package server import ( "bytes" "errors" "fmt" "github.com/emersion/go-smtp" "io" "mime" "mime/multipart" "net" "net/http" "net/http/httptest" "net/mail" "strings" "sync" ) var ( errInvalidDomain = errors.New("invalid domain") errInvalidAddress = errors.New("invalid address") errInvalidTopic = errors.New("invalid topic") errTooManyRecipients = errors.New("too many recipients") errUnsupportedContentType = errors.New("unsupported content type") ) // smtpBackend implements SMTP server methods. type smtpBackend struct { config *Config handler func(http.ResponseWriter, *http.Request) success int64 failure int64 mu sync.Mutex } var _ smtp.Backend = (*smtpBackend)(nil) var _ smtp.Session = (*smtpSession)(nil) func newMailBackend(conf *Config, handler func(http.ResponseWriter, *http.Request)) *smtpBackend { return &smtpBackend{ config: conf, handler: handler, } } func (b *smtpBackend) NewSession(conn *smtp.Conn) (smtp.Session, error) { logem(conn).Debug("Incoming mail") return &smtpSession{backend: b, conn: conn}, nil } func (b *smtpBackend) Counts() (total int64, success int64, failure int64) { b.mu.Lock() defer b.mu.Unlock() return b.success + b.failure, b.success, b.failure } // smtpSession is returned after EHLO. type smtpSession struct { backend *smtpBackend conn *smtp.Conn topic string mu sync.Mutex } func (s *smtpSession) AuthPlain(username, _ string) error { logem(s.conn).Field("smtp_username", username).Debug("AUTH PLAIN (with username %s)", username) return nil } func (s *smtpSession) Mail(from string, opts *smtp.MailOptions) error { logem(s.conn).Field("smtp_mail_from", from).Debug("MAIL FROM: %s", from) return nil } func (s *smtpSession) Rcpt(to string) error { logem(s.conn).Field("smtp_rcpt_to", to).Debug("RCPT TO: %s", to) return s.withFailCount(func() error { conf := s.backend.config addressList, err := mail.ParseAddressList(to) if err != nil { return err } else if len(addressList) != 1 { return errTooManyRecipients } to = addressList[0].Address if !strings.HasSuffix(to, "@"+conf.SMTPServerDomain) { return errInvalidDomain } to = strings.TrimSuffix(to, "@"+conf.SMTPServerDomain) if conf.SMTPServerAddrPrefix != "" { if !strings.HasPrefix(to, conf.SMTPServerAddrPrefix) { return errInvalidAddress } to = strings.TrimPrefix(to, conf.SMTPServerAddrPrefix) } if !topicRegex.MatchString(to) { return errInvalidTopic } s.mu.Lock() s.topic = to s.mu.Unlock() return nil }) } func (s *smtpSession) Data(r io.Reader) error { return s.withFailCount(func() error { conf := s.backend.config b, err := io.ReadAll(r) // Protected by MaxMessageBytes if err != nil { return err } ev := logem(s.conn) if ev.IsTrace() { ev.Field("smtp_data", string(b)).Trace("DATA") } else if ev.IsDebug() { ev.Field("smtp_data_len", len(b)).Debug("DATA") } msg, err := mail.ReadMessage(bytes.NewReader(b)) if err != nil { return err } body, err := readMailBody(msg) if err != nil { return err } body = strings.TrimSpace(body) if len(body) > conf.MessageLimit { body = body[:conf.MessageLimit] } m := newDefaultMessage(s.topic, body) subject := strings.TrimSpace(msg.Header.Get("Subject")) if subject != "" { dec := mime.WordDecoder{} subject, err := dec.DecodeHeader(subject) if err != nil { return err } m.Title = subject } if m.Title != "" && m.Message == "" { m.Message = m.Title // Flip them, this makes more sense m.Title = "" } if err := s.publishMessage(m); err != nil { return err } s.backend.mu.Lock() s.backend.success++ s.backend.mu.Unlock() return nil }) } func (s *smtpSession) publishMessage(m *message) error { // Extract remote address (for rate limiting) remoteAddr, _, err := net.SplitHostPort(s.conn.Conn().RemoteAddr().String()) if err != nil { remoteAddr = s.conn.Conn().RemoteAddr().String() } // Call HTTP handler with fake HTTP request url := fmt.Sprintf("%s/%s", s.backend.config.BaseURL, m.Topic) req, err := http.NewRequest("POST", url, strings.NewReader(m.Message)) req.RequestURI = "/" + m.Topic // just for the logs req.RemoteAddr = remoteAddr // rate limiting!! req.Header.Set("X-Forwarded-For", remoteAddr) if err != nil { return err } if m.Title != "" { req.Header.Set("Title", m.Title) } rr := httptest.NewRecorder() s.backend.handler(rr, req) if rr.Code != http.StatusOK { return errors.New("error: " + rr.Body.String()) } return nil } func (s *smtpSession) Reset() { s.mu.Lock() s.topic = "" s.mu.Unlock() } func (s *smtpSession) Logout() error { return nil } func (s *smtpSession) withFailCount(fn func() error) error { err := fn() s.backend.mu.Lock() defer s.backend.mu.Unlock() if err != nil { // Almost all of these errors are parse errors, and user input errors. // We do not want to spam the log with WARN messages. logem(s.conn).Err(err).Debug("Incoming mail error") s.backend.failure++ } return err } func readMailBody(msg *mail.Message) (string, error) { if msg.Header.Get("Content-Type") == "" { return readPlainTextMailBody(msg) } contentType, params, err := mime.ParseMediaType(msg.Header.Get("Content-Type")) if err != nil { return "", err } if contentType == "text/plain" { return readPlainTextMailBody(msg) } else if strings.HasPrefix(contentType, "multipart/") { return readMultipartMailBody(msg, params) } return "", errUnsupportedContentType } func readPlainTextMailBody(msg *mail.Message) (string, error) { body, err := io.ReadAll(msg.Body) if err != nil { return "", err } return string(body), nil } func readMultipartMailBody(msg *mail.Message, params map[string]string) (string, error) { mr := multipart.NewReader(msg.Body, params["boundary"]) for { part, err := mr.NextPart() if err != nil { // may be io.EOF return "", err } partContentType, _, err := mime.ParseMediaType(part.Header.Get("Content-Type")) if err != nil { return "", err } if partContentType != "text/plain" { continue } body, err := io.ReadAll(part) if err != nil { return "", err } return string(body), nil } }