Looks like this goes back to the days when Internet with TCP/IP was not yet the only option and when there was also other networks available (Chaosnet). [1]
For the A records you can different classes of addresses. Nowadays we are using the "IN", but the spec also supports others like "CH" (Chaos) and "HS" (Hesiod).[2]
MX record points to mail.example.org (A-record). mail.example.org could then point to two different addresses in the different networks, using the different classes. Depending on what network the client is connected to, it can request specify the class in the query and get address on the correct network.
I guess they could have baked this in to the MX records as well, but that would have made it more complicated.
No. If you read the top-level comment, it sounds like the response directly indicates that these days it would be useful to use the defunct multiple classes (IN vs CH) functionality to instead reference different IP types (v4 or v6) in today's Internet, hence "nowadays". That isn't needed because we can already mix v4 and v6 records within the IN class itself through A/AAAA records.
I see, I haven't had to dig too deep into DNS so it wasn't obvious to me that those were separated like that. I guess, then, that it's only useful to allow us to someday upgrade to some post-TCP/IP world if there's ever a reason for that.
DNS standards tend to favor using aliases rather than IP's. My observation is that the overall goal is to have the IP listed as few times as possible. Otherwise there'd be no need for CNAME's in general, and everything would be an A record or an IP. But, MX records are supposed to refer to an A record not an IP.
The author is correct- if somebody is doing this, they're simply doing it wrong. Standards exist so that we can say "This configuration is wrong" instead of "Lets complicate software to allow people to use any configuration they want."
There is no such thing as an ALIAS record. That’s something your DNS provider made up and shows to you in the interface, but behind the scenes they are providing the same old AAAA and A records as always, along with some sort of auto-updating feature which they will run to detect changes in the address of the name in the “ALIAS record”. How often this will update is anyone’s guess as it’s up to the DNS provider.
RFC 5321 is ambivalent of A vs. AAAA. When discussing the matter, 5321 says "A or AAAA RR". It later says about IPv6 that "The appropriate actions to be taken either will depend on local circumstances." When I re-wrote Gmail's DNS libraries about 6 years ago, I wrote it to start with AAAA and fall back to A. So if you have an MX and AAAA records for those names, you should notice all your inbound from Google comes via IPv6.
In my travels there's a significant portion of the internet which is not IPv6 capable; for example the World Community Grid failed for me when trying to use pure IPv6, and they send emails about things. I'd currently not risk using an IPv6 record for my MX, I suspect a high failure rate delivering email from every service on the planet will follow.
I wake up and see the HackerNews downvote train has hit myself and you, when what you said is 100% correct; gmail-smtp-in.l.google.com is both IPv4 and IPv6.
The network I'm on right now (VPN to work) cannot service IPv6 and I cannot connect to the IPv6 IP for that MX host, but I can easily connect to the IPv4 endpoint.
If it’s an IP address, you’re restricting it to IPv4 or IPv6. If it’s a name, then it can work on both IPv4 and IPv6.
You want pretty much everything but A and AAAA records to use domain names for pointing, so that they’re not tied to a particular version of IP. Keep the IP resolution to the last step only, and let everything else use domain names.
The RFC doesn’t “forbid” using IP addresses in MX records – it’s physically impossible to have them. The fact that the IPv4 address syntax is also parseable as a domain name is unrelated; what is physically stored inside the MX record is a textual domain name.
The RFC is pretty readable, but the tldr is that DNS is more strongly typed than that. It's not just a string-lookup service. There's a wire format for names, which some records carry; there are wire formats for v4 and v6 addresses, which other records carry, but in no situation do the strings "foo.example.com" or "10.1.0.1" actually go over the network when doing a DNS resolution.
