cmd/pubsubhelper/pubsubhelper.go - build - Git at Google

 // Copyright 2017 The Go Authors. All rights reserved.
 // Use of this source code is governed by the Apache 2.0
 // license that can be found in the LICENSE file.

 // The pubsubhelper is an SMTP server for Gerrit updates and an HTTP
 // server for Github webhook updates. It then lets other clients subscribe
 // to those changes.
 package main

 import (
 	"bufio"
 	"bytes"
 	"context"
 	"crypto/tls"
 	"encoding/json"
 	"errors"
 	"flag"
 	"fmt"
 	"io"
 	"log"
 	"net"
 	"net/http"
 	"net/textproto"
 	"os"
 	"os/signal"
 	"strconv"
 	"strings"
 	"sync"
 	"time"

 	"github.com/bradfitz/go-smtpd/smtpd"
 	"github.com/jellevandenhooff/dkim"
 	"go4.org/types"
 	"golang.org/x/build/cmd/pubsubhelper/pubsubtypes"
 	"golang.org/x/crypto/acme/autocert"
 )

 var (
 	botEmail   = flag.String("rcpt", "\x67\x6f\x70\x68\x65\x72\x62\x6f\x74@pubsubhelper.golang.org", "email address of bot. incoming emails must be to this address.")
 	httpListen = flag.String("http", ":80", "HTTP listen address")
 	acmeDomain = flag.String("autocert", "pubsubhelper.golang.org", "If non-empty, listen on port 443 and serve HTTPS with a LetsEncrypt cert for this domain.")
 	smtpListen = flag.String("smtp", ":25", "SMTP listen address")
 )

 func main() {
 	flag.Parse()

 	ch := make(chan os.Signal, 1)
 	signal.Notify(ch, os.Interrupt)
 	go func() {
 		sig := <-ch
 		log.Printf("Signal %v received; exiting with status 0.", sig)
 		os.Exit(0)
 	}()

 	http.HandleFunc("/", handleRoot)
 	http.HandleFunc("/waitevent", handleWaitEvent)
 	http.HandleFunc("/recent", handleRecent)
 	http.HandleFunc("/github-webhook", handleGithubWebhook)

 	errc := make(chan error)
 	go func() {
 		log.Printf("running pubsubhelper on %s", *smtpListen)
 		s := &smtpd.Server{
 			Addr:            *smtpListen,
 			OnNewMail:       onNewMail,
 			OnNewConnection: onNewConnection,
 			ReadTimeout:     time.Minute,
 		}
 		err := s.ListenAndServe()
 		errc <- fmt.Errorf("SMTP ListenAndServe: %v", err)
 	}()
 	m := &autocert.Manager{
 		Prompt:     autocert.AcceptTOS,
 		HostPolicy: autocert.HostWhitelist(*acmeDomain),
 	}
 	go func() {
 		if *acmeDomain == "" {
 			return
 		}
 		if _, err := os.Stat("/autocert-cache"); err == nil {
 			m.Cache = autocert.DirCache("/autocert-cache")
 		} else {
 			log.Printf("Warning: running acme/autocert without cache")
 		}
 		log.Printf("running pubsubhelper HTTPS on :443 for %s", *acmeDomain)
 		s := &http.Server{
 			Addr:              ":https",
 			TLSConfig:         &tls.Config{GetCertificate: m.GetCertificate},
 			ReadHeaderTimeout: 10 * time.Second,
 			WriteTimeout:      5 * time.Minute,
 			IdleTimeout:       5 * time.Minute,
 		}
 		err := s.ListenAndServeTLS("", "")
 		errc <- fmt.Errorf("HTTPS ListenAndServeTLS: %v", err)
 	}()
 	go func() {
 		log.Printf("running pubsubhelper HTTP on %s", *httpListen)
 		s := &http.Server{
 			Addr:              *httpListen,
 			ReadHeaderTimeout: 10 * time.Second,
 			WriteTimeout:      5 * time.Minute,
 			IdleTimeout:       5 * time.Minute,
 			Handler:           m.HTTPHandler(http.DefaultServeMux),
 		}
 		err := s.ListenAndServe()
 		errc <- fmt.Errorf("HTTP ListenAndServe: %v", err)
 	}()

 	log.Fatal(<-errc)
 }

 func handleRoot(w http.ResponseWriter, r *http.Request) {
 	if r.URL.Path != "/" {
 		http.NotFound(w, r)
 		return
 	}
 	io.WriteString(w, `<html>
 <body>
   This is <a href="https://godoc.org/golang.org/x/build/cmd/pubsubhelper">pubsubhelper</a>.

