The Photon operating system must disable systemd fallback Domain Name System (DNS).

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-256590	SRG-OS-000480-GPOS-00227	PHTN-30-000245	SV-256590r991589_rule	2024-12-16	1

Description
Systemd contains an ability to set fallback DNS servers. This is used for DNS lookups in the event no system-level DNS servers are configured or other DNS servers are specified in the systemd "resolved.conf" file. If uncommented, this configuration contains Google DNS servers by default and could result in DNS leaking information unknowingly in the event DNS is absent or misconfigured at the system level.

Description

Systemd contains an ability to set fallback DNS servers. This is used for DNS lookups in the event no system-level DNS servers are configured or other DNS servers are specified in the systemd "resolved.conf" file. If uncommented, this configuration contains Google DNS servers by default and could result in DNS leaking information unknowingly in the event DNS is absent or misconfigured at the system level.

ℹ️ Check
At the command line, run the following command: # resolvectl status \| grep 'Fallback DNS' If the output indicates that fallback DNS servers are configured, this is a finding.

✔️ Fix
Navigate to and open: /etc/systemd/resolved.conf Add or update the "FallbackDNS" entry to the following: FallbackDNS= Restart the systemd resolved service by running the following command: # systemctl restart systemd-resolved Note: If this option is not given, a compiled-in list of DNS servers is used instead, which is undesirable.