SERVFAIL errors with FreeBSD using unbound only

[C.L. Martinez]

Hi all,

  I have configured unbound as a cache nameserver in a FreeBSD 10.2 
amd64 (fully patched) host. At the same time, I am using nsd to resolve 
names for my internal hosts.

  But there is a problem: unbound doesn't works/redirect reverse queries 
for IP address to nsd daemon.

  My current unbound.conf:

server:
	interface: 127.0.0.1
	interface: 172.21.55.14
	interface: ::1
	do-ip6: no
	username: unbound
	directory: /var/unbound
	chroot: /var/unbound
	pidfile: /var/run/local_unbound.pid
	auto-trust-anchor-file: /var/unbound/root.key
	access-control: 0.0.0.0/0 refuse
	access-control: 127.0.0.0/8 allow
	access-control: 172.21.55.0/28 allow
	access-control: ::0/0 refuse
	access-control: ::1 allow
	hide-identity: yes
	hide-version: yes
	do-not-query-localhost: no
	

include: /var/unbound/forward.conf
#include: /var/unbound/lan-zones.conf
include: /var/unbound/control.conf
#include: /var/unbound/conf.d/*.conf

stub-zone:
	name: "mydom.org"
	stub-addr: 127.0.0.1 at 5353

stub-zone:
	name: "21.172.in-addr.arpa"
	stub-addr: 127.0.0.1 at 5353


nsd is listening on localhost, port 5353. When I try to do some reverse 
query from a linux client:

[root at cstbbvn01 ~]# nslookup
 > 172.21.55.14
Server:		172.21.55.14
Address:	172.21.55.14#53

** server can't find 14.55.21.172.in-addr.arpa: SERVFAIL
 > 172.21.55.1
Server:		172.21.55.14
Address:	172.21.55.14#53

** server can't find 1.55.21.172.in-addr.arpa: SERVFAIL
 > exit


Every time, a servfail is displayed. All other queries works ok. 
Disabling unbound and using nsd only, all works ok also.

Then, what am I doing wrong with unbound??

Thanks.