CNAME Record in Additional section

Question

RFC 1034+1035 state the CNAME records should cause no additional section processing. But I am seeing an increasing trend of services like wordpress sending a CNAME chain with one part of the chain in the additional section. So, without parsing the additional section you cannot decode the DNS response.

Example:

;; QUESTION SECTION:
;after12.failblog.org.      IN  A

;; ANSWER SECTION:
after12.failblog.org.   3600    IN  CNAME   chzallnighter.wordpress.com.
vip-lb.wordpress.com.   300 IN  A   72.233.104.123
vip-lb.wordpress.com.   300 IN  A   76.74.255.117
vip-lb.wordpress.com.   300 IN  A   74.200.247.187
vip-lb.wordpress.com.   300 IN  A   74.200.247.59
vip-lb.wordpress.com.   300 IN  A   76.74.255.123
vip-lb.wordpress.com.   300 IN  A   72.233.127.217

;; AUTHORITY SECTION:
wordpress.com.      14400   IN  NS  ns1.wordpress.com.
wordpress.com.      14400   IN  NS  ns2.wordpress.com.
wordpress.com.      14400   IN  NS  ns3.wordpress.com.
wordpress.com.      14400   IN  NS  ns4.wordpress.com.
wordpress.com.      14400   IN  NS  ns5.wordpress.com.
wordpress.com.      14400   IN  NS  ns6.wordpress.com.

;; ADDITIONAL SECTION:
chzallnighter.wordpress.com. 300 IN CNAME   vip-lb.wordpress.com.
ns1.wordpress.com.  14400   IN  A   72.233.69.14
ns2.wordpress.com.  14400   IN  A   76.74.159.137
ns3.wordpress.com.  14400   IN  A   64.34.177.159
ns4.wordpress.com.  14400   IN  A   72.233.104.98
ns5.wordpress.com.  14400   IN  A   69.174.248.140
ns6.wordpress.com.  14400   IN  A   64.34.174.135

Note chzallnighter.wordpress.com. 300 IN CNAME vip-lb.wordpress.com. is in the Additional section and without it you cannot get to A records in the Answer.

Why are providers using such a tactic and what does it really buy them ?

Answer 1

The server you're talking to happens to be authoritative for both domains and gives you whatever it has. A smart client could optimize for this type of response, avoiding round-trips. One of those stretch-the-spec "make the web faster" type things.

A naive client might not notice that the server is authoritative for the CNAMEd domain, discard the additional data and re-query - ensuring a correct, secure result. A smart client can short-circuit a second query by noticing that it had asked server X for the original domain and that server X is also in the list of authoritative servers for the CNAMEd domain. Therefore the CNAME in the additional section is trustable.