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

Jonas Schäfer jonas at wielicki.name
Sun Jun 30 08:39:27 UTC 2019


On Samstag, 29. Juni 2019 23:32:41 CEST Dave Cridland wrote:
> 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
> > >XMPP
> > >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.

Thanks for your feedback folks.

Here’s a proposal: https://github.com/xsf/xeps/pull/796 

Please comment on-list.

kind regards,
Jonas
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