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/names_test.go

@@ -0,0 +1,108 @@
+// 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_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.probo.inc/probo/pkg/dnsclient"
+)
+
+func TestEqualNames(t *testing.T) {
+	t.Parallel()
+
+	assert.True(t, dnsclient.EqualNames("trust.example.com", "trust.example.com."))
+	assert.True(t, dnsclient.EqualNames("Trust.Example.COM", "trust.example.com"))
+	assert.False(t, dnsclient.EqualNames("trust.example.com", "example.com"))
+}
+
+func TestHostnamesForCAA(t *testing.T) {
+	t.Parallel()
+
+	t.Run("subdomain walks past etld plus one through tld", func(t *testing.T) {
+		t.Parallel()
+
+		names, err := dnsclient.HostnamesForCAA("trust.example.com")
+
+		require.NoError(t, err)
+		assert.Equal(t, []string{"trust.example.com", "example.com", "com"}, names)
+	})
+
+	t.Run("apex continues through tld", func(t *testing.T) {
+		t.Parallel()
+
+		names, err := dnsclient.HostnamesForCAA("example.com")
+
+		require.NoError(t, err)
+		assert.Equal(t, []string{"example.com", "com"}, names)
+	})
+
+	t.Run("nested subdomain walks each parent through tld", func(t *testing.T) {
+		t.Parallel()
+
+		names, err := dnsclient.HostnamesForCAA("portal.trust.example.com")
+
+		require.NoError(t, err)
+		assert.Equal(
+			t,
+			[]string{
+				"portal.trust.example.com",
+				"trust.example.com",
+				"example.com",
+				"com",
+			},
+			names,
+		)
+	})
+
+	t.Run("multi-label public suffix continues past etld plus one", func(t *testing.T) {
+		t.Parallel()
+
+		names, err := dnsclient.HostnamesForCAA("app.example.co.uk")
+
+		require.NoError(t, err)
+		assert.Equal(
+			t,
+			[]string{"app.example.co.uk", "example.co.uk", "co.uk", "uk"},
+			names,
+		)
+	})
+
+	t.Run("single label is only entry", func(t *testing.T) {
+		t.Parallel()
+
+		names, err := dnsclient.HostnamesForCAA("com")
+
+		require.NoError(t, err)
+		assert.Equal(t, []string{"com"}, names)
+	})
+
+	t.Run("empty hostname errors", func(t *testing.T) {
+		t.Parallel()
+
+		names, err := dnsclient.HostnamesForCAA("  ")
+
+		require.Error(t, err)
+		assert.Nil(t, names)
+	})
+}