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lmtp.go
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lmtp.go
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package ulist
import (
"io"
"log"
"net/mail"
"strings"
"sync/atomic"
"time"
"github.com/emersion/go-smtp" // not to be confused with golang's net/smtp
"github.com/wansing/ulist/mailutil"
)
func NewLMTPServer(ul *Ulist) *smtp.Server {
s := smtp.NewServer(&LMTPBackend{ul})
s.LMTP = true
s.Domain = "localhost"
s.WriteTimeout = 10 * time.Second
s.ReadTimeout = 10 * time.Second
s.MaxMessageBytes = 50 * 1024 * 1024 // 50 MiB sounds reasonable (note that base64 encoding costs 33%)
s.MaxRecipients = 50 // value in go-smtp example code is 50, postfix lmtp_destination_recipient_limit is 50
s.AllowInsecureAuth = true
return s
}
type LMTPBackend struct {
Ulist *Ulist
}
func (lb LMTPBackend) NewSession(_ *smtp.Conn) (smtp.Session, error) {
return &lmtpSession{
Ulist: lb.Ulist,
}, nil
}
// implements smtp.Session
type lmtpSession struct {
Ulist *Ulist
Lists []*List
isBounce bool // indicated by empty Envelope-From
logId uint32
}
func (s *lmtpSession) logf(format string, a ...interface{}) {
log.Printf("% 7d: "+format, append([]interface{}{s.logId}, a...)...)
}
// "RSET". Aborts the current mail transaction.
func (s *lmtpSession) Reset() {
s.Lists = nil
s.isBounce = false
}
func (*lmtpSession) Logout() error {
return nil
}
func (*lmtpSession) AuthPlain(username, password string) error {
return smtp.ErrAuthUnsupported
}
// "MAIL FROM". Starts a new mail transaction.
func (s *lmtpSession) Mail(envelopeFrom string, _ *smtp.MailOptions) error {
s.Reset() // just in case
s.logId = atomic.AddUint32(&s.Ulist.LastLogID, 1)
s.logf("envelope-from: %s", envelopeFrom)
if strings.TrimSpace(envelopeFrom) == "" {
s.isBounce = true
}
return nil
}
// "RCPT TO". Can be called multiple times for multiple recipients.
func (s *lmtpSession) Rcpt(to string, _ *smtp.RcptOptions) error {
err := s.rcpt(to)
if err != nil {
s.logf("\033[1;31mrcpt error: %v\033[0m", err) // red color
}
return err
}
func (s *lmtpSession) rcpt(toStr string) error {
s.logf("envelope-to: %s", toStr)
to, err := mailutil.ParseAddress(toStr)
if err != nil {
return SMTPErrorf(510, "parsing envelope-to address: %v", err) // 510 Bad email address
}
toBounce := strings.HasSuffix(to.Local, BounceAddressSuffix)
switch {
case toBounce && !s.isBounce:
return SMTPErrorf(541, "bounce address accepts only bounce notifications (with empty envelope-from)") // 541 The recipient address rejected your message
case !toBounce && s.isBounce:
return SMTPErrorf(541, "got bounce notification (with empty envelope-from) to non-bounce address") // 541 The recipient address rejected your message
case toBounce && s.isBounce:
to.Local = strings.TrimSuffix(to.Local, BounceAddressSuffix)
}
list, err := s.Ulist.Lists.GetList(to)
if err != nil {
return SMTPErrorf(451, "getting list from database: %v", err) // 451 Aborted – Local error in processing
}
if list == nil {
return SMTPErrUserNotExist
}
s.Lists = append(s.Lists, list)
return nil
}
// "DATA". Finishes a transaction.
