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/client.go

@@ -0,0 +1,94 @@
+// 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"
+
+	"codeberg.org/miekg/dns"
+)
+
+type (
+	// Client performs DNS lookups used to verify domain ownership and
+	// certificate prerequisites.
+	Client struct {
+		ResolverAddr string
+		exchange     exchangeFunc
+	}
+
+	exchangeFunc func(ctx context.Context, msg *dns.Msg, network string) (*dns.Msg, error)
+)
+
+// NewClient returns a client that resolves names through resolverAddr.
+func NewClient(resolverAddr string) *Client {
+	return &Client{ResolverAddr: resolverAddr}
+}
+
+func (c *Client) exchangeUDP(ctx context.Context, msg *dns.Msg) (*dns.Msg, error) {
+	if c.exchange != nil {
+		return c.exchange(ctx, msg, "udp")
+	}
+
+	client := dns.NewClient()
+
+	resp, _, err := client.Exchange(ctx, msg, "udp", c.ResolverAddr)
+	if err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}
+
+func (c *Client) exchangeTCP(ctx context.Context, msg *dns.Msg) (*dns.Msg, error) {
+	if c.exchange != nil {
+		return c.exchange(ctx, msg, "tcp")
+	}
+
+	client := dns.NewClient()
+
+	resp, _, err := client.Exchange(ctx, msg, "tcp", c.ResolverAddr)
+	if err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}
+
+func (c *Client) query(ctx context.Context, msg *dns.Msg) (*dns.Msg, error) {
+	resp, err := c.exchangeUDP(ctx, msg)
+	if err != nil {
+		return nil, fmt.Errorf("cannot exchange dns message: %w", err)
+	}
+
+	if resp.Truncated {
+		resp, err = c.exchangeTCP(ctx, msg)
+		if err != nil {
+			return nil, fmt.Errorf("cannot exchange dns message over TCP: %w", err)
+		}
+	}
+
+	if resp.Truncated {
+		return nil, fmt.Errorf("cannot exchange dns message: truncated response")
+	}
+
+	return resp, nil
+}