 <ul>
    <li><b><a href="/waitevent">/waitevent</a></b>: long-poll wait 30s for next event (use ?after=[RFC3339Nano] to resume at point)</li>
    <li><b><a href="/recent">/recent</a></b>: recent events, without long-polling.</li>
 </ul>

 </body>
 </html>
 `)
 }

 func handleWaitEvent(w http.ResponseWriter, r *http.Request) {
 	if r.Method != "GET" {
 		http.Error(w, "requires GET", http.StatusBadRequest)
 		return
 	}

 	ch := make(chan *eventAndJSON, 1)

 	var after time.Time
 	if v := r.FormValue("after"); v != "" {
 		var err error
 		after, err = time.Parse(time.RFC3339Nano, v)
 		if err != nil {
 			http.Error(w, "'after' parameter is not in time.RFC3339Nano format", http.StatusBadRequest)
 			return
 		}
 	} else {
 		after = time.Now()
 	}

 	register(ch, after)
 	defer unregister(ch)
 	ctx := r.Context()

 	timer := time.NewTimer(30 * time.Second)
 	defer timer.Stop()

 	var e *eventAndJSON
 	select {
 	case <-ctx.Done():
 		return
 	case <-timer.C:
 		e = newEventAndJSON(&pubsubtypes.Event{
 			LongPollTimeout: true,
 		})
 	case e = <-ch:
 	}

 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 	io.WriteString(w, e.json)
 }

 func handleRecent(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")

 	var after time.Time
 	if v := r.FormValue("after"); v != "" {
 		var err error
 		after, err = time.Parse(time.RFC3339Nano, v)
 		if err != nil {
 			http.Error(w, "'after' parameter is not in time.RFC3339Nano format", http.StatusBadRequest)
 			return
 		}
 	}

 	var buf bytes.Buffer
 	mu.Lock()
 	buf.WriteString("[\n")
 	n := 0
 	for i := len(recent) - 1; i >= 0; i-- {
 		ev := recent[i]
 		if ev.Time.Time().Before(after) {
 			continue
 		}
 		if n > 0 {
 			buf.WriteString(",\n")
 		}
 		n++
 		buf.WriteString(ev.json)
 	}
 	buf.WriteString("\n]\n")
 	mu.Unlock()
 	w.Write(buf.Bytes())
 }

 type env struct {
 	from    smtpd.MailAddress
 	body    bytes.Buffer
 	conn    smtpd.Connection
 	tooBig  bool
 	hasRcpt bool
 }

 func (e *env) BeginData() error {
 	if !e.hasRcpt {
 		return smtpd.SMTPError("554 5.5.1 Error: no valid recipients")
 	}
 	return nil
 }

 func (e *env) AddRecipient(rcpt smtpd.MailAddress) error {
 	if e.hasRcpt {
 		return smtpd.SMTPError("554 5.5.1 Error: dup recipients")
 	}
 	to := rcpt.Email()
 	if to != *botEmail {
 		return errors.New("bogus recipient")
 	}
 	e.hasRcpt = true
 	return nil
 }

 func (e *env) Write(line []byte) error {
 	const maxSize = 5 << 20
 	if e.body.Len() > maxSize {
 		e.tooBig = true
 		return nil
 	}
 	e.body.Write(line)
 	return nil
 }

 var (
 	headerSep           = []byte("\r\n\r\n")
 	dkimSignatureHeader = []byte("\nDKIM-Signature:")
 )

 func (e *env) Close() error {
 	if e.tooBig {
 		log.Printf("Ignoring too-large email from %q", e.from)
 		return nil
 	}
 	from := e.from.Email()
 	bodyBytes := e.body.Bytes()
 	if !bytes.Contains(bodyBytes, dkimSignatureHeader) {
 		log.Printf("Ignoring unsigned (~spam) email from %q", from)
 		return nil
 	}

