バインドの DNS が、応答に SRV レコードの A レコードを追加しない

Issue

[root@vm183 ~]# dig _sip._tcp.example.com.SRV 

; <<>> DiG 9.9.4-RedHat-9.9.4-18.el7 <<>> _sip._tcp.example.com.SRV
;; global options:+cmd
;; Got answer:
;; ->>HEADER<<- opcode:QUERY, status:NOERROR, id:31510
;; flags: qr aa rd ra; QUERY:1, ANSWER:1, AUTHORITY:2, ADDITIONAL:4

;; OPT PSEUDOSECTION:
; EDNS: version:0, flags:; udp:4096
;; QUESTION SECTION:
;_sip._tcp.example.com.     IN  SRV

;; ANSWER SECTION:
_sip._tcp.example.com.  86400   IN  SRV 10 10 5060 sip.example.in.

;; AUTHORITY SECTION:
example.com.        86400   IN  NS  test1.example.com.
example.com.        86400   IN  NS  test2.example.com.

;; ADDITIONAL SECTION:
test1.example.com.  86400   IN  A   192.168.2.174
test2.example.com.  86400   IN  A   192.168.2.190

;; Query time:0 msec
;; SERVER:127.0.0.1#53(127.0.0.1)
;; WHEN:Mon Nov 30 17:22:06 IST 2015
;; MSG SIZE  rcvd:173

• srv ドメインの A レコード

[root@vm183 ~]# 
[root@vm183 ~]# dig sip.example.com +short 
10.10.10.10
[root@vm183 ~]#