NNPS / TCP port 433

[Russ Allbery]

Julien ÉLIE <julien at trigofacile.com> writes:

>    To aid with this choice, sites SHOULD offer both sets of IMAP (_imap
>    and/or _imaps) and POP3 (_pop3 and/or _pop3s) SRV records in their
>    DNS and set the priority for those sets of records such that the
>    "preferred" service has a lower-numbered priority value than the
>    other.  When an MUA supports both IMAP and POP3, it SHOULD retrieve
>    records for both services and then use the service with the lowest
>    priority value.  If the priority is the same for both services, MUAs
>    are free to choose whichever one is appropriate.  When considering
>    multiple records for different protocols at the same priority but
>    with different weights, the client MUST first select the protocol it
>    intends to use, then perform the weight selection algorithm given in
>    [RFC2782] on the records associated with the selected protocol.

Ah, yes, using this for NNTP is a neat idea.  However, it probably also
implies that we'd want to write a short RFC to document this behavior
(once someone implemented it), since I think by default SRV records are
only interpreted for a single protocol.

> Use of SRV records is not wide-spread at all...

They're widely used in a few places.  Kerberos uses them heavily.  But it
varies wildly based on protocol.

> It seems like e-mail clients haven't implemented it but use other
> mechanisms of autodiscovery or like.

I think email clients mostly use manual configuration, even.  I've yet to
work somewhere where the email servers were autodiscovered.  There's
always some documentation somewhere saying what to enter into the various
fields.

The most natural way to use SRV records, particularly across protocols, is
to ask DNS for the values of all the SRV records in question and then sort
and apply logic to them within the client.  That's what Kerberos does, for
example.  It unfortunately means handling the DNS lookups directly in the
client and not outsourcing them to a program like netcat or telnet that
isn't aware of what protocol you're using.

-- 
Russ Allbery (eagle at eyrie.org)             <https://www.eyrie.org/~eagle/>

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