On Fri, Aug 22, 2014 at 7:36 AM, Tom Thorogood
<me(_at_)tomthorogood(_dot_)co(_dot_)uk> wrote:
On 21 Aug 2014, at 1:28 am, Phillip Hallam-Baker
<phill(_at_)hallambaker(_dot_)com> wrote:
It is now possible to make a complicated DNS discovery request for the
same latency cost as traditional A record look up:
Traditional query:
example.com ? A
Complex discovery
example.com ? A
example.com ? AAAA
_http._tcp.example.com ? SRV
_http._tcp.example.com ? POLICY
_80.example.com ? TLSA
Just to weigh in solely on your example here. I don't believe it makes
complete sense to query A/AAAA records for example.com at that point. Until
the SRV record has been queried you can't know what server the http protocol
is handled by. Or is this a form of collateral where that query takes place
to quicken legacy lookups? (Those lacking SRV records).
Apologies if any of this is off track at all.
The reason that you would do that is that it allows an SRV record to
dominate the A/AAAA when it is present. So the algorithm for discovery
would be
* Was an SRV record returned? If so us it and expect that the A/AAAA
record would be returned as an additional RR
* Was a AAAA record returned? If IPv6 is available, use that.
* Fallback to using the A record result.
Making the discovery mechanism standard and protocol independent
allows an intelligent DNS resolver or authoritative to return the
result most likely to be useful because it knows what else is likely
to be relevant.