[dns-operations] Introducing CNAME Flattening: RFC-Compliant CNAMEs at a Domain's Root

[Mark Andrews]

In message <CAAF6GDeFhst9DyW1jpgb7JkrPG-KmDSM49ey_Ny+F=wsR_yKrQ at mail.gmail.com>,
 =?ISO-8859-1?Q?Colm_MacC=E1rthaigh?= writes:
> On Sat, Apr 5, 2014 at 3:44 PM, Mark Andrews <marka at isc.org> wrote:
> 
> >
> > ; EDNS: version: 0, flags:; udp: 4096
> >
> 
> ...
> 
> 
> > ;; ANSWER SECTION:
> > _http._tcp.pkg.freebsd.OrG. 3485 IN     SRV     50 10 80
> > pkg0.bme.freebsd.org.
> > _http._tcp.pkg.freebsd.OrG. 3485 IN     SRV     10 10 80
> > pkg0.isc.freebsd.org.
> > _http._tcp.pkg.freebsd.OrG. 3485 IN     SRV     90 10 80
> > pkg0.ydx.freebsd.org.
> > _http._tcp.pkg.freebsd.OrG. 3485 IN     SRV     20 10 80
> > pkg0.nyi.freebsd.org.
> >
> 
> ...
> 
> 
> > ;; ADDITIONAL SECTION:
> > gns0.freebsd.OrG.       3484    IN      A       8.8.178.30
> >
> 
> It'd probably be beneficial for the additional section to contain the
> A/AAAA records sets for the SRV targets here (which are in bailiwick, and
> there's no other zonecut as far as I can tell), and could help avoid
> another round-trip before the connect() can even be called.
> 
> It's speculation on my part, but that behavior might be holding back SRV.
> It's probably a hard-sell to expect browsers to perform two queries instead
> of one, and to double the time they spend in DNS resolution.
> 
> -- 
> Colm

They are marginally more expensive over a raw A/AAAA record and
just as expensive as a CNAME record.  You either wait for the
recursive server to follow the CNAME chain in the first lookup or
you wait for the second lookup.  If the recursive server as the
addresses you don't wait in either case and you get the address
records either in the answer or additional sections.

Add a little more smarts to the recurive server and it can prioritize
the records it add to the additional section based on SRV values.
The brower can do any missing address records lookups while doing
the initial connect.

Mark
-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org