smtp/conn.go

   1 package smtp
   2
   3 import (
   4         "bytes"
   5         "crypto/rand"
   6         "crypto/tls"
   7         "encoding/base64"
   8         "fmt"
   9         "net"
  10         "net/mail"
  11         "net/textproto"
  12         "strings"
  13         "time"
  14
  15         "github.com/uber-go/zap"
  16 )
  17
  18 type state int
  19
  20 const (
  21         stateNew state = iota // Before EHLO.
  22         stateInitial
  23         stateMail
  24         stateRecipient
  25         stateData
  26 )
  27
  28 type delivery int
  29
  30 func (d delivery) String() string {
  31         switch d {
  32         case deliverUnknown:
  33                 return "unknown"
  34         case deliverInbound:
  35                 return "inbound"
  36         case deliverOutbound:
  37                 return "outbound"
  38         }
  39         panic("Unknown delivery")
  40 }
  41
  42 const (
  43         deliverUnknown  delivery = iota
  44         deliverInbound           // Mail is not from one of this server's domains.
  45         deliverOutbound          // Mail IS from one of this server's domains.
  46 )
  47
  48 type connection struct {
  49         server Server
  50
  51         tp *textproto.Conn
  52
  53         nc         net.Conn
  54         remoteAddr net.Addr
  55
  56         esmtp bool
  57         tls   *tls.ConnectionState
  58
  59         // The authcid from a PLAIN SASL login. Non-empty iff tls is non-nil and
  60         // doAUTH() succeeded.
  61         authc string
  62
  63         log zap.Logger
  64
  65         state
  66         line string
  67
  68         delivery
  69         // For deliverOutbound, replaces the From and Reply-To values.
  70         sendAs *mail.Address
  71
  72         ehlo     string
  73         mailFrom *mail.Address
  74         rcptTo   []mail.Address
  75 }
  76
  77 func AcceptConnection(netConn net.Conn, server Server, log zap.Logger) {
  78         conn := connection{
  79                 server:     server,
  80                 tp:         textproto.NewConn(netConn),
  81                 nc:         netConn,
  82                 remoteAddr: netConn.RemoteAddr(),
  83                 log:        log.With(zap.Stringer("client", netConn.RemoteAddr())),
  84                 state:      stateNew,
  85         }
  86
  87         conn.log.Info("accepted connection")
  88         conn.writeReply(220, fmt.Sprintf("%s ESMTP [%s] (mailpopbox)",
  89                 server.Name(), netConn.LocalAddr()))
  90
  91         for {
  92                 var err error
  93                 conn.line, err = conn.tp.ReadLine()
  94                 if err != nil {
  95                         conn.log.Error("ReadLine()", zap.Error(err))
  96                         conn.tp.Close()
  97                         return
  98                 }
  99
 100                 lineForLog := conn.line
 101                 const authPlain = "AUTH PLAIN "
 102                 if strings.HasPrefix(conn.line, authPlain) {
 103                         lineForLog = authPlain + "[redacted]"
 104                 }
 105                 conn.log.Info("ReadLine()", zap.String("line", lineForLog))
 106
 107                 var cmd string
 108                 if _, err = fmt.Sscanf(conn.line, "%s", &cmd); err != nil {
 109                         conn.reply(ReplyBadSyntax)
 110                         continue
 111                 }
 112
 113                 switch strings.ToUpper(cmd) {
 114                 case "QUIT":
 115                         conn.writeReply(221, "Goodbye")
 116                         conn.tp.Close()
 117                         return
 118                 case "HELO":
 119                         conn.esmtp = false
 120                         fallthrough
 121                 case "EHLO":
 122                         conn.