add TransportDirect transport

The `TransportDirect` transport allows to tweak outgoing SMTP connexions
to remote servers. Currently, it only allows to select network IP family
(ipv4, ipv6 or both).

For example, to disable ipv6 for all outgoing SMTP connexions:
- add these lines in mox.conf to create a new transport named
"disableipv6":
```
Transports:
  disableipv6:
    Direct:
      DisableIpv6: true
```
- then add these lines in domains.conf to use this transport:
```
Routes:
  -
    Transport: disableipv6
```

fix #149

config/config.go

@@ -222,10 +222,11 @@ type WebService struct {
 // be non-nil. The non-nil field represents the type of transport. For a
 // transport with all fields nil, regular email delivery is done.
 type Transport struct {
-	Submissions *TransportSMTP  `sconf:"optional" sconf-doc:"Submission SMTP over a TLS connection to submit email to a remote queue."`
-	Submission  *TransportSMTP  `sconf:"optional" sconf-doc:"Submission SMTP over a plain TCP connection (possibly with STARTTLS) to submit email to a remote queue."`
-	SMTP        *TransportSMTP  `sconf:"optional" sconf-doc:"SMTP over a plain connection (possibly with STARTTLS), typically for old-fashioned unauthenticated relaying to a remote queue."`
-	Socks       *TransportSocks `sconf:"optional" sconf-doc:"Like regular direct delivery, but makes outgoing connections through a SOCKS proxy."`
+	Submissions *TransportSMTP   `sconf:"optional" sconf-doc:"Submission SMTP over a TLS connection to submit email to a remote queue."`
+	Submission  *TransportSMTP   `sconf:"optional" sconf-doc:"Submission SMTP over a plain TCP connection (possibly with STARTTLS) to submit email to a remote queue."`
+	SMTP        *TransportSMTP   `sconf:"optional" sconf-doc:"SMTP over a plain connection (possibly with STARTTLS), typically for old-fashioned unauthenticated relaying to a remote queue."`
+	Socks       *TransportSocks  `sconf:"optional" sconf-doc:"Like regular direct delivery, but makes outgoing connections through a SOCKS proxy."`
+	Direct      *TransportDirect `sconf:"optional" sconf-doc:"Like regular direct delivery, but allows to tweak outgoing connections."`
 }
 
 // TransportSMTP delivers messages by "submission" (SMTP, typically
@@ -262,6 +263,13 @@ type TransportSocks struct {
 	Hostname dns.Domain `sconf:"-" json:"-"` // Parsed form of RemoteHostname
 }
 
+type TransportDirect struct {
+	DisableIpv4 bool `sconf:"optional" sconf-doc:"If set, outgoing SMTP connexions will *NOT* use IPv4 addresses to connect to remote SMTP servers."`
+	DisableIpv6 bool `sconf:"optional" sconf-doc:"If set, outgoing SMTP connexions will *NOT* use IPv6 addresses to connect to remote SMTP servers."`
+
+	IPFamily string `sconf:"-" json:"-"`
+}
+
 type Domain struct {
 	Description                string  `sconf:"optional" sconf-doc:"Free-form description of domain."`
 	ClientSettingsDomain       string  `sconf:"optional" sconf-doc:"Hostname for client settings instead of the mail server hostname. E.g. mail.<domain>. For future migration to another mail operator without requiring all clients to update their settings, it is convenient to have client settings that reference a subdomain of the hosted domain instead of the hostname of the server where the mail is currently hosted. If empty, the hostname of the mail server is used for client configurations. Unicode name."`

config/doc.go

@@ -638,6 +638,18 @@ See https://pkg.go.dev/github.com/mjl-/sconf for details.
 				# typically the hostname of the host in the Address field.
 				RemoteHostname:
 
+			# Like regular direct delivery, but allows to tweak outgoing connections.
+			# (optional)
+			Direct:
+
+				# If set, outgoing SMTP connexions will *NOT* use IPv4 addresses to connect to
+				# remote SMTP servers. (optional)
+				DisableIpv4: false
+
+				# If set, outgoing SMTP connexions will *NOT* use IPv6 addresses to connect to
+				# remote SMTP servers. (optional)
+				DisableIpv6: false
+
 	# Do not send DMARC reports (aggregate only). By default, aggregate reports on
 	# DMARC evaluations are sent to domains if their DMARC policy requests them.
 	# Reports are sent at whole hours, with a minimum of 1 hour and maximum of 24

main.go

@@ -1505,7 +1505,7 @@ sharing most of its code.
 
 		log.Printf("attempting to connect to %s", host)
 
