[Standards] XEP-0368: What does a . for a target mean in _xmpps-client/server records?

Dave Cridland dave at cridland.net
Sat Jun 29 21:32:41 UTC 2019

On Sat, 29 Jun 2019 at 16:56, Ralph Meijer <ralphm at ik.nu> wrote:

> On June 29, 2019 4:32:15 PM GMT+02:00, "Jonas Schäfer" <
> jonas at wielicki.name> wrote:
> >Hi list,
> >
> >It is not clear to me how to interpret, in a library connecting to an
> >service, a single SRV record for _xmpps-{client,server} which has `.`
> >as the
> >target.
> >
> >For RFC 6120 _xmpp-{client,server} records (note the missing `s`), a
> >`.`
> >indicates that the domain does not host an XMPP service at all, so
> >attempting
> >to form a connection should stop right there (most notably, no fallback
> >to
> >domainpart A/AAAA lookup).
> >
> >How should this be interpreted for XEP-0368? Should a `.` indicate "I
> >do not
> >speak direct TLS, but try _xmpp-client records"? Or should it indicate,
> >right
> >away, that there is no XMPP service on the domain?
> According to RFC 2782 it means the service xmpps-client is not available
> at this domain. So I think the answer should be the former. If there is a
> similar record for xmpp-client, though, you can't connect the regular way
> either. Maybe there's still another binding (BOSH, WebSocket) that could
> succeed, but
> defining all possible permutations is a bit much.
I think:

1) A client ought to, if possible, send the two DNS queries in parallel.
2) If this isn't the case, there's no "right" order.
3) Therefore it'd be possible to obtain some records from _xmpp-client, but
afterward get a '.' from _xmpps-client.
4) Therefore the only sensible interpretation is that it says direct TLS
(xmpps-client) is not supported, and says nothing about the traditional
method (xmpp-client).

> >Whatever the consensus is, this should be written down in the XEP I
> >think.
> Agreed.
I'm always for documenting things.

May as well note here that the '.' target explicitly prevents use of the
fallback A/AAAA resolution.

> --
> Cheers,
> ralphm
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