BIND9 resolver SERVFAIL in IPv4 only network

- Problem statement

BIND9 resolver returns SERVFAIL if following all conditions are met:

 1. BIND 9 resolver [1][2] in IPv4 only network environment [3].
 2. Resolving a zone whose all NS’s are dual stack.
 3. When all A RRSets of NS hosts expired, while AAAA not expired
    in resolver cache.

BIND9 in IPv4-only network is very common. Also, expiration of all NS's A RRset
can occur especially when A RRsets TTL are shorter than AAAA RRset TTL.

[1] I confirmed this problem with 9.9.4-P2 and 9.8.6-P2
[2] I confirmed this problem with FreeBSD 10.0 and CentOS 6.4
[3] Only IPv4 address is assigned to network interface.
   (IPv6 link local addresses may be assigned)

- How to repeat

I've set up and published a zone (hdais.net) for you can repeat
this problem easily; the zone’s NS are all dual stack, whose A RRset TTL is
much smaller than AAAA RRset TTL.

 1. Set up BIND9 resolver in IPv4 only network environment.
 2. Launch resolver: "named -c /dev/null"
 3. "dig @127.0.0.1 www.hdais.net" for some times.


- Additional Information

This problem can be avoided with these workarounds:

 * For resolver side, launch resolver with "named -4 -c /dev/null"
 * For authoritative server, configure one of NS hosts
   as single stack (A-RRset only host)

Of course this problem doesn't occur if resolver is running
in dual stack network environment.

- Test log example

# First query for www.hdais.net results proper answer.
#
freebsd10:~ % drill @127.0.0.1 www.hdais.net
;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 42947
;; flags: qr rd ra ; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 4
;; QUESTION SECTION:
;; www.hdais.net.	IN	A

;; ANSWER SECTION:
www.hdais.net.	5	IN	A	49.212.156.90

;; AUTHORITY SECTION:
hdais.net.	86400	IN	NS	ns2.autosign.info.
hdais.net.	86400	IN	NS	ns1.autosign.info.

;; ADDITIONAL SECTION:
ns1.autosign.info.	10	IN	A	27.120.111.185
ns1.autosign.info.	86400	IN	AAAA	2001:2e8:67a:0:2:1:0:39
ns2.autosign.info.	10	IN	A	49.212.156.90
ns2.autosign.info.	86400	IN	AAAA	2001:e41:31d4:9c5a::1

;; Query time: 0 msec
;; SERVER: 127.0.0.1
;; WHEN: Sun Jan 26 21:35:47 2014
;; MSG SIZE  rcvd: 184


# 10 seconds later, ns[12].autosign.info A RRSet expired.
# Named starts to return SERVFAIL
#
freebsd10:~% drill @127.0.0.1 www.hdais.net
;; ->>HEADER<<- opcode: QUERY, rcode: SERVFAIL, id: 10738
;; flags: qr rd ra ; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;; www.hdais.net.	IN	A
<snip>

# Further www.hdais.net queries also result SERVFAIL
#
freebsd10:~ % drill @127.0.0.1 www.hdais.net
;; ->>HEADER<<- opcode: QUERY, rcode: SERVFAIL, id: 2128
;; flags: qr rd ra ; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;; www.hdais.net.	IN	A
<snip>