do not lookup cname after looking up the txt for mta-sts, and follow cnames for mocks

because the txt would already follow cnames.
the additional cname lookup didn't hurt, it just didn't do anything.
i probably didn't realize that before looking deeper into dns.

dns/mock.go

@@ -184,20 +184,20 @@ func (r MockResolver) LookupHost(ctx context.Context, host string) ([]string, ad
 
 func (r MockResolver) LookupIP(ctx context.Context, network, host string) ([]net.IP, adns.Result, error) {
 	mr := mockReq{"ip", host}
-	_, result, err := r.result(ctx, mr)
+	name, result, err := r.result(ctx, mr)
 	if err != nil {
 		return nil, result, err
 	}
 	var ips []net.IP
 	switch network {
 	case "ip", "ip4":
-		for _, ip := range r.A[host] {
+		for _, ip := range r.A[name] {
 			ips = append(ips, net.ParseIP(ip))
 		}
 	}
 	switch network {
 	case "ip", "ip6":
-		for _, ip := range r.AAAA[host] {
+		for _, ip := range r.AAAA[name] {
 			ips = append(ips, net.ParseIP(ip))
 		}
 	}
@@ -209,7 +209,7 @@ func (r MockResolver) LookupIP(ctx context.Context, network, host string) ([]net
 
 func (r MockResolver) LookupMX(ctx context.Context, name string) ([]*net.MX, adns.Result, error) {
 	mr := mockReq{"mx", name}
-	_, result, err := r.result(ctx, mr)
+	name, result, err := r.result(ctx, mr)
 	if err != nil {
 		return nil, result, err
 	}
@@ -222,7 +222,7 @@ func (r MockResolver) LookupMX(ctx context.Context, name string) ([]*net.MX, adn
 
 func (r MockResolver) LookupTXT(ctx context.Context, name string) ([]string, adns.Result, error) {
 	mr := mockReq{"txt", name}
-	_, result, err := r.result(ctx, mr)
+	name, result, err := r.result(ctx, mr)
 	if err != nil {
 		return nil, result, err
 	}
@@ -241,7 +241,7 @@ func (r MockResolver) LookupTLSA(ctx context.Context, port int, protocol string,
 		name = fmt.Sprintf("_%d._%s.%s", port, protocol, host)
 	}
 	mr := mockReq{"tlsa", name}
-	_, result, err := r.result(ctx, mr)
+	name, result, err := r.result(ctx, mr)
 	if err != nil {
 		return nil, result, err
 	}

mtasts/mtasts.go

@@ -162,45 +162,26 @@ var (
 )
 
 // LookupRecord looks up the MTA-STS TXT DNS record at "_mta-sts.<domain>",
-// following CNAME records, and returns the parsed MTA-STS record, the DNS TXT
-// record and any CNAMEs that were followed.
-func LookupRecord(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (rrecord *Record, rtxt string, rcnames []string, rerr error) {
+// following CNAME records, and returns the parsed MTA-STS record and the DNS TXT
+// record.
+func LookupRecord(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (rrecord *Record, rtxt string, rerr error) {
 	log := xlog.WithContext(ctx)
 	start := time.Now()
 	defer func() {
-		log.Debugx("mtasts lookup result", rerr, mlog.Field("domain", domain), mlog.Field("record", rrecord), mlog.Field("cnames", rcnames), mlog.Field("duration", time.Since(start)))
+		log.Debugx("mtasts lookup result", rerr, mlog.Field("domain", domain), mlog.Field("record", rrecord), mlog.Field("duration", time.Since(start)))
 	}()
 
 	// ../rfc/8461:289
 	// ../rfc/8461:351
-	// We lookup the txt record, but must follow CNAME records when the TXT does not exist.
-	var cnames []string
+	// We lookup the txt record, but must follow CNAME records when the TXT does not
+	// exist. LookupTXT follows CNAMEs.
 	name := "_mta-sts." + domain.ASCII + "."
 	var txts []string
-	for {
-		var err error
-		txts, _, err = dns.WithPackage(resolver, "mtasts").LookupTXT(ctx, name)
+	txts, _, err := dns.WithPackage(resolver, "mtasts").LookupTXT(ctx, name)
 	if dns.IsNotFound(err) {
-			// DNS has no specified limit on how many CNAMEs to follow. Chains of 10 CNAMEs
-			// have been seen on the internet.
-			if len(cnames) > 16 {
-				return nil, "", cnames, fmt.Errorf("too many cnames")
-			}
-			cname, _, err := dns.WithPackage(resolver, "mtasts").LookupCNAME(ctx, name)
-			if dns.IsNotFound(err) {
-				return nil, "", cnames, ErrNoRecord
-			}
-			if err != nil {
-				return nil, "", cnames, fmt.Errorf("%w: %s", ErrDNS, err)
-			}
-			cnames = append(cnames, cname)
-			name = cname
-			continue
+		return nil, "", ErrNoRecord
 	} else if err != nil {
-			return nil, "", cnames, fmt.Errorf("%w: %s", ErrDNS, err)
-		} else {
-			break
-		}
+		return nil, "", fmt.Errorf("%w: %s", ErrDNS, err)
 	}
 