 	headerBytes := bodyBytes
 	if i := bytes.Index(headerBytes, headerSep); i == -1 {
 		log.Printf("Ignoring email without header separator from %q", from)
 		return nil
 	} else {
 		headerBytes = headerBytes[:i+len(headerSep)]
 	}

 	ve, err := dkim.ParseAndVerify(string(headerBytes), dkim.HeadersOnly, dnsClient{})
 	if err != nil {
 		log.Printf("Email from %q didn't pass DKIM verifications: %v", from, err)
 		return nil
 	}
 	if !strings.HasSuffix(ve.Signature.Domain, "google.com") {
 		log.Printf("Ignoring DKIM-verified Gerrit email from non-Google domain %q", ve.Signature.Domain)
 		return nil
 	}
 	tp := textproto.NewReader(bufio.NewReader(bytes.NewReader(headerBytes)))
 	hdr, err := tp.ReadMIMEHeader()
 	if err != nil {
 		log.Printf("Ignoring ReadMIMEHeader error: %v    from email:\n%s", err, headerBytes)
 		return nil
 	}
 	if e.from.Hostname() != "gerritcodereview.bounces.google.com" {
 		log.Printf("Ignoring signed, DKIM-verified, non-Gerrit email from %q:\n%s", from, bodyBytes)
 		return nil
 	}
 	changeNum, _ := strconv.Atoi(hdr.Get("X-Gerrit-Change-Number"))

 	// Extract gerrit project "oauth2" from List-Id header like:
 	// List-Id: <gerrit-oauth2.go-review.googlesource.com>
 	project := strings.TrimPrefix(hdr.Get("List-Id"), "<gerrit-")
 	if i := strings.IndexByte(project, '.'); i == -1 {
 		project = ""
 	} else {
 		project = project[:i]
 	}
 	publish(&pubsubtypes.Event{
 		Gerrit: &pubsubtypes.GerritEvent{
 			URL:          strings.Trim(hdr.Get("X-Gerrit-ChangeURL"), "<>"),
 			Project:      project,
 			CommitHash:   hdr.Get("X-Gerrit-Commit"),
 			ChangeNumber: changeNum,
 		},
 	})
 	return nil
 }

 type eventAndJSON struct {
 	*pubsubtypes.Event
 	json string // JSON MarshalIndent of Event
 }

 var (
 	mu      sync.Mutex      // guards following
 	recent  []*eventAndJSON // newest at end
 	waiting = map[chan *eventAndJSON]struct{}{}
 )

 const (
 	keepMin = 50
 	maxAge  = 1 * time.Hour
 )

 func register(ch chan *eventAndJSON, after time.Time) {
 	mu.Lock()
 	defer mu.Unlock()
 	for _, e := range recent {
 		if e.Time.Time().After(after) {
 			ch <- e
 			return
 		}
 	}
 	waiting[ch] = struct{}{}
 }

 func unregister(ch chan *eventAndJSON) {
 	mu.Lock()
 	defer mu.Unlock()
 	delete(waiting, ch)
 }

 // numOldInRecentLocked returns how many leading items of recent are
 // too old.
 func numOldInRecentLocked() int {
 	if len(recent) <= keepMin {
 		return 0
 	}
 	n := 0
 	tooOld := time.Now().Add(-maxAge)
 	for _, e := range recent {
 		if e.Time.Time().After(tooOld) {
 			break
 		}
 		n++
 	}
 	return n
 }

 func newEventAndJSON(e *pubsubtypes.Event) *eventAndJSON {
 	e.Time = types.Time3339(time.Now())
 	j, err := json.MarshalIndent(e, "", "\t")
 	if err != nil {
 		log.Printf("JSON error: %v", err)
 	}
 	return &eventAndJSON{
 		Event: e,
 		json:  string(j),
 	}
 }

 func publish(e *pubsubtypes.Event) {
 	ej := newEventAndJSON(e)
 	log.Printf("Event: %s", ej.json)

 	mu.Lock()
 	defer mu.Unlock()

 	recent = append(recent, ej)
 	// Trim old ones off the front of recent
 	if n := numOldInRecentLocked(); n > 0 {
 		copy(recent, recent[n:])
 		recent = recent[:len(recent)-n]
 	}

 	for ch := range waiting {
 		ch <- ej
 		delete(waiting, ch)
 	}
 }

 type dnsClient struct{}

 var resolver = &net.Resolver{PreferGo: true}

 func (dnsClient) LookupTxt(hostname string) ([]string, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
 	defer cancel()
 	return resolver.LookupTXT(ctx, hostname)
 }

