Fix DMARC guidance for send-only relays with optional rua= settings.

The panel now suggests policy-only DMARC by default, lets operators configure a default and per-domain report address, and DNS-checks hub _report._dmarc records. Future in-panel report ingestion is tracked as dmarc-reports in the roadmap.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/dnscheck/dnscheck.go

@@ -56,12 +56,13 @@ type Server struct {
 
 // Domain is the published-DNS state of one sending domain.
 type Domain struct {
-	Name      string
-	DKIM      Result
-	SPF       Result
-	DMARC     Result
-	Overall   health.Status
-	CheckedAt time.Time
+	Name            string
+	DKIM            Result
+	SPF             Result
+	DMARC           Result
+	DMARCReportAuth Result // zero when external rua= is not used
+	Overall         health.Status
+	CheckedAt       time.Time
 }
 
 // Query describes the domain to check. ExpectedDKIM is the TXT value the panel
@@ -69,11 +70,12 @@ type Domain struct {
 // compares DNS against the key this server actually signs with. Hostname and
 // ServerIPs identify this server and come from a preceding Server check.
 type Query struct {
-	Name         string
-	Selector     string
-	ExpectedDKIM string
-	Hostname     string
-	ServerIPs    []string
+	Name              string
+	Selector          string
+	ExpectedDKIM      string
+	Hostname          string
+	ServerIPs         []string
+	DMARCReportEmail  string // resolved rua= destination; empty = policy-only template
 }
 
 // resolver is the slice of *net.Resolver this package uses, as an interface so
@@ -178,12 +180,19 @@ func (c *Checker) Forget(domainName string) {
 func (c *Checker) checkDomain(ctx context.Context, q Query) Domain {
 	d := Domain{Name: q.Name, CheckedAt: time.Now()}
 	var wg sync.WaitGroup
-	wg.Add(3)
+	wg.Add(4)
 	go func() { defer wg.Done(); d.DKIM = c.checkDKIM(ctx, q) }()
 	go func() { defer wg.Done(); d.SPF = c.checkSPF(ctx, q) }()
-	go func() { defer wg.Done(); d.DMARC = c.checkDMARC(ctx, q.Name) }()
+	go func() { defer wg.Done(); d.DMARC = c.checkDMARC(ctx, q) }()
+	go func() {
+		defer wg.Done()
+		hub := EmailDomain(q.DMARCReportEmail)
+		if hub != "" && !strings.EqualFold(hub, q.Name) {
+			d.DMARCReportAuth = c.checkReportAuth(ctx, hub)
+		}
+	}()
 	wg.Wait()
-	d.Overall = health.Worst(d.DKIM.Status, d.SPF.Status, d.DMARC.Status)
+	d.Overall = health.Worst(d.DKIM.Status, d.SPF.Status, d.DMARC.Status, d.DMARCReportAuth.Status)
 	return d
 }
 

internal/dnscheck/dnscheck_test.go

@@ -293,7 +293,7 @@ func TestDMARC(t *testing.T) {
 				txt["_dmarc.example.com"] = c.record
 			}
 			f := &fakeResolver{txt: txt}
-			got := newTestChecker(f).checkDMARC(context.Background(), "example.com")
+			got := newTestChecker(f).checkDMARC(context.Background(), Query{Name: "example.com"})
 			if got.Status != c.want {
 				t.Errorf("status = %q, want %q (%s)", got.Status, c.want, got.Detail)
 			}
@@ -303,7 +303,7 @@ func TestDMARC(t *testing.T) {
 
 func TestDMARCNonePolicyIsExplained(t *testing.T) {
 	f := &fakeResolver{txt: map[string][]string{"_dmarc.example.com": {"v=DMARC1; p=none"}}}
-	got := newTestChecker(f).checkDMARC(context.Background(), "example.com")
+	got := newTestChecker(f).checkDMARC(context.Background(), Query{Name: "example.com"})
 	if !strings.Contains(got.Detail, "monitoring only") {
 		t.Errorf("p=none is not explained: %s", got.Detail)
 	}
@@ -357,3 +357,20 @@ func TestForgetDropsTheCachedDomain(t *testing.T) {
 		t.Error("Forget did not drop the cached result")
 	}
 }
+
+func TestReportAuth(t *testing.T) {
+	f := &fakeResolver{txt: map[string][]string{"_report._dmarc.hub.example": {"v=DMARC1;"}}}
+	got := newTestChecker(f).checkReportAuth(context.Background(), "hub.example")
+	if got.Status != health.StatusOK {
+		t.Fatalf("status = %q (%s)", got.Status, got.Detail)
+	}
+
+	f = &fakeResolver{}
+	got = newTestChecker(f).checkReportAuth(context.Background(), "hub.example")
+	if got.Status != health.StatusWarn {
+		t.Fatalf("missing = %q, want warn", got.Status)
+	}
+	if !strings.Contains(got.Detail, ReportAuthExample()) {
+		t.Errorf("advice %q should cite %q", got.Detail, ReportAuthExample())
+	}
+}

internal/dnscheck/examples.go

@@ -1,6 +1,7 @@
 package dnscheck
 
 import (
+	"database/sql"
 	"net"
 	"strings"
 )
@@ -16,6 +17,13 @@ import (
 // helper: it is published at the domain itself.)
 func DMARCRecordName(domainName string) string { return "_dmarc." + domainName }
 
+// ReportAuthRecordName is where a report-receiving domain authorises external
+// DMARC aggregate destinations (RFC 7489 §7.1).
+func ReportAuthRecordName(hubDomain string) string { return "_report._dmarc." + hubDomain }
+
+// ReportAuthExample is the TXT value a hub domain publishes to accept reports.
+func ReportAuthExample() string { return "v=DMARC1;" }
+
 // SPFExample is the SPF record this server expects for a sending domain: the
 // addresses its mail actually leaves from, and "-all" to say that nothing else
 // is authorised. When the server's own addresses are not known (its hostname
@@ -40,10 +48,44 @@ func SPFExample(hostname string, serverIPs []string) string {
 	return "v=spf1 " + strings.Join(mechanisms, " ") + " -all"
 }
 
-// DMARCExample is the least a domain should publish: monitoring only, with an
-// address the aggregate reports go to. p=none is deliberate — it changes
-// nothing about delivery, so it is safe to publish before the reports have
-// shown that DKIM and SPF pass everywhere.
-func DMARCExample(domainName string) string {
-	return "v=DMARC1; p=none; rua=mailto:dmarc@" + domainName
+// ResolveDMARCRua picks the rua= mailbox for a sending domain: per-domain
+// override wins, then the administrator profile, then policy-only (empty).
+func ResolveDMARCRua(domainRua sql.NullString, profileEmail string) string {
+	if domainRua.Valid {
+		return domainRua.String
+	}
+	return profileEmail
+}
+
+// EmailDomain returns the lower-case domain part of addr, or "" when invalid.
+func EmailDomain(addr string) string {
+	addr = strings.TrimSpace(addr)
+	at := strings.LastIndex(addr, "@")
+	if at < 0 || at == len(addr)-1 {
+		return ""
+	}
+	return strings.ToLower(strings.TrimSpace(addr[at+1:]))
+}
+
+// DMARCExample is the DMARC TXT record this server suggests for a sending
+// domain. p=none is deliberate — it changes nothing about delivery. rua= is
+// included only when reportEmail is set; SelfPost is send-only and most
+// operators have no inbox on the sending domain itself.
+func DMARCExample(reportEmail string) string {
+	base := "v=DMARC1; p=none"
+	if reportEmail == "" {
+		return base
+	}
+	return base + "; rua=mailto:" + reportEmail
+}
+
+// ExternalReportAuth reports whether the hub domain must publish a
+// _report._dmarc authorisation for aggregate reports sent to reportEmail from
+// sendingDomain.
+func ExternalReportAuth(sendingDomain, reportEmail string) (name, value string, ok bool) {
+	hub := EmailDomain(reportEmail)
+	if hub == "" || strings.EqualFold(hub, sendingDomain) {
+		return "", "", false
+	}
+	return ReportAuthRecordName(hub), ReportAuthExample(), true
 }

internal/dnscheck/examples_test.go

@@ -2,6 +2,7 @@ package dnscheck
 
 import (
 	"context"
+	"database/sql"
 	"strings"
 	"testing"
 
@@ -18,8 +19,6 @@ func TestSPFExample(t *testing.T) {
 		{"ipv4", "mail.example.com", []string{"203.0.113.10"}, "v=spf1 ip4:203.0.113.10 -all"},
 		{"both families", "mail.example.com", []string{"203.0.113.10", "2001:db8::1"},
 			"v=spf1 ip4:203.0.113.10 ip6:2001:db8::1 -all"},
-		// The hostname does not resolve, so there is no address to name; an "a:"
-		// mechanism still gives the operator a publishable record.
 		{"no addresses", "mail.example.com", nil, "v=spf1 a:mail.example.com -all"},
 		{"unparsable addresses", "mail.example.com", []string{"not-an-ip"}, "v=spf1 a:mail.example.com -all"},
 	}
@@ -32,6 +31,41 @@ func TestSPFExample(t *testing.T) {
 	}
 }
 
+func TestDMARCExample(t *testing.T) {
+	if got := DMARCExample(""); got != "v=DMARC1; p=none" {
+		t.Errorf("empty = %q", got)
+	}
+	want := "v=DMARC1; p=none; rua=mailto:reports@hub.example"
+	if got := DMARCExample("reports@hub.example"); got != want {
+		t.Errorf("with rua = %q, want %q", got, want)
+	}
+}
+
+func TestResolveDMARCRua(t *testing.T) {
+	inherit := sql.NullString{}
+	if got := ResolveDMARCRua(inherit, "a@b.com"); got != "a@b.com" {
+		t.Errorf("inherit profile = %q", got)
+	}
+	none := sql.NullString{Valid: true}
+	if got := ResolveDMARCRua(none, "a@b.com"); got != "" {
+		t.Errorf("explicit none = %q", got)
+	}
+	custom := sql.NullString{Valid: true, String: "x@y.com"}
+	if got := ResolveDMARCRua(custom, "a@b.com"); got != "x@y.com" {
+		t.Errorf("custom = %q", got)
+	}
+}
+
+func TestExternalReportAuth(t *testing.T) {
+	name, value, ok := ExternalReportAuth("shop.com", "reports@hub.com")
+	if !ok || name != "_report._dmarc.hub.com" || value != "v=DMARC1;" {
+		t.Fatalf("external = (%q, %q, %v)", name, value, ok)
+	}
+	if _, _, ok := ExternalReportAuth("shop.com", "dmarc@shop.com"); ok {
+		t.Fatal("same domain should not need external auth")
+	}
+}
+
 // The record the panel shows and the one a failed check suggests must be the
 // same string, or the operator is told two different things on one page.
 func TestMissingRecordChecksSuggestTheShownExample(t *testing.T) {
@@ -50,11 +84,11 @@ func TestMissingRecordChecksSuggestTheShownExample(t *testing.T) {
 		t.Errorf("SPF advice %q does not suggest %q", spf.Detail, want)
 	}
 
-	dmarc := c.checkDMARC(context.Background(), "example.com")
+	dmarc := c.checkDMARC(context.Background(), Query{Name: "example.com"})
 	if dmarc.Status != health.StatusWarn {
 		t.Fatalf("DMARC status = %q, want warn (%s)", dmarc.Status, dmarc.Detail)
 	}
-	if want := DMARCExample("example.com"); !strings.Contains(dmarc.Detail, want) {
+	if want := DMARCExample(""); !strings.Contains(dmarc.Detail, want) {
 		t.Errorf("DMARC advice %q does not suggest %q", dmarc.Detail, want)
 	}
 }

internal/dnscheck/records.go

@@ -60,13 +60,15 @@ func (c *Checker) checkDKIM(ctx context.Context, q Query) Result {
 
 // checkDMARC reports whether the domain publishes a DMARC policy. DMARC is not
 // required for delivery, so its absence is advice (warn), not a fault.
-func (c *Checker) checkDMARC(ctx context.Context, domainName string) Result {
-	name := DMARCRecordName(domainName)
+func (c *Checker) checkDMARC(ctx context.Context, q Query) Result {
+	name := DMARCRecordName(q.Name)
 	txt, found, err := c.lookupTXT(ctx, name)
 	if err != nil {
 		return lookupFailed("the DMARC record", err)
 	}
 
+	example := DMARCExample(q.DMARCReportEmail)
+
 	var records []string
 	for _, rec := range txt {
 		if strings.HasPrefix(strings.ToLower(strings.TrimSpace(rec)), "v=dmarc1") {
@@ -74,10 +76,11 @@ func (c *Checker) checkDMARC(ctx context.Context, domainName string) Result {
 		}
 	}
 	if !found || len(records) == 0 {
-		return Result{
-			Status: health.StatusWarn,
-			Detail: fmt.Sprintf("No DMARC record at %s. Delivery works without one, but publishing at least %q tells receivers what to do with mail that fails DKIM and gets you reports.", name, DMARCExample(domainName)),
+		detail := fmt.Sprintf("No DMARC record at %s. Delivery works without one, but publishing %q tells receivers what to do with mail that fails authentication.", name, example)
+		if q.DMARCReportEmail == "" {
+			detail += " Aggregate reports (rua=) are optional on a send-only relay — omit rua= unless a mailbox that receives inbound mail is configured."
 		}
+		return Result{Status: health.StatusWarn, Detail: detail}
 	}
 	if len(records) > 1 {
 		return Result{
@@ -102,6 +105,52 @@ func (c *Checker) checkDMARC(ctx context.Context, domainName string) Result {
 	return Result{Status: health.StatusOK, Detail: detail, Records: records}
 }
 
+// checkReportAuth verifies the hub domain publishes _report._dmarc for external
+// aggregate-report destinations. Missing authorisation does not affect outbound
+// delivery, only whether reports reach the rua= mailbox.
+func (c *Checker) checkReportAuth(ctx context.Context, hubDomain string) Result {
+	name := ReportAuthRecordName(hubDomain)
+	expected := ReportAuthExample()
+	txt, found, err := c.lookupTXT(ctx, name)
+	if err != nil {
+		return lookupFailed("the DMARC report-authorisation record", err)
+	}
+
+	var records []string
+	for _, rec := range txt {
+		if strings.HasPrefix(strings.ToLower(strings.TrimSpace(rec)), "v=dmarc1") {
+			records = append(records, rec)
+		}
+	}
+	if !found || len(records) == 0 {
+		return Result{
+			Status: health.StatusWarn,
+			Detail: fmt.Sprintf("No report-authorisation record at %s. Aggregate DMARC reports sent to a mailbox on %s will not be delivered until %q is published there.", name, hubDomain, expected),
+		}
+	}
+	if len(records) > 1 {
+		return Result{
+			Status:  health.StatusError,
+			Detail:  fmt.Sprintf("More than one DMARC report-authorisation record is published at %s. Keep exactly one.", name),
+			Records: records,
+		}
+	}
+	return Result{
+		Status:  health.StatusOK,
+		Detail:  fmt.Sprintf("Published at %s — aggregate reports addressed to %s are authorised.", name, hubDomain),
+		Records: records,
+	}
+}
+
+// ReportAuth checks whether hubDomain authorises external DMARC aggregate
+// reports. It is used on the settings page for the administrator profile.
+func (c *Checker) ReportAuth(ctx context.Context, hubDomain string) Result {
+	if hubDomain == "" {
+		return Result{}
+	}
+	return c.checkReportAuth(ctx, hubDomain)
+}
+
 // publicKeyTag extracts the p= (public key) tag of a DKIM record, with all
 // whitespace removed: DNS providers and TXT chunking freely insert spaces and
 // line breaks into the base64, none of which are part of the key.