As per the above article, DNS information will store the previous resolutions till the cache ttl expiry, it is correct but FQDN will not work based on this feature.
For example, firewallgeeks.com's first resolved IP is 1.2.3.4 with DNS TTL as 20 seconds with global DNS cache TTL as 1800 seconds, it will store 1.2.3.4 until 20 seconds, and it will not store until 30 minutes.
If there is a need to store the previous resolution for more time, it is necessary to change the cache ttl under a specific FQDN configuration.
Now in the v7.0.16 above and v7.2.x, there is a feature added 'fqdn-cache-ttl' under global configuration, using this feature it is possible to store the FQDN resolve IP address for a long time irrespective of TTL sent by the DNS server. In addition, each FQDN can store up to 3000 addresses and the limit is hard-coded.
FQDN cache time to live (TTL), in seconds (0 - 86400, default = 0).
This is the amount of time an FQDN's address record can live if not refreshed. This setting applies globally, across all VDOMs, to FQDNs that have unspecified firewall address cache-ttl settings. If the cache-ttl value is configured for an FQDN address, it will supersede the fqdn-cache-ttl setting for that address.
For example, configure the FQDN cache TTL on the global VDOM:
config system dns
set fqdn-cache-ttl 2000
end
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