 func onNewMail(c smtpd.Connection, from smtpd.MailAddress) (smtpd.Envelope, error) {
 	return &env{
 		from: from,
 		conn: c,
 	}, nil
 }

 func onNewConnection(c smtpd.Connection) error {
 	log.Printf("smtpd: new connection from %v", c.Addr())
 	return nil
 }
	// Copyright 2017 The Go Authors. All rights reserved.
	// Use of this source code is governed by the Apache 2.0
	// license that can be found in the LICENSE file.

	// The pubsubhelper is an SMTP server for Gerrit updates and an HTTP
	// server for Github webhook updates. It then lets other clients subscribe
	// to those changes.
	package main

	import (
	"bufio"
	"bytes"
	"context"
	"crypto/tls"
	"encoding/json"
	"errors"
	"flag"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
	"net/textproto"
	"os"
	"os/signal"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/bradfitz/go-smtpd/smtpd"
	"github.com/jellevandenhooff/dkim"
	"go4.org/types"
	"golang.org/x/build/cmd/pubsubhelper/pubsubtypes"
	"golang.org/x/crypto/acme/autocert"
	)

	var (
	botEmail = flag.String("rcpt", "\x67\x6f\x70\x68\x65\x72\x62\x6f\x74@pubsubhelper.golang.org", "email address of bot. incoming emails must be to this address.")
	httpListen = flag.String("http", ":80", "HTTP listen address")
	acmeDomain = flag.String("autocert", "pubsubhelper.golang.org", "If non-empty, listen on port 443 and serve HTTPS with a LetsEncrypt cert for this domain.")
	smtpListen = flag.String("smtp", ":25", "SMTP listen address")
	)

	func main() {
	flag.Parse()

	ch := make(chan os.Signal, 1)
	signal.Notify(ch, os.Interrupt)
	go func() {
	sig := <-ch
	log.Printf("Signal %v received; exiting with status 0.", sig)
	os.Exit(0)
	}()

	http.HandleFunc("/", handleRoot)
	http.HandleFunc("/waitevent", handleWaitEvent)
	http.HandleFunc("/recent", handleRecent)
	http.HandleFunc("/github-webhook", handleGithubWebhook)

	errc := make(chan error)
	go func() {
	log.Printf("running pubsubhelper on %s", *smtpListen)
	s := &smtpd.Server{
	Addr: *smtpListen,
	OnNewMail: onNewMail,
	OnNewConnection: onNewConnection,
	ReadTimeout: time.Minute,
	}
	err := s.ListenAndServe()
	errc <- fmt.Errorf("SMTP ListenAndServe: %v", err)
	}()
	m := &autocert.Manager{
	Prompt: autocert.AcceptTOS,
	HostPolicy: autocert.HostWhitelist(*acmeDomain),
	}
	go func() {
	if *acmeDomain == "" {
	return
	}
	if _, err := os.Stat("/autocert-cache"); err == nil {
	m.Cache = autocert.DirCache("/autocert-cache")
	} else {
	log.Printf("Warning: running acme/autocert without cache")
	}
	log.Printf("running pubsubhelper HTTPS on :443 for %s", *acmeDomain)
	s := &http.Server{
	Addr: ":https",
	TLSConfig: &tls.Config{GetCertificate: m.GetCertificate},
	ReadHeaderTimeout: 10 * time.Second,
	WriteTimeout: 5 * time.Minute,
	IdleTimeout: 5 * time.Minute,
	}
	err := s.ListenAndServeTLS("", "")
	errc <- fmt.Errorf("HTTPS ListenAndServeTLS: %v", err)
	}()
	go func() {
	log.Printf("running pubsubhelper HTTP on %s", *httpListen)
	s := &http.Server{
	Addr: *httpListen,
	ReadHeaderTimeout: 10 * time.Second,
	WriteTimeout: 5 * time.Minute,
	IdleTimeout: 5 * time.Minute,
	Handler: m.HTTPHandler(http.DefaultServeMux),
	}
	err := s.ListenAndServe()
	errc <- fmt.Errorf("HTTP ListenAndServe: %v", err)
	}()

	log.Fatal(<-errc)
	}

	func handleRoot(w http.ResponseWriter, r *http.Request) {
	if r.URL.Path != "/" {
	http.NotFound(w, r)
	return
	}
	io.WriteString(w, `<html>
	<body>
	This is <a href="https://godoc.org/golang.org/x/build/cmd/pubsubhelper">pubsubhelper</a>.