func (s *lmtpSession) Data(r io.Reader) error {
s.Ulist.Waiting.Add(1)
defer s.Ulist.Waiting.Done()
if err := s.data(r); err != nil {
s.logf("\033[1;31mdata error: %v\033[0m", err) // red color
return err
}
return nil
}
func (s *lmtpSession) data(r io.Reader) error {
// check s.Lists again (in case MAIL FROM and RCPT TO have not been called before)
if len(s.Lists) == 0 {
return SMTPErrUserNotExist
}
message, err := mailutil.ReadMessage(r)
if err != nil {
return SMTPErrorf(442, "reading message: %v", err) // 442 The connection was dropped during the transmission
}
// logging
if from := mailutil.RobustWordDecode(message.Header.Get("From")); from != "" {
s.logf("from: %s", from)
}
if to := mailutil.RobustWordDecode(message.Header.Get("To")); to != "" {
s.logf("to: %s", to)
}
if cc := mailutil.RobustWordDecode(message.Header.Get("Cc")); cc != "" {
s.logf("cc: %s", cc)
}
if subject := mailutil.RobustWordDecode(message.Header.Get("Subject")); subject != "" {
s.logf("subject: %s", subject)
}
// do as many checks (and maybe rejections) as possible before sending any email
// check that lists are in to or cc, avoiding bcc spam
if !s.isBounce {
tos, err := mailutil.ParseAddressesFromHeader(message.Header, "To", 10000)
if err != nil {
return SMTPErrorf(510, "parsing to addresses: %v", err)
}
ccs, err := mailutil.ParseAddressesFromHeader(message.Header, "Cc", 10000)
if err != nil {
return SMTPErrorf(510, "parsing cc addresses: %v", err)
}
nextList:
for _, list := range s.Lists {
for _, to := range tos {
if list.Equals(to) {
continue nextList
}
}
for _, cc := range ccs {
if list.Equals(cc) {
continue nextList
}
}
return SMTPErrorf(541, "list address %s is not in To or Cc", list) // 541 The recipient address rejected your message
}
}
// check for mailing list loops
for _, field := range message.Header["List-Id"] {
listId, err := mailutil.ParseAddress(field)
if err != nil {
return SMTPErrorf(510, `parsing list-id field "%s": %v`, field, err) // 510 Bad email address
}
for _, list := range s.Lists {
if list.Equals(listId) {
return SMTPErrorf(554, "email loop detected: %s", list)
}
}
}
// process mail
for _, list := range s.Lists {
// if it's a bounce, forward it to all admins
if s.isBounce {
notifieds, err := s.Ulist.Lists.BounceNotifieds(list)
if err != nil {
return SMTPErrorf(451, "getting list bounce notifieds from database: %v", err) // 451 Aborted – Local error in processing
}
header := make(mail.Header)
header["Content-Type"] = []string{"text/plain; charset=utf-8"}
header["From"] = []string{list.RFC5322NameAddr()}
header["Message-Id"] = []string{list.NewMessageId()}
header["Subject"] = []string{"[" + list.DisplayOrLocal() + "] Bounce notification: " + message.Header.Get("Subject")}
header["To"] = []string{list.BounceAddress()}
err = s.Ulist.MTA.Send("", notifieds, header, message.BodyReader()) // empty envelope-from, so if this mail gets bounced, that won't cause a bounce loop
if err != nil {
s.logf("error forwarding bounce notification: %v", err)
}
s.logf("forwarded bounce to notifieds of %s through %s", list, s.Ulist.MTA)
continue // to next list
}
// catch special subjects
command := strings.ToLower(strings.TrimSpace(message.Header.Get("Subject")))
if command == "join" || command == "leave" {
// Join and leave can only be asked personally. So there must be one From address and no different Sender address.
froms, err := mailutil.ParseAddressesFromHeader(message.Header, "From", 10)
if err != nil {
return SMTPErrorf(510, `error parsing "From" header "%s": %s"`, message.Header.Get("From"), err) // 510 Bad email address
}
if len(froms) != 1 {
return SMTPErrorf(513, `expected exactly one "From" address in join/leave email, got %d`, len(froms))
}
if senders, err := mailutil.ParseAddressesFromHeader(message.Header, "Sender", 2); len(senders) > 0 && err == nil {
if froms[0].Equals(senders[0]) {
return SMTPErrorf(513, "From and Sender addresses differ in join/leave email: %s and %s", froms[0], senders[0])
}
}
personalFrom := froms[0]
m, err := s.Ulist.Lists.GetMembership(list, personalFrom)
if err != nil {
return SMTPErrorf(451, "getting membership from database: %v", err)
}
// public signup check is crucial, as SendJoinCheckback sends a confirmation link which allows the receiver to join
if list.PublicSignup && !m.Member && command == "join" {
if err = s.Ulist.SendJoinCheckback(list, personalFrom); err != nil {
return SMTPErrorf(451, "sending join checkback: %v", err)
}
continue // next list
}
if m.Member && command == "leave" {
if _, err = s.Ulist.SendLeaveCheckback(list, personalFrom); err != nil {
return SMTPErrorf(451, "sending leave checkback: %v", err)
}
continue // next list
}
return SMTPErrorf(554, "unknown command")
}
// determine action
froms, err := mailutil.ParseAddressesFromHeader(message.Header, "From", 10) // 10 for DoS mitigation
if err != nil {
return SMTPErrorf(510, `error parsing "From" header: %s"`, err) // 510 Bad email address
}
if len(froms) == 0 {
return SMTPErrorf(510, `no "From" addresses given`) // 510 Bad email address
}
action, reason, err := s.Ulist.GetAction(list, message.Header, froms)
if err != nil {
return SMTPErrorf(451, "error getting status from database: %v", err) // 451 Aborted – Local error in processing
}
s.logf("list: %s, action: %s, reason: %s", list, action, reason)
// do action
switch action {
case Reject:
return SMTPErrUserNotExist
case Pass:
if err := s.Ulist.Forward(list, message); err != nil {
return SMTPErrorf(451, "sending email: %v", err)
}
s.logf("sent email through %s", s.Ulist.MTA)
case Mod:
if err := s.Ulist.Save(list, message); err != nil {
return SMTPErrorf(471, "saving email to file: %v", err)
}
notifieds, err := s.Ulist.Lists.Notifieds(list)
if err != nil {
return SMTPErrorf(451, "getting notifieds from database: %v", err) // 451 Aborted – Local error in processing
}
if err = s.Ulist.NotifyMods(list, notifieds); err != nil {
s.logf("sending moderation notificiation: %v", err)
}
s.logf("stored email for moderation")
}
}
return nil
}