-		authentic, expandedAuthentic, expandedHost, ips, _, err := smtpclient.GatherIPs(ctxbg, c.log.Logger, resolver, host, dialedIPs)
+		authentic, expandedAuthentic, expandedHost, ips, _, err := smtpclient.GatherIPs(ctxbg, c.log.Logger, resolver, "ip", host, dialedIPs)
 		if err != nil {
 			log.Printf("resolving ips for %s: %v, skipping", host, err)
 			continue

mox-/config.go

@@ -929,6 +929,19 @@ func PrepareStaticConfig(ctx context.Context, log mlog.Log, configFile string, c
 		}
 	}
 
+	checkTransportDirect := func(name string, t *config.TransportDirect) {
+		if t.DisableIpv4 && t.DisableIpv6 {
+			addErrorf("transport %s: both IPv4 and IPv6 are disabled, enable at least one", name)
+		}
+		t.IPFamily = "ip"
+		if t.DisableIpv4 {
+			t.IPFamily = "ip6"
+		}
+		if t.DisableIpv6 {
+			t.IPFamily = "ip4"
+		}
+	}
+
 	for name, t := range c.Transports {
 		n := 0
 		if t.Submissions != nil {
@@ -947,6 +960,9 @@ func PrepareStaticConfig(ctx context.Context, log mlog.Log, configFile string, c
 			n++
 			checkTransportSocks(name, t.Socks)
 		}
+		if t.Direct != nil {
+			checkTransportDirect(name, t.Direct)
+		}
 		if n > 1 {
 			addErrorf("transport %s: cannot have multiple methods in a transport", name)
 		}

queue/direct.go

@@ -19,6 +19,7 @@ import (
 	"github.com/mjl-/adns"
 	"github.com/mjl-/bstore"
 
+	"github.com/mjl-/mox/config"
 	"github.com/mjl-/mox/dns"
 	"github.com/mjl-/mox/dsn"
 	"github.com/mjl-/mox/mlog"
@@ -110,7 +111,7 @@ type msgResp struct {
 // domain (MTA-STS), its policy type can be empty, in which case there is no
 // information (e.g. internal failure). hostResults are per-host details (DANE, one
 // per MX target).
-func deliverDirect(qlog mlog.Log, resolver dns.Resolver, dialer smtpclient.Dialer, ourHostname dns.Domain, transportName string, msgs []*Msg, backoff time.Duration) (recipientDomainResult tlsrpt.Result, hostResults []tlsrpt.Result) {
+func deliverDirect(qlog mlog.Log, resolver dns.Resolver, dialer smtpclient.Dialer, ourHostname dns.Domain, transportName string, transport config.Transport, msgs []*Msg, backoff time.Duration) (recipientDomainResult tlsrpt.Result, hostResults []tlsrpt.Result) {
 	// High-level approach:
 	// - Resolve domain to deliver to (CNAME), and determine hosts to try to deliver to (MX)
 	// - Get MTA-STS policy for domain (optional). If present, only deliver to its
@@ -252,7 +253,7 @@ func deliverDirect(qlog mlog.Log, resolver dns.Resolver, dialer smtpclient.Diale
 			msgResps[i] = &msgResp{msg: msgs[i]}
 		}
 
-		result := deliverHost(nqlog, resolver, dialer, ourHostname, transportName, h, enforceMTASTS, haveMX, origNextHopAuthentic, origNextHop, expandedNextHopAuthentic, expandedNextHop, msgResps, tlsMode, tlsPKIX, &recipientDomainResult)
+		result := deliverHost(nqlog, resolver, dialer, ourHostname, transportName, transport, h, enforceMTASTS, haveMX, origNextHopAuthentic, origNextHop, expandedNextHopAuthentic, expandedNextHop, msgResps, tlsMode, tlsPKIX, &recipientDomainResult)
 
 		var zerotype tlsrpt.PolicyType
 		if result.hostResult.Policy.Type != zerotype {
@@ -279,7 +280,7 @@ func deliverDirect(qlog mlog.Log, resolver dns.Resolver, dialer smtpclient.Diale
 				slog.Bool("enforcemtasts", enforceMTASTS),
 				slog.Bool("tlsdane", result.tlsDANE),
 				slog.Any("requiretls", m0.RequireTLS))
-			result = deliverHost(nqlog, resolver, dialer, ourHostname, transportName, h, enforceMTASTS, haveMX, origNextHopAuthentic, origNextHop, expandedNextHopAuthentic, expandedNextHop, msgResps, smtpclient.TLSSkip, false, &tlsrpt.Result{})
+			result = deliverHost(nqlog, resolver, dialer, ourHostname, transportName, transport, h, enforceMTASTS, haveMX, origNextHopAuthentic, origNextHop, expandedNextHopAuthentic, expandedNextHop, msgResps, smtpclient.TLSSkip, false, &tlsrpt.Result{})
 		}
 