	<ul>
	<li><b><a href="/waitevent">/waitevent</a></b>: long-poll wait 30s for next event (use ?after=[RFC3339Nano] to resume at point)</li>
	<li><b><a href="/recent">/recent</a></b>: recent events, without long-polling.</li>
	</ul>

	</body>
	</html>
	`)
	}

	func handleWaitEvent(w http.ResponseWriter, r *http.Request) {
	if r.Method != "GET" {
	http.Error(w, "requires GET", http.StatusBadRequest)
	return
	}

	ch := make(chan *eventAndJSON, 1)

	var after time.Time
	if v := r.FormValue("after"); v != "" {
	var err error
	after, err = time.Parse(time.RFC3339Nano, v)
	if err != nil {
	http.Error(w, "'after' parameter is not in time.RFC3339Nano format", http.StatusBadRequest)
	return
	}
	} else {
	after = time.Now()
	}

	register(ch, after)
	defer unregister(ch)
	ctx := r.Context()

	timer := time.NewTimer(30 * time.Second)
	defer timer.Stop()

	var e *eventAndJSON
	select {
	case <-ctx.Done():
	return
	case <-timer.C:
	e = newEventAndJSON(&pubsubtypes.Event{
	LongPollTimeout: true,
	})
	case e = <-ch:
	}

	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	io.WriteString(w, e.json)
	}

	func handleRecent(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "application/json; charset=utf-8")

	var after time.Time
	if v := r.FormValue("after"); v != "" {
	var err error
	after, err = time.Parse(time.RFC3339Nano, v)
	if err != nil {
	http.Error(w, "'after' parameter is not in time.RFC3339Nano format", http.StatusBadRequest)
	return
	}
	}

	var buf bytes.Buffer
	mu.Lock()
	buf.WriteString("[\n")
	n := 0
	for i := len(recent) - 1; i >= 0; i-- {
	ev := recent[i]
	if ev.Time.Time().Before(after) {
	continue
	}
	if n > 0 {
	buf.WriteString(",\n")
	}
	n++
	buf.WriteString(ev.json)
	}
	buf.WriteString("\n]\n")
	mu.Unlock()
	w.Write(buf.Bytes())
	}

	type env struct {
	from smtpd.MailAddress
	body bytes.Buffer
	conn smtpd.Connection
	tooBig bool
	hasRcpt bool
	}

	func (e *env) BeginData() error {
	if !e.hasRcpt {
	return smtpd.SMTPError("554 5.5.1 Error: no valid recipients")
	}
	return nil
	}

	func (e *env) AddRecipient(rcpt smtpd.MailAddress) error {
	if e.hasRcpt {
	return smtpd.SMTPError("554 5.5.1 Error: dup recipients")
	}
	to := rcpt.Email()
	if to != *botEmail {
	return errors.New("bogus recipient")
	}
	e.hasRcpt = true
	return nil
	}

	func (e *env) Write(line []byte) error {
	const maxSize = 5 << 20
	if e.body.Len() > maxSize {
	e.tooBig = true
	return nil
	}
	e.body.Write(line)
	return nil
	}

	var (
	headerSep = []byte("\r\n\r\n")
	dkimSignatureHeader = []byte("\nDKIM-Signature:")
	)

	func (e *env) Close() error {
	if e.tooBig {
	log.Printf("Ignoring too-large email from %q", e.from)
	return nil
	}
	from := e.from.Email()
	bodyBytes := e.body.Bytes()
	if !bytes.Contains(bodyBytes, dkimSignatureHeader) {
	log.Printf("Ignoring unsigned (~spam) email from %q", from)
	return nil
	}

	headerBytes := bodyBytes
	if i := bytes.Index(headerBytes, headerSep); i == -1 {
	log.Printf("Ignoring email without header separator from %q", from)
	return nil
	} else {
	headerBytes = headerBytes[:i+len(headerSep)]
	}

