implement dnssec-awareness throughout code, and dane for incoming/outgoing mail delivery

the vendored dns resolver code is a copy of the go stdlib dns resolver, with
awareness of the "authentic data" (i.e. dnssec secure) added, as well as support
for enhanced dns errors, and looking up tlsa records (for dane). ideally it
would be upstreamed, but the chances seem slim.

dnssec-awareness is added to all packages, e.g. spf, dkim, dmarc, iprev. their
dnssec status is added to the Received message headers for incoming email.

but the main reason to add dnssec was for implementing dane. with dane, the
verification of tls certificates can be done through certificates/public keys
published in dns (in the tlsa records). this only makes sense (is trustworthy)
if those dns records can be verified to be authentic.

mox now applies dane to delivering messages over smtp. mox already implemented
mta-sts for webpki/pkix-verification of certificates against the (large) pool
of CA's, and still enforces those policies when present. but it now also checks
for dane records, and will verify those if present. if dane and mta-sts are
both absent, the regular opportunistic tls with starttls is still done. and the
fallback to plaintext is also still done.

mox also makes it easy to setup dane for incoming deliveries, so other servers
can deliver with dane tls certificate verification. the quickstart now
generates private keys that are used when requesting certificates with acme.
the private keys are pre-generated because they must be static and known during
setup, because their public keys must be published in tlsa records in dns.
autocert would generate private keys on its own, so had to be forked to add the
option to provide the private key when requesting a new certificate. hopefully
upstream will accept the change and we can drop the fork.

with this change, using the quickstart to setup a new mox instance, the checks
at internet.nl result in a 100% score, provided the domain is dnssec-signed and
the network doesn't have any issues.

mox-/admin.go

@@ -3,9 +3,11 @@ package mox
 import (
 	"bytes"
 	"context"
+	"crypto"
 	"crypto/ed25519"
 	cryptorand "crypto/rand"
 	"crypto/rsa"
+	"crypto/sha256"
 	"crypto/x509"
 	"encoding/pem"
 	"fmt"
@@ -19,6 +21,8 @@ import (
 
 	"golang.org/x/exp/maps"
 
+	"github.com/mjl-/adns"
+
 	"github.com/mjl-/mox/config"
 	"github.com/mjl-/mox/dkim"
 	"github.com/mjl-/mox/dmarc"
@@ -446,7 +450,7 @@ func WebserverConfigSet(ctx context.Context, domainRedirects map[string]string,
 
 // DomainRecords returns text lines describing DNS records required for configuring
 // a domain.
-func DomainRecords(domConf config.Domain, domain dns.Domain) ([]string, error) {
+func DomainRecords(domConf config.Domain, domain dns.Domain, hasDNSSEC bool) ([]string, error) {
 	d := domain.ASCII
 	h := Conf.Static.HostnameDomain.ASCII
 
@@ -457,6 +461,60 @@ func DomainRecords(domConf config.Domain, domain dns.Domain) ([]string, error) {
 		"",
 	}
 
+	if public, ok := Conf.Static.Listeners["public"]; ok && public.TLS != nil && (len(public.TLS.HostPrivateRSA2048Keys) > 0 || len(public.TLS.HostPrivateECDSAP256Keys) > 0) {
+		records = append(records,
+			"; DANE: These records indicate that a remote mail server trying to deliver email",
+			"; with SMTP (TCP port 25) must verify the TLS certificate with DANE-EE (3), based",
+			"; on the certificate public key (\"SPKI\", 1) that is SHA2-256-hashed (1) to the",
+			"; hexadecimal hash. DANE-EE verification means only the certificate or public",
+			"; key is verified, not whether the certificate is signed by a (centralized)",
+			"; certificate authority (CA), is expired, or matches the host name.",
+			";",
+			"; NOTE: Create the records below only once: They are for the machine, and apply",
+			"; to all hosted domains.",
+		)
+		if !hasDNSSEC {
+			records = append(records,
+				";",
+				"; WARNING: Domain does not appear to be DNSSEC-signed. To enable DANE, first",
+				"; enable DNSSEC on your domain, then add the TLSA records. Records below have been",
+				"; commented out.",
+			)
+		}
+		addTLSA := func(privKey crypto.Signer) error {
+			spkiBuf, err := x509.MarshalPKIXPublicKey(privKey.Public())
+			if err != nil {
+				return fmt.Errorf("marshal SubjectPublicKeyInfo for DANE record: %v", err)
+			}
+			sum := sha256.Sum256(spkiBuf)
+			tlsaRecord := adns.TLSA{
+				Usage:     adns.TLSAUsageDANEEE,
+				Selector:  adns.TLSASelectorSPKI,
+				MatchType: adns.TLSAMatchTypeSHA256,
+				CertAssoc: sum[:],
+			}
+			var s string
+			if hasDNSSEC {
+				s = fmt.Sprintf("_25._tcp.%-*s IN TLSA %s", 20+len(d)-len("_25._tcp."), h+".", tlsaRecord.Record())
+			} else {
+				s = fmt.Sprintf(";; _25._tcp.%-*s IN TLSA %s", 20+len(d)-len(";; _25._tcp."), h+".", tlsaRecord.Record())
+			}
+			records = append(records, s)
+			return nil
+		}
+		for _, privKey := range public.TLS.HostPrivateECDSAP256Keys {
+			if err := addTLSA(privKey); err != nil {
+				return nil, err
+			}
+		}
+		for _, privKey := range public.TLS.HostPrivateRSA2048Keys {
+			if err := addTLSA(privKey); err != nil {
+				return nil, err
+			}
+		}
+		records = append(records, "")
+	}
+
 	if d != h {
 		records = append(records,
 			"; For the machine, only needs to be created once, for the first domain added.",
@@ -537,7 +595,9 @@ func DomainRecords(domConf config.Domain, domain dns.Domain) ([]string, error) {
 
 	if sts := domConf.MTASTS; sts != nil {
 		records = append(records,
-			"; TLS must be used when delivering to us.",
+			"; Remote servers can use MTA-STS to verify our TLS certificate with the",
+			"; WebPKI pool of CA's (certificate authorities) when delivering over SMTP with",
+			"; STARTTLSTLS.",
 			fmt.Sprintf(`mta-sts.%s.            IN CNAME %s.`, d, h),
 			fmt.Sprintf(`_mta-sts.%s.           IN TXT "v=STSv1; id=%s"`, d, sts.PolicyID),
 			"",