the exist from 1.4.1 to 1.6.x...

simple check do not use host_dns if "use dns is checked" but use ip defined...

to correct this you must modifiy check_simple.c as is:

diff -u -r zabbix-1.4.1/src/zabbix_server/poller/checks_simple.c zabbix-1.4.1-patched/src/zabbix_server/poller/checks_simple.c
— zabbix-1.4.1/src/zabbix_server/poller/checks_simple.c 2007-06-29 22:50:26.000000000 +0200
+++ zabbix-1.4.1-patched/src/zabbix_server/poller/checks_simple.c 2007-07-25 02:00:30.836587875 +0200
@@ -59,7 +59,7 @@

{ zbx_snprintf(c,sizeof(c),"net.tcp.service[%s,%s]", item->key, - item->host_ip); + item->host_dns); }

}
}