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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] Fo
by jpalomaki 6y ago
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.
Disclaimer: Just googled this up.
[1] https://serverfault.com/questions/663112/why-cant-mx-records-point-to-an-ip-address https://serverfault.com/questions/663112/why-cant-mx-records...
[2] https://serverfault.com/questions/220775/what-does-the-in-mean-in-a-zone-file/220787 https://serverfault.com/questions/220775/what-does-the-in-me...
- Natsu 6y agoI guess nowadays that should still be useful to give both an IPv4 and IPv6 address for the mail server?
- deleted 6y ago[deleted]
- aussiegeek 6y agoYep, just have A and AAAA records for the hostname the MX points to
- herpderperator 6y agoWhy? You can just make mail.example.com resolve to two IPs: an IPv4 and IPv6 address. Nothing wrong with that.
- remram 6y agoIsn't that what Natsu said?
- herpderperator 6y agoNo. 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. If I misunderstood the comment, then my bad.
- Natsu 6y agoI 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.