The earliest recollection of this bug is traced back to PAN-OS 9.1.7 - July 29, 2024. This bug is fixed in PAN-OS versions 9.1.7. Fixed an issue where the firewall changed the TTL (time-to-live) value in DNS responses to 0 when the firewall failed to resolve the DNS Security service, which caused a large amount of DNS requests to be sent to the DNS server. For more information: https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-release-notes/pan-os-9-1-addressed-issues/pan-os-9-1-7-addressed-issues