On Sun 15/Dec/2019 17:13:38 +0100 Keith Moore wrote:
On 12/15/19 6:42 AM, Alessandro Vesely wrote:
If we reject [A.B.C.D], why don't we also reject foo.example?
The reason why SMTP requires to accept such stuff, AFAIUI, is to
allow a broken server to still be able to do its job. Is it so?
Compare that to, e.g., DANE, where a certificate error can shut a
IMO, hosts should not be required as a matter of SMTP protocol to use
DNS names, not even to send mail.
In a server to server connection, the HELO name can be used to provide
DNS is simply a convenient way of finding IP addresses and allowing
applications to continue to operate across infrequent changes in IP
To that end, host files would suffice. In addition to associating an
IP number to a name, DNS establishes a delegation hierarchy.
It is not "a control plane for the Internet" (as some are now
saying), it is not a call setup protocol, and it is nowhere nearly
Hmm... not universal. However, the global Internet, given that you
spell it with a capital I, is, well, global. Multiple, non-global
DNSes were one of the doom future scenarios ISOC hypothesized a while
There are very many IP networks that do not use DNS but which still
use SMTP to send mail, even if only to a forwarder. Before web
browsers were commonplace, many networks did not use DNS except to
forward email to the Internet. The popularity of web browsers has
resulted in much wider support for DNS, but there are still many
networks that aren't intended to support humans using web browsers.
For some of those networks, DNS is not a feature, it is a liability -
it adds nothing of value for them and is something that can break and
cause (say) assembly lines to fail.
In a walled garden it makes sense to accept mail from anyone who is
able to connect. To do so globally has proven to be unaffordable.
Anti-spam techniques vary from dropping messages at random to
assessing senders reputation, each has its FPs.
Of course if an organization wants to refuse mail based on a MAIL FROM
address containing an IP address literal, it is free to do so. But
there is no particular reason I can see that an outgoing mail relay
needs to know its own DNS name, or even to have one.
The reason to have a name is that you can port it, along with your
reputation, across IP address changes.
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