As this is happening with secdeb in the standard configuration, this means apt- cacher-ng should be downloading from:
deb.debian.org/debian-security security.debian.org cdn-fastly.deb.debian.org/debian-security which, according to host, resolve to: deb.debian.org is an alias for debian.map.fastlydns.net. debian.map.fastlydns.net has address 199.232.58.132 debian.map.fastlydns.net has IPv6 address 2a04:4e42:4b::644 (which is the same for cdn-fastly.deb.debian.org) security.debian.org has address 151.101.194.132 security.debian.org has address 151.101.2.132 security.debian.org has address 151.101.66.132 security.debian.org has address 151.101.130.132 security.debian.org has IPv6 address 2a04:4e42::644 security.debian.org has IPv6 address 2a04:4e42:200::644 security.debian.org has IPv6 address 2a04:4e42:400::644 security.debian.org has IPv6 address 2a04:4e42:600::644 security.debian.org mail is handled by 10 mitropoulos.debian.org. security.debian.org mail is handled by 10 mailly.debian.org. security.debian.org mail is handled by 10 muffat.debian.org.
