When using Server=host.domain in zabbix_agentd.conf, the agent sends AAAA queries without IPv6 available on the operating system.

IPv6 totally disabled using:

• Kernel command line: ipv6.disable=1
• sysctl.conf parameters:
  net.ipv6.conf.all.disable_ipv6 = 1
  net.ipv6.conf.default.disable_ipv6 = 1
  net.ipv6.conf.lo.disable_ipv6 = 1
• ifconfig -a
  eth0 Link encap:Ethernet HWaddr 00:26:36:11:11:a6
  inet addr:192.168.1.13 Bcast:192.168.1.255 Mask:255.255.255.0
  UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
  RX packets:1547619 errors:0 dropped:0 overruns:0 frame:0
  TX packets:1455500 errors:0 dropped:0 overruns:0 carrier:0
  collisions:0 txqueuelen:1000
  RX bytes:139664869 (139.6 MB) TX bytes:132665509 (132.6 MB)

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:12945 errors:0 dropped:0 overruns:0 frame:0
TX packets:12945 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:1778023 (1.7 MB) TX bytes:1778023 (1.7 MB)

tcpdump sample:
14:01:16.162879 IP 192.168.1.15.53 > 192.168.1.13.44963: 35160 NXDomain* 0/1/0 (98)
14:01:16.162967 IP 192.168.1.13.59965 > 192.168.1.15.53: 43502+ A? zabbix.elabsis.com. (36)
14:01:16.163875 IP 192.168.1.15.53 > 192.168.1.13.59965: 43502* 1/2/2 A[|domain]
14:01:16.292863 IP 192.168.1.13.45479 > 192.168.1.15.53: 3614+ AAAA? zabbix.elabsis.com. (36)
14:01:16.293466 IP 192.168.1.15.53 > 192.168.1.13.45479: 3614* 0/1/0 (98)
14:01:16.293538 IP 192.168.1.13.43333 > 192.168.1.15.53: 54497+ AAAA? zabbix.elabsis.com.elabsis.com. (48)
14:01:16.294384 IP 192.168.1.15.53 > 192.168.1.13.43333: 54497 NXDomain* 0/1/0 (98)
14:01:16.294435 IP 192.168.1.13.41446 > 192.168.1.15.53: 45609+ A? zabbix.elabsis.com. (36)
14:01:16.295289 IP 192.168.1.15.53 > 192.168.1.13.41446: 45609* 1/2/2 A[|domain]
14:01:16.378012 IP 192.168.1.13.50143 > 192.168.1.15.53: 446+ AAAA? zabbix.elabsis.com. (36)
14:01:16.378635 IP 192.168.1.15.53 > 192.168.1.13.50143: 446* 0/1/0 (98)
14:01:16.378704 IP 192.168.1.13.53937 > 192.168.1.15.53: 49180+ AAAA? zabbix.elabsis.com.elabsis.com. (48)
14:01:16.379620 IP 192.168.1.15.53 > 192.168.1.13.53937: 49180 NXDomain* 0/1/0 (98)
14:01:16.379670 IP 192.168.1.13.54214 > 192.168.1.15.53: 2279+ A? zabbix.elabsis.com. (36)
14:01:16.380582 IP 192.168.1.15.53 > 192.168.1.13.54214: 2279* 1/2/2 A[|domain]
14:01:16.450368 IP 192.168.1.13.39695 > 192.168.1.15.53: 61037+ AAAA? zabbix.elabsis.com. (36)
14:01:16.451051 IP 192.168.1.15.53 > 192.168.1.13.39695: 61037* 0/1/0 (98)
14:01:16.451329 IP 192.168.1.13.43782 > 192.168.1.15.53: 35967+ AAAA? zabbix.elabsis.com.elabsis.com. (48)
14:01:16.452111 IP 192.168.1.15.53 > 192.168.1.13.43782: 35967 NXDomain* 0/1/0 (98)
14:01:16.452192 IP 192.168.1.13.60385 > 192.168.1.15.53: 12860+ A? zabbix.elabsis.com. (36)
14:01:16.453088 IP 192.168.1.15.53 > 192.168.1.13.60385: 12860* 1/2/2 A[|domain]
14:01:16.470602 IP 192.168.1.13.37713 > 192.168.1.15.53: 39599+ AAAA? zabbix.elabsis.com. (36)
14:01:16.471580 IP 192.168.1.15.53 > 192.168.1.13.37713: 39599* 0/1/0 (98)
14:01:16.471671 IP 192.168.1.13.38240 > 192.168.1.15.53: 34748+ AAAA? zabbix.elabsis.com.elabsis.com. (48)
14:01:16.472410 IP 192.168.1.15.53 > 192.168.1.13.38240: 34748 NXDomain* 0/1/0 (98)

Recompiling the agent without the IPv6 support, the AAAA queries don´t occur anymore.

But if the agent is compiled with IPv6 support (using Debian/Ubuntu package) and IPv6 is disabled on the operating system, AAAA queries are sent anyway, doing extra load on DNS servers.