	ve, err := dkim.ParseAndVerify(string(headerBytes), dkim.HeadersOnly, dnsClient{})
	if err != nil {
	log.Printf("Email from %q didn't pass DKIM verifications: %v", from, err)
	return nil
	}
	if !strings.HasSuffix(ve.Signature.Domain, "google.com") {
	log.Printf("Ignoring DKIM-verified Gerrit email from non-Google domain %q", ve.Signature.Domain)
	return nil
	}
	tp := textproto.NewReader(bufio.NewReader(bytes.NewReader(headerBytes)))
	hdr, err := tp.ReadMIMEHeader()
	if err != nil {
	log.Printf("Ignoring ReadMIMEHeader error: %v from email:\n%s", err, headerBytes)
	return nil
	}
	if e.from.Hostname() != "gerritcodereview.bounces.google.com" {
	log.Printf("Ignoring signed, DKIM-verified, non-Gerrit email from %q:\n%s", from, bodyBytes)
	return nil
	}
	changeNum, _ := strconv.Atoi(hdr.Get("X-Gerrit-Change-Number"))

	// Extract gerrit project "oauth2" from List-Id header like:
	// List-Id: <gerrit-oauth2.go-review.googlesource.com>
	project := strings.TrimPrefix(hdr.Get("List-Id"), "<gerrit-")
	if i := strings.IndexByte(project, '.'); i == -1 {
	project = ""
	} else {
	project = project[:i]
	}
	publish(&pubsubtypes.Event{
	Gerrit: &pubsubtypes.GerritEvent{
	URL: strings.Trim(hdr.Get("X-Gerrit-ChangeURL"), "<>"),
	Project: project,
	CommitHash: hdr.Get("X-Gerrit-Commit"),
	ChangeNumber: changeNum,
	},
	})
	return nil
	}

	type eventAndJSON struct {
	*pubsubtypes.Event
	json string // JSON MarshalIndent of Event
	}

	var (
	mu sync.Mutex // guards following
	recent []*eventAndJSON // newest at end
	waiting = map[chan *eventAndJSON]struct{}{}
	)

	const (
	keepMin = 50
	maxAge = 1 * time.Hour
	)

	func register(ch chan *eventAndJSON, after time.Time) {
	mu.Lock()
	defer mu.Unlock()
	for _, e := range recent {
	if e.Time.Time().After(after) {
	ch <- e
	return
	}
	}
	waiting[ch] = struct{}{}
	}

	func unregister(ch chan *eventAndJSON) {
	mu.Lock()
	defer mu.Unlock()
	delete(waiting, ch)
	}

	// numOldInRecentLocked returns how many leading items of recent are
	// too old.
	func numOldInRecentLocked() int {
	if len(recent) <= keepMin {
	return 0
	}
	n := 0
	tooOld := time.Now().Add(-maxAge)
	for _, e := range recent {
	if e.Time.Time().After(tooOld) {
	break
	}
	n++
	}
	return n
	}

	func newEventAndJSON(e pubsubtypes.Event) eventAndJSON {
	e.Time = types.Time3339(time.Now())
	j, err := json.MarshalIndent(e, "", "\t")
	if err != nil {
	log.Printf("JSON error: %v", err)
	}
	return &eventAndJSON{
	Event: e,
	json: string(j),
	}
	}

	func publish(e *pubsubtypes.Event) {
	ej := newEventAndJSON(e)
	log.Printf("Event: %s", ej.json)

	mu.Lock()
	defer mu.Unlock()

	recent = append(recent, ej)
	// Trim old ones off the front of recent
	if n := numOldInRecentLocked(); n > 0 {
	copy(recent, recent[n:])
	recent = recent[:len(recent)-n]
	}

	for ch := range waiting {
	ch <- ej
	delete(waiting, ch)
	}
	}

	type dnsClient struct{}

	var resolver = &net.Resolver{PreferGo: true}

	func (dnsClient) LookupTxt(hostname string) ([]string, error) {
	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
	defer cancel()
	return resolver.LookupTXT(ctx, hostname)
	}

	func onNewMail(c smtpd.Connection, from smtpd.MailAddress) (smtpd.Envelope, error) {
	return &env{
	from: from,
	conn: c,
	}, nil
	}

	func onNewConnection(c smtpd.Connection) error {
	log.Printf("smtpd: new connection from %v", c.Addr())
	return nil
	}