fix: derive kubelet clusterDNS for a custom serviceSubnet (C2)#23
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…1, C2) When serviceSubnet is non-default, the kubelet clusterDNS must be the DNS service IP of that CIDR, not the default 10.96.0.10. The provider previously relied on kubeadm's implicit derivation; this makes it correct-by-construction and unit-testable (ADR-17). - DeriveDNSIP(serviceSubnet): pure function returning the 10th host IP of the service CIDR, matching kubeadm's GetDNSIP. Uses the primary (first) CIDR for a dual-stack value, big-endian byte addition so IPv4/IPv6 and non-zero-aligned bases are correct, and fails fast on empty/malformed/too-small/empty-secondary CIDRs rather than guessing. - KubeletConfiguration type + BuildKubeletConfiguration: emit clusterDNS only when the operator set a custom serviceSubnet; empty subnet emits no document so the default path is byte-for-byte unchanged (kubeadm derives it). - Emitted in the init flow and in the ADR-12-R1 kubelet-start repair (both regenerate /var/lib/kubelet/config.yaml); the join path omits it, as workers inherit clusterDNS from the kubelet-config ConfigMap uploaded at init. Tests: full DeriveDNSIP matrix (default/custom IPv4, IPv6, dual-stack primary, non-zero-aligned bases, /28 boundary, and error cases), a builder test, an action behavior test (init emits the derived clusterDNS for a custom subnet, omits it for the default), and a join-omits regression test. Credential-free; no security gate. Co-Authored-By: Claude <noreply@anthropic.com> Signed-off-by: William Rizzo <william.rizzo@gmail.com>
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What
When
serviceSubnetis non-default, the kubeletclusterDNSmust be the DNSservice IP of that CIDR (the 10th host IP), not the default
10.96.0.10. Theprovider previously relied on kubeadm's implicit derivation; this makes the
provider own the value, correct-by-construction and hardware-free testable.
Closes kairos-io/kairos#4201 (pitfall C2).
How (ADR-17, option B)
DeriveDNSIP(serviceSubnet)(internal/kubeadmconfig/dns.go): pure functionreturning the 10th host IP of the service CIDR, matching kubeadm's
GetDNSIP.Uses the primary (first) CIDR for a dual-stack value; big-endian byte
addition so IPv4/IPv6 and non-zero-aligned bases are correct (e.g.
10.96.0.64/26 -> 10.96.0.74); fails fast on empty / malformed / too-small /empty-secondary CIDRs rather than guessing.
KubeletConfigurationtype +BuildKubeletConfiguration: emitsclusterDNSonly when the operator set a custom
serviceSubnet; an empty subnet emits nodocument, so the default path is byte-for-byte unchanged (kubeadm derives it).
regenerate
/var/lib/kubelet/config.yaml); the join path omits it, sinceworkers inherit
clusterDNSfrom the kubelet-config ConfigMap kubeadm uploads atinit.
Tests
DeriveDNSIPmatrix: default/custom IPv4, IPv6, dual-stack primary,non-zero-aligned bases,
/28boundary, and all error cases (empty, garbage,too-small
/29&/125, three CIDRs, empty primary/secondary).clusterDNS; empty -> none).clusterDNSfor a custom subnetand omits the document for the default; join never emits it.
go build/go test ./.../vet/golangci-lintall green. Credential-free,so no security-architect gate.
Review
staff-engineer code-reviewed this and ran a differential test reimplementing
kubeadm's
GetDNSIP/GetIndexedIPfromk8s.io/utils/netagainstDeriveDNSIPacross the matrix + 9 extra edge cases — byte-for-byte parity on value and error.
Both should-fix items (repair-path consistency; reject empty-secondary CIDR) were
actioned before this PR.