Steps to reproduce:

• Set up simple DNS server on a different VM that resolves the test.dns name 
• Configure a host in Zabbix to use test.dns for connecting to Zabbix agent
• Start Zabbix server (without having the DNS server used, for now)
• Connection to agent shows as failed (as expected)
• Edit /etc/resolv.conf to point to the DNS server like: nameserver 192.168.8.115
• Initially the Agents shows as connected (as expected)

Result:

• After a while it shows as red again with error in DNS resolution

Expected:

• After a while DNS resolved and connection restores