> (i) RFC 2821 can be read (and was intended to be read) > to prohibit the use of an address literal in a HELO or > EHLO command unless the relevant host has no DNS name. > (sections 3.6, 4.1.1.1, 4.1.4)
these days it's sort of odd to think that a host has a distinguished DNS name - hosts quite ordinarily have either an emphemeral DNS name, multiple DNS names, or no DNS name. > (ii) The use of address literals is described as a > mechanism to bypass a barrier, not one for normal use > (RFC2821, section 4.1.3) right. about the only reasonable use of an address literal is for testing, or to reach the postmaster at a particular host associated with a particular address (since "postmaster" is the only address that is guaranteed to be valid when associated with an address literal - and even this is often not true in practice) > (iii) On the other hand, the address literal "should" > still be provided in the From clause of a Received > field. Received field information is expected to not be > picked up by other software and protocols, but the > inclusion of address information there is very > leak-friendly. this is different than using address literals in addresses. email addresses are defined relative to DNS names because you cannot properly send mail to an email address without an MX lookup. OTOH MTAs are still expected to be hosts with addresses. > > of course it is possible to write apps that do not use DNS, > > but this is rarely done. > > Yep. And as pointed out earlier, we have pushed back strongly > against such protocol proposals and implementations. many apps that are used in practice are not standardized; we need to be careful about believing that what's good for standard apps is good for every app. I could certainly make a case for some apps to have their own naming systems and their own name-to-address lookup mechanisms independent of DNS, or more generally, for alternate means of mapping resource names (say URNs) to IP addresses that did not involve DNS names or DNS queries. But it's difficult to believe that such apps would not employ DNS names at all - if nothing else, for initial configuration.
