implement outgoing tls reports

we were already accepting, processing and displaying incoming tls reports. now
we start tracking TLS connection and security-policy-related errors for
outgoing message deliveries as well. we send reports once a day, to the
reporting addresses specified in TLSRPT records (rua) of a policy domain. these
reports are about MTA-STS policies and/or DANE policies, and about
STARTTLS-related failures.

sending reports is enabled by default, but can be disabled through setting
NoOutgoingTLSReports in mox.conf.

only at the end of the implementation process came the realization that the
TLSRPT policy domain for DANE (MX) hosts are separate from the TLSRPT policy
for the recipient domain, and that MTA-STS and DANE TLS/policy results are
typically delivered in separate reports. so MX hosts need their own TLSRPT
policies.

config for the per-host TLSRPT policy should be added to mox.conf for existing
installs, in field HostTLSRPT. it is automatically configured by quickstart for
new installs. with a HostTLSRPT config, the "dns records" and "dns check" admin
pages now suggest the per-host TLSRPT record. by creating that record, you're
requesting TLS reports about your MX host.

gathering all the TLS/policy results is somewhat tricky. the tentacles go
throughout the code. the positive result is that the TLS/policy-related code
had to be cleaned up a bit. for example, the smtpclient TLS modes now reflect
reality better, with independent settings about whether PKIX and/or DANE
verification has to be done, and/or whether verification errors have to be
ignored (e.g. for tls-required: no header). also, cached mtasts policies of
mode "none" are now cleaned up once the MTA-STS DNS record goes away.

mtasts/mtasts.go

@@ -23,6 +23,8 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promauto"
 
+	"github.com/mjl-/adns"
+
 	"github.com/mjl-/mox/dns"
 	"github.com/mjl-/mox/metrics"
 	"github.com/mjl-/mox/mlog"
@@ -153,6 +155,30 @@ func (p *Policy) Matches(host dns.Domain) bool {
 	return false
 }
 
+// TLSReportFailureReason returns a concise error for known error types, or an
+// empty string. For use in TLSRPT.
+func TLSReportFailureReason(err error) string {
+	// If this is a DNSSEC authentication error, we'll collect it for TLS reporting.
+	// We can also use this reason for STS, not only DANE. ../rfc/8460:580
+	var errCode adns.ErrorCode
+	if errors.As(err, &errCode) && errCode.IsAuthentication() {
+		return fmt.Sprintf("dns-extended-error-%d-%s", errCode, strings.ReplaceAll(errCode.String(), " ", "-"))
+	}
+
+	for _, e := range mtastsErrors {
+		if errors.Is(err, e) {
+			s := strings.TrimPrefix(e.Error(), "mtasts: ")
+			return strings.ReplaceAll(s, " ", "-")
+		}
+	}
+	return ""
+}
+
+var mtastsErrors = []error{
+	ErrNoRecord, ErrMultipleRecords, ErrDNS, ErrRecordSyntax, // Lookup
+	ErrNoPolicy, ErrPolicyFetch, ErrPolicySyntax, // Fetch
+}
+
 // Lookup errors.
 var (
 	ErrNoRecord        = errors.New("mtasts: no mta-sts dns txt record") // Domain does not implement MTA-STS. If a cached non-expired policy is available, it should still be used.
@@ -262,7 +288,8 @@ func FetchPolicy(ctx context.Context, domain dns.Domain) (policy *Policy, policy
 		return nil, "", ErrNoPolicy
 	}
 	if err != nil {
-		return nil, "", fmt.Errorf("%w: http get: %s", ErrPolicyFetch, err)
+		// We pass along underlying TLS certificate errors.
+		return nil, "", fmt.Errorf("%w: http get: %w", ErrPolicyFetch, err)
 	}
 	metrics.HTTPClientObserve(ctx, "mtasts", req.Method, resp.StatusCode, err, start)
 	defer resp.Body.Close()
@@ -302,7 +329,7 @@ func FetchPolicy(ctx context.Context, domain dns.Domain) (policy *Policy, policy
 // record is still returned.
 //
 // Also see Get in package mtastsdb.
-func Get(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (record *Record, policy *Policy, err error) {
+func Get(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (record *Record, policy *Policy, policyText string, err error) {
 	log := xlog.WithContext(ctx)
 	start := time.Now()
 	result := "lookuperror"
@@ -313,15 +340,15 @@ func Get(ctx context.Context, resolver dns.Resolver, domain dns.Domain) (record
 
 	record, _, err = LookupRecord(ctx, resolver, domain)
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, "", err
 	}
 
 	result = "fetcherror"
-	policy, _, err = FetchPolicy(ctx, domain)
+	policy, policyText, err = FetchPolicy(ctx, domain)
 	if err != nil {
-		return record, nil, err
+		return record, nil, "", err
 	}
 
 	result = "ok"
-	return record, policy, nil
+	return record, policy, policyText, nil
 }