.
Best Answer
CNAMEs at the root aren't really supported by any standard, but some providers does it since it's quite handy. The problem, as you've noticed, is that since it's not a standardised solution, the implementations vary, like for example querying the name in the CNAME, resolving it and inserting the IP as an A-record.
I wouldn't recommend this practice, since there's actual reasons for why the root record needs to be an A or AAAA record.
The proper ways to handle this is either to pony up and pay for a static IP from Azure, or alternatively, if you were hosting a lot of these sites, is to get a single static IP for a load balancer/reverse proxy, and let it forward requests to your dynamic web applications.
Or, you could use Cloudflare, since they've actually managed to make an RFC-compliant CNAME at root implementation. https://blog.cloudflare.com/introducing-cname-flattening-rfc-compliant-cnames-at-a-domains-root/