 	var text string
@@ -215,18 +196,18 @@ func LookupRecord(ctx context.Context, resolver dns.Resolver, domain dns.Domain)
 			continue
 		}
 		if err != nil {
-			return nil, "", cnames, err
+			return nil, "", err
 		}
 		if record != nil {
-			return nil, "", cnames, ErrMultipleRecords
+			return nil, "", ErrMultipleRecords
 		}
 		record = r
 		text = txt
 	}
 	if record == nil {
-		return nil, "", cnames, ErrNoRecord
+		return nil, "", ErrNoRecord
 	}
-	return record, text, cnames, nil
+	return record, text, nil
 }
 
 // Policy fetch errors.
@@ -330,7 +311,7 @@ func Get(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (record
 		log.Debugx("mtasts get result", err, mlog.Field("domain", domain), mlog.Field("record", record), mlog.Field("policy", policy), mlog.Field("duration", time.Since(start)))
 	}()
 
-	record, _, _, err = LookupRecord(ctx, resolver, domain)
+	record, _, err = LookupRecord(ctx, resolver, domain)
 	if err != nil {
 		return nil, nil, err
 	}

mtasts/mtasts_test.go

@@ -21,9 +21,12 @@ import (
 	"github.com/mjl-/adns"
 
 	"github.com/mjl-/mox/dns"
+	"github.com/mjl-/mox/mlog"
 )
 
 func TestLookup(t *testing.T) {
+	mlog.SetConfig(map[string]mlog.Level{"": mlog.LevelDebug})
+
 	resolver := dns.MockResolver{
 		TXT: map[string][]string{
 			"_mta-sts.a.example.":         {"v=STSv1; id=1"},
@@ -37,39 +40,37 @@ func TestLookup(t *testing.T) {
 		CNAME: map[string]string{
 			"_mta-sts.a.cnames.example.":        "_mta-sts.b.cnames.example.",
 			"_mta-sts.b.cnames.example.":        "_mta-sts.c.cnames.example.",
-			"_mta-sts.followtemperror.example.": "_mta-sts.cnametemperror.example.",
+			"_mta-sts.followtemperror.example.": "_mta-sts.temperror.example.",
 		},
 		Fail: []string{
 			"txt _mta-sts.temperror.example.",
-			"cname _mta-sts.cnametemperror.example.",
 		},
 	}
 
-	test := func(host string, expRecord *Record, expCNAMEs []string, expErr error) {
+	test := func(host string, expRecord *Record, expErr error) {
 		t.Helper()
 
-		record, _, cnames, err := LookupRecord(context.Background(), resolver, dns.Domain{ASCII: host})
+		record, _, err := LookupRecord(context.Background(), resolver, dns.Domain{ASCII: host})
 		if (err == nil) != (expErr == nil) || err != nil && !errors.Is(err, expErr) {
 			t.Fatalf("lookup: got err %#v, expected %#v", err, expErr)
 		}
 		if err != nil {
 			return
 		}
-		if !reflect.DeepEqual(record, expRecord) || !reflect.DeepEqual(cnames, expCNAMEs) {
-			t.Fatalf("lookup: got record %#v, cnames %#v, expected %#v %#v", record, cnames, expRecord, expCNAMEs)
+		if !reflect.DeepEqual(record, expRecord) {
+			t.Fatalf("lookup: got record %#v, expected %#v", record, expRecord)
 		}
 	}
 
-	test("absent.example", nil, nil, ErrNoRecord)
-	test("other.example", nil, nil, ErrNoRecord)
-	test("a.example", &Record{Version: "STSv1", ID: "1"}, nil, nil)
-	test("one.example", &Record{Version: "STSv1", ID: "1"}, nil, nil)
-	test("bad.example", nil, nil, ErrRecordSyntax)
-	test("multiple.example", nil, nil, ErrMultipleRecords)
-	test("a.cnames.example", &Record{Version: "STSv1", ID: "1"}, []string{"_mta-sts.b.cnames.example.", "_mta-sts.c.cnames.example."}, nil)
-	test("temperror.example", nil, nil, ErrDNS)
-	test("cnametemperror.example", nil, nil, ErrDNS)
-	test("followtemperror.example", nil, nil, ErrDNS)
+	test("absent.example", nil, ErrNoRecord)
+	test("other.example", nil, ErrNoRecord)
+	test("a.example", &Record{Version: "STSv1", ID: "1"}, nil)
+	test("one.example", &Record{Version: "STSv1", ID: "1"}, nil)
+	test("bad.example", nil, ErrRecordSyntax)
+	test("multiple.example", nil, ErrMultipleRecords)
+	test("a.cnames.example", &Record{Version: "STSv1", ID: "1"}, nil)
+	test("temperror.example", nil, ErrDNS)
+	test("followtemperror.example", nil, ErrDNS)
 }
 
 func TestMatches(t *testing.T) {

mtastsdb/db.go

@@ -263,7 +263,7 @@ func Get(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (policy
 	policy = &cachedPolicy.Policy
 	nctx, cancel := context.WithTimeout(ctx, 30*time.Second)
 	defer cancel()
-	record, _, _, err := mtasts.LookupRecord(nctx, resolver, domain)
+	record, _, err := mtasts.LookupRecord(nctx, resolver, domain)
 	if err != nil {
 		if !errors.Is(err, mtasts.ErrNoRecord) {
 			// Could be a temporary DNS or configuration error.

mtastsdb/refresh.go

@@ -125,7 +125,7 @@ func refreshDomain(ctx context.Context, db *bstore.DB, resolver dns.Resolver, pr
 		return
 	}
 	log.Debug("refreshing mta-sts policy for domain", mlog.Field("domain", d))
-	record, _, _, err := mtasts.LookupRecord(ctx, resolver, d)
+	record, _, err := mtasts.LookupRecord(ctx, resolver, d)
 	if err == nil && record.ID == pr.RecordID {
 		qup := bstore.QueryDB[PolicyRecord](ctx, db)
 		qup.FilterNonzero(PolicyRecord{Domain: pr.Domain, LastUpdate: pr.LastUpdate})

webadmin/admin.go

@@ -325,7 +325,6 @@ type MTASTSRecord struct {
 	mtasts.Record
 }
 type MTASTSCheckResult struct {
-	CNAMEs     []string
 	TXT        string
 	Record     *MTASTSRecord
 	PolicyText string
@@ -1180,15 +1179,10 @@ Ensure a DNS TXT record like the following exists:
 		defer logPanic(ctx)
 		defer wg.Done()
 
-		record, txt, cnames, err := mtasts.LookupRecord(ctx, resolver, domain)
+		record, txt, err := mtasts.LookupRecord(ctx, resolver, domain)
 		if err != nil {
 			addf(&r.MTASTS.Errors, "Looking up MTA-STS record: %s", err)
 		}
-		if cnames != nil {
-			r.MTASTS.CNAMEs = cnames
-		} else {
-			r.MTASTS.CNAMEs = []string{}
-		}
 		r.MTASTS.TXT = txt
 		if record != nil {
 			r.MTASTS.Record = &MTASTSRecord{*record}

webadmin/admin.html

@@ -951,8 +951,7 @@ const domainDNSCheck = async (d) => {
 	const detailsTLSRPT = !checks.TLSRPT.TXT ? [] : [
 		dom.div('TXT record: ' + checks.TLSRPT.TXT),
 	]
-	const detailsMTASTS = empty(checks.MTASTS.CNAMEs) && !checks.MTASTS.TXT && !checks.MTASTS.PolicyText ? [] : [
-		dom.div('CNAMEs followed: ' + (checks.MTASTS.CNAMEs.join(', ') || '(none)')),
+	const detailsMTASTS = !checks.MTASTS.TXT && !checks.MTASTS.PolicyText ? [] : [
 		!checks.MTASTS.TXT ? [] : dom.div('MTA-STS record: ' + checks.MTASTS.TXT),
 		!checks.MTASTS.PolicyText ? [] : dom.div('MTA-STS policy: ', dom('pre.literal', style({maxWidth: '60em'}), checks.MTASTS.PolicyText)),
 	]

webadmin/adminapi.json

@@ -1626,14 +1626,6 @@
 			"Name": "MTASTSCheckResult",
 			"Docs": "",
 			"Fields": [
-				{
-					"Name": "CNAMEs",
-					"Docs": "",
-					"Typewords": [
-						"[]",
-						"string"
-					]
-				},
 				{
 					"Name": "TXT",
 					"Docs": "",