[dns-operations] DNS forwarder behavior on response with cname

[Mukund Sivaraman]

On Fri, Dec 07, 2018 at 04:06:29PM +0800, Davey Song(宋林健) wrote:
> Hi folks,
> 
>  
> 
> I noticed my local resolver (set forwarder for . ) send queries for cname
> after it received a response which contains cname in answer section. 
> 
> For example when I dig A www.youku.com @my local resolver . It returns two
> cname RR and A RR in answer section. It is observed that local resolver sent
> a A query on the first cname : ipv6-aserver-heyi.m.taobao.com. The local
> resolver response the client only after it got answer from the cname
> response. It definite introduces a delay in between. Is it normal ?  which
> RFC specify this or is it a implementation consideration? For what purpose ?

See RFC 2181 section 5.4.1:

   Note that the answer section of an authoritative answer normally
   contains only authoritative data.  However when the name sought is an
   alias (see section 10.1.1) only the record describing that alias is
   necessarily authoritative.  Clients should assume that other records
   may have come from the server's cache.  Where authoritative answers
   are required, the client should query again, using the canonical name
   associated with the alias.

		Mukund