Harden DNS CAA and TXT verification

Extract shared DNS checks into dnsclient and fail closed on
truncated or non-success CAA responses. Climb past eTLD+1,
validate RFC 8659 issue-value syntax, and map NXDOMAIN TXT
lookups to the pending-verification path.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/dnsclient/cname.go

@@ -0,0 +1,76 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package dnsclient
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"codeberg.org/miekg/dns"
+)
+
+// CheckCNAME verifies that hostname has a single CNAME record owned by that
+// name and pointing at expectedTarget.
+func (c *Client) CheckCNAME(ctx context.Context, hostname, expectedTarget string) error {
+	owner := ToFQDN(hostname)
+	target := ToFQDN(expectedTarget)
+
+	msg := &dns.Msg{MsgHeader: dns.MsgHeader{ID: dns.ID(), RecursionDesired: true}}
+	msg.Question = []dns.RR{&dns.CNAME{Hdr: dns.Header{Name: owner, Class: dns.ClassINET}}}
+
+	resp, err := c.query(ctx, msg)
+	if err != nil {
+		return err
+	}
+
+	if len(resp.Answer) == 0 {
+		return fmt.Errorf("no cname records found for domain %q", hostname)
+	}
+
+	if len(resp.Answer) > 1 {
+		return fmt.Errorf("multiple cname records found for domain %q", hostname)
+	}
+
+	resolvedRecord, ok := resp.Answer[0].(*dns.CNAME)
+	if !ok {
+		return fmt.Errorf("first answer is not a cname record for domain %q", hostname)
+	}
+
+	if !EqualNames(resolvedRecord.Hdr.Name, owner) {
+		return fmt.Errorf(
+			"cname owner mismatch: domain %q has record owned by %q",
+			hostname,
+			strings.TrimSuffix(resolvedRecord.Hdr.Name, "."),
+		)
+	}
+
+	if !EqualNames(resolvedRecord.Target, target) {
+		return fmt.Errorf(
+			"cname target mismatch: domain %q resolves to %q, expected %q",
+			hostname,
+			strings.TrimSuffix(resolvedRecord.Target, "."),
+			strings.TrimSuffix(expectedTarget, "."),
+		)
+	}
+
+	return nil
+}