implement accepting dmarc & tls reports for other domains

to accept reports for another domain, first add that domain to the config,
leaving all options empty except DMARC/TLSRPT in which you configure a Domain.

the suggested DNS DMARC/TLSRPT records will show the email address with
configured domain. for DMARC, the dnscheck functionality will verify that the
destination domain has opted in to receiving reports.

there is a new command-line subcommand "mox dmarc checkreportaddrs" that
verifies if dmarc reporting destination addresses have opted in to received
reports.

this also changes the suggested dns records (in quickstart, and through admin
pages and cli subcommand) to take into account whether DMARC and TLSRPT is
configured, and with which localpart/domain (previously it always printed
records as if reporting was enabled for the domain). and when generating the
suggested DNS records, the dmarc.Record and tlsrpt.Record code is used, with
proper uri-escaping.

dmarc/dmarc.go

@@ -146,6 +146,63 @@ func lookupRecord(ctx context.Context, resolver dns.Resolver, domain dns.Domain)
 	return StatusNone, record, text, rerr
 }
 
+func lookupReportsRecord(ctx context.Context, resolver dns.Resolver, dmarcDomain, extDestDomain dns.Domain) (Status, *Record, string, error) {
+	name := dmarcDomain.ASCII + "._report._dmarc." + extDestDomain.ASCII + "."
+	txts, err := dns.WithPackage(resolver, "dmarc").LookupTXT(ctx, name)
+	if err != nil && !dns.IsNotFound(err) {
+		return StatusTemperror, nil, "", fmt.Errorf("%w: %s", ErrDNS, err)
+	}
+	var record *Record
+	var text string
+	var rerr error = ErrNoRecord
+	for _, txt := range txts {
+		r, isdmarc, err := ParseRecordNoRequired(txt)
+		// Examples in the RFC use "v=DMARC1", even though it isn't a valid DMARC record.
+		// Accept the specific example.
+		// ../rfc/7489-eid5440
+		if !isdmarc && txt == "v=DMARC1" {
+			xr := DefaultRecord
+			r, isdmarc, err = &xr, true, nil
+		}
+		if !isdmarc {
+			// ../rfc/7489:1374
+			continue
+		} else if err != nil {
+			return StatusPermerror, nil, text, fmt.Errorf("%w: %s", ErrSyntax, err)
+		}
+		if record != nil {
+			// ../ ../rfc/7489:1388
+			return StatusNone, nil, "", ErrMultipleRecords
+		}
+		text = txt
+		record = r
+		rerr = nil
+	}
+	return StatusNone, record, text, rerr
+}
+
+// LookupExternalReportsAccepted returns whether the extDestDomain has opted in
+// to receiving dmarc reports for dmarcDomain (where the dmarc record was found),
+// through a "._report._dmarc." DNS TXT DMARC record.
+//
+// Callers should look at status for interpretation, not err, because err will
+// be set to ErrNoRecord when the DNS TXT record isn't present, which means the
+// extDestDomain does not opt in (not a failure condition).
+//
+// The normally invalid "v=DMARC1" record is accepted since it is used as
+// example in RFC 7489.
+func LookupExternalReportsAccepted(ctx context.Context, resolver dns.Resolver, dmarcDomain dns.Domain, extDestDomain dns.Domain) (accepts bool, status Status, record *Record, txt string, rerr error) {
+	log := xlog.WithContext(ctx)
+	start := time.Now()
+	defer func() {
+		log.Debugx("dmarc externalreports result", rerr, mlog.Field("accepts", accepts), mlog.Field("dmarcdomain", dmarcDomain), mlog.Field("extdestdomain", extDestDomain), mlog.Field("record", record), mlog.Field("duration", time.Since(start)))
+	}()
+
+	status, record, txt, rerr = lookupReportsRecord(ctx, resolver, dmarcDomain, extDestDomain)
+	accepts = rerr == nil
+	return accepts, status, record, txt, rerr
+}
+
 // Verify evaluates the DMARC policy for the domain in the From-header of a
 // message given the DKIM and SPF evaluation results.
 //