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)
I would just let your DNS server do what DNS was designed to do. Don't try to configure the domain in your DNS server. Your DNS server will cache the DNS entries for the TTL specified by the provider. At worst, you will have a stale entry for the TTL specified by the provider.
If you do set up the entry in your DNS, set the TTL to a value no larger than the provider specified. You will still need an system to check the provider's DNS and update your DNS. This will add more complexity to your setup, and increase the risk of failure. You will also generate at least as much DNS traffic as specified above.
It is possible to define TTL on a per entry basis, although I don't have experience doing so in active directory. Your provider is likely doing so on their server.
Best Answer
Typically you would explicitly configure the DNS root servers similar to the snippet below:
Where you observe that zone type is not the common
masternorslavebut a special zone type calledhint.When the name server starts up, it only uses the root hints to find a responding root name server and will then get the current list of root name servers from there. Those are the root servers that will be actually be used during operation.
Since those are cached they will need a TTL and as you have observed: that TTL will decrease like any other cached DNS record.
If no hint zone is specified for class IN, the server uses a compiled-in default set of root servers hints. Source: the Bind Administrator Reference Manual.
As Brian explained in his answer the root zone does change, and as long as at least one name server remains valid the hint zone allows for such changes in the root name servers without requiring all existing name servers to update their static configuration files.