[dns-operations] HTTP SRV, was Adding CNAME for the root domain issue

Tony Finch dot at dotat.at
Fri Apr 29 10:47:20 UTC 2016

John Levine <johnl at taugh.com> wrote:

> >But SRV additional data can't tell a client if the apparently missing
> >target addresses are actually missing. To avoid the extra round trip, the
> >SRV answer needs to be able to say, this target has no A / this target has
> >no AAAA.
> I don't understand why this is a problem.  Wouldn't the client prefer
> the SRV answer to the A/AAAA, so if there's a SRV answer, it doesn't
> matter what the A/AAAA answer is?

I'm talking about the addresses of the SRV target, not the addresses at
the SRV domain. e.g.

_imap._tcp.example.com. SRV target.example.net.

The client requires two round trips, first to query for example.com,
second to query for target.example.net. It can't eliminate the second
round trip because it doesn't know the target name until after the first
round trip, and it can't rely on the additional data being complete.

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