 		remoteMTA = dsn.NameIP{Name: h.XString(false), IP: remoteIP}
@@ -375,7 +376,7 @@ type deliverResult struct {
 //
 // deliverHost may send a message multiple times: if the server doesn't accept
 // multiple recipients for a message.
-func deliverHost(log mlog.Log, resolver dns.Resolver, dialer smtpclient.Dialer, ourHostname dns.Domain, transportName string, host dns.IPDomain, enforceMTASTS, haveMX, origNextHopAuthentic bool, origNextHop dns.Domain, expandedNextHopAuthentic bool, expandedNextHop dns.Domain, msgResps []*msgResp, tlsMode smtpclient.TLSMode, tlsPKIX bool, recipientDomainResult *tlsrpt.Result) (result deliverResult) {
+func deliverHost(log mlog.Log, resolver dns.Resolver, dialer smtpclient.Dialer, ourHostname dns.Domain, transportName string, transport config.Transport, host dns.IPDomain, enforceMTASTS, haveMX, origNextHopAuthentic bool, origNextHop dns.Domain, expandedNextHopAuthentic bool, expandedNextHop dns.Domain, msgResps []*msgResp, tlsMode smtpclient.TLSMode, tlsPKIX bool, recipientDomainResult *tlsrpt.Result) (result deliverResult) {
 	// About attempting delivery to multiple addresses of a host: ../rfc/5321:3898
 
 	m0 := msgResps[0].msg
@@ -451,7 +452,14 @@ func deliverHost(log mlog.Log, resolver dns.Resolver, dialer smtpclient.Dialer,
 	}
 
 	metricDestinations.Inc()
-	authentic, expandedAuthentic, expandedHost, ips, dualstack, err := smtpclient.GatherIPs(ctx, log.Logger, resolver, host, m0.DialedIPs)
+	network := "ip"
+	if transport.Direct != nil {
+		if network != transport.Direct.IPFamily {
+			log.Debug("set custom IP network family for direct transport", slog.Any("network", transport.Direct.IPFamily))
+			network = transport.Direct.IPFamily
+		}
+	}
+	authentic, expandedAuthentic, expandedHost, ips, dualstack, err := smtpclient.GatherIPs(ctx, log.Logger, resolver, network, host, m0.DialedIPs)
 	destAuthentic := err == nil && authentic && origNextHopAuthentic && (!haveMX || expandedNextHopAuthentic) && host.IsDomain()
 	if !destAuthentic {
 		log.Debugx("not attempting verification with dane", err, slog.Bool("authentic", authentic), slog.Bool("expandedauthentic", expandedAuthentic))
@@ -645,6 +653,7 @@ func deliverHost(log mlog.Log, resolver dns.Resolver, dialer smtpclient.Dialer,
 			// attempt and remote has both IPv4 and IPv6, we'll give it
 			// another try. Our first IP may be in a block list, the address for
 			// the other family perhaps is not.
+
 			if cerr.Permanent && m0.Attempts == 1 && dualstack && strings.HasPrefix(cerr.Secode, "7.") {
 				cerr.Permanent = false
 			}

queue/queue.go

@@ -1075,7 +1075,7 @@ func deliver(log mlog.Log, resolver dns.Resolver, m Msg) {
 			}
 			ourHostname = transport.Socks.Hostname
 		}
-		recipientDomainResult, hostResults = deliverDirect(qlog, resolver, dialer, ourHostname, transportName, msgs, backoff)
+		recipientDomainResult, hostResults = deliverDirect(qlog, resolver, dialer, ourHostname, transportName, transport, msgs, backoff)
 	}
 }
 

queue/submit.go

@@ -101,7 +101,7 @@ func deliverSubmit(qlog mlog.Log, resolver dns.Resolver, dialer smtpclient.Diale
 		msgs[0].DialedIPs = map[string][]net.IP{}
 		m0 = msgs[0]
 	}
-	_, _, _, ips, _, err := smtpclient.GatherIPs(dialctx, qlog.Logger, resolver, dns.IPDomain{Domain: transport.DNSHost}, m0.DialedIPs)
+	_, _, _, ips, _, err := smtpclient.GatherIPs(dialctx, qlog.Logger, resolver, "ip", dns.IPDomain{Domain: transport.DNSHost}, m0.DialedIPs)
 	var conn net.Conn
 	if err == nil {
 		conn, _, err = smtpclient.Dial(dialctx, qlog.Logger, dialer, dns.IPDomain{Domain: transport.DNSHost}, ips, port, m0.DialedIPs, mox.Conf.Static.SpecifiedSMTPListenIPs)

smtpclient/dial_test.go

@@ -37,7 +37,7 @@ func TestDialHost(t *testing.T) {
 	}
 
 	dialedIPs := map[string][]net.IP{}
-	_, _, _, ips, dualstack, err := GatherIPs(ctxbg, log.Logger, resolver, ipdomain("dualstack.example"), dialedIPs)
+	_, _, _, ips, dualstack, err := GatherIPs(ctxbg, log.Logger, resolver, "ip", ipdomain("dualstack.example"), dialedIPs)
 	if err != nil || !reflect.DeepEqual(ips, []net.IP{net.ParseIP("10.0.0.1"), net.ParseIP("2001:db8::1")}) || !dualstack {
 		t.Fatalf("expected err nil, address 10.0.0.1,2001:db8::1, dualstack true, got %v %v %v", err, ips, dualstack)
 	}
@@ -46,7 +46,7 @@ func TestDialHost(t *testing.T) {
 		t.Fatalf("expected err nil, address 10.0.0.1, dualstack true, got %v %v %v", err, ip, dualstack)
 	}
 
-	_, _, _, ips, dualstack, err = GatherIPs(ctxbg, log.Logger, resolver, ipdomain("dualstack.example"), dialedIPs)
+	_, _, _, ips, dualstack, err = GatherIPs(ctxbg, log.Logger, resolver, "ip", ipdomain("dualstack.example"), dialedIPs)
 	if err != nil || !reflect.DeepEqual(ips, []net.IP{net.ParseIP("2001:db8::1"), net.ParseIP("10.0.0.1")}) || !dualstack {
 		t.Fatalf("expected err nil, address 2001:db8::1,10.0.0.1, dualstack true, got %v %v %v", err, ips, dualstack)
 	}

smtpclient/gather.go

@@ -170,7 +170,7 @@ func GatherDestinations(ctx context.Context, elog *slog.Logger, resolver dns.Res
 // GatherIPs looks up the IPs to try for connecting to host, with the IPs ordered
 // to take previous attempts into account. For use with DANE, the CNAME-expanded
 // name is returned, and whether the DNS responses were authentic.
-func GatherIPs(ctx context.Context, elog *slog.Logger, resolver dns.Resolver, host dns.IPDomain, dialedIPs map[string][]net.IP) (authentic bool, expandedAuthentic bool, expandedHost dns.Domain, ips []net.IP, dualstack bool, rerr error) {
+func GatherIPs(ctx context.Context, elog *slog.Logger, resolver dns.Resolver, network string, host dns.IPDomain, dialedIPs map[string][]net.IP) (authentic bool, expandedAuthentic bool, expandedHost dns.Domain, ips []net.IP, dualstack bool, rerr error) {
 	log := mlog.New("smtpclient", elog)
 
 	if len(host.IP) > 0 {
@@ -216,16 +216,16 @@ func GatherIPs(ctx context.Context, elog *slog.Logger, resolver dns.Resolver, ho
 		}
 	}
 
-	ipaddrs, result, err := resolver.LookupIPAddr(ctx, name)
+	ipaddrs, result, err := resolver.LookupIP(ctx, network, name)
 	authentic = authentic && result.Authentic
 	expandedAuthentic = expandedAuthentic && result.Authentic
 	if err != nil || len(ipaddrs) == 0 {
 		return authentic, expandedAuthentic, expandedHost, nil, false, fmt.Errorf("looking up %q: %w", name, err)
 	}
 	var have4, have6 bool
 	for _, ipaddr := range ipaddrs {
-		ips = append(ips, ipaddr.IP)
-		if ipaddr.IP.To4() == nil {
+		ips = append(ips, ipaddr)
+		if ipaddr.To4() == nil {
 			have6 = true
 		} else {
 			have4 = true

smtpclient/gather_test.go

@@ -155,7 +155,7 @@ func TestGatherIPs(t *testing.T) {
 			"temperror-cname.example.":      "absent.example.",
 		},
 		Fail: []string{
-			"host temperror-a.example.",
+			"ip temperror-a.example.",
 			"cname temperror-cname.example.",
 		},
 		Inauthentic: []string{"cname cnameinauthentic.example."},
@@ -164,7 +164,7 @@ func TestGatherIPs(t *testing.T) {
 	test := func(host dns.IPDomain, expAuthic, expAuthicExp bool, expHostExp dns.Domain, expIPs []net.IP, expErr any) {
 		t.Helper()
 
-		authic, authicExp, hostExp, ips, _, err := GatherIPs(ctxbg, log.Logger, resolver, host, nil)
+		authic, authicExp, hostExp, ips, _, err := GatherIPs(ctxbg, log.Logger, resolver, "ip", host, nil)
 		if (err == nil) != (expErr == nil) || err != nil && !(errors.Is(err, expErr.(error)) || errors.As(err, &expErr)) {
 			// todo: could also check the individual errors?
 			t.Fatalf("gather hosts: %v, expected %v", err, expErr)