#17: implement DANE TLSA, MTA-STS policy resolution, ARC sealing and mail authentication

services/protocols/internal/infrastructure/netdns/tlsa_resolver.go

@@ -0,0 +1,77 @@
+package netdns
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/miekg/dns"
+
+	"lambdamail/protocols/internal/domain/valueobject"
+)
+
+// TlsaResolver looks up DANE associations through a validating resolver. It
+// reports whether the answer carried the AD bit, because a TLSA record that
+// was not DNSSEC-validated is worthless: an attacker able to spoof DNS could
+// otherwise choose the policy we apply (PLAN.md section 5.1).
+type TlsaResolver struct {
+	// serverAddr is the validating resolver to ask, as host:port.
+	serverAddr string
+	client     *dns.Client
+}
+
+func NewTlsaResolver(serverAddr string) *TlsaResolver {
+	if serverAddr == "" {
+		serverAddr = "1.1.1.1:53"
+	}
+	if !strings.Contains(serverAddr, ":") {
+		serverAddr += ":53"
+	}
+	return &TlsaResolver{
+		serverAddr: serverAddr,
+		client:     &dns.Client{Timeout: 10 * time.Second},
+	}
+}
+
+func (r *TlsaResolver) LookupTLSA(ctx context.Context, host string, port int) ([]valueobject.TLSARecord, bool, error) {
+	name := dns.Fqdn(fmt.Sprintf("_%d._tcp.%s", port, strings.TrimSuffix(host, ".")))
+
+	msg := new(dns.Msg)
+	msg.SetQuestion(name, dns.TypeTLSA)
+	// Ask the resolver to validate and to report the result in the AD bit.
+	msg.SetEdns0(4096, true)
+	msg.AuthenticatedData = true
+
+	reply, _, err := r.client.ExchangeContext(ctx, msg, r.serverAddr)
+	if err != nil {
+		return nil, false, fmt.Errorf("query TLSA for %s: %w", name, err)
+	}
+
+	switch reply.Rcode {
+	case dns.RcodeSuccess:
+		// Fall through to parsing.
+	case dns.RcodeNameError:
+		// No TLSA record: the destination is simply not DANE-protected.
+		return nil, reply.AuthenticatedData, nil
+	default:
+		return nil, false, fmt.Errorf("TLSA query for %s returned %s", name, dns.RcodeToString[reply.Rcode])
+	}
+
+	var records []valueobject.TLSARecord
+	for _, answer := range reply.Answer {
+		tlsa, ok := answer.(*dns.TLSA)
+		if !ok {
+			continue
+		}
+		// Usage 2 is rejected by explicit decision (PLAN.md section 5.1), so
+		// unsupported associations are skipped rather than failing the lookup.
+		record, err := valueobject.NewTLSARecord(tlsa.Usage, tlsa.Selector, tlsa.MatchingType, strings.ToLower(tlsa.Certificate))
+		if err != nil {
+			continue
+		}
+		records = append(records, record)
+	}
+
+	return records, reply.AuthenticatedData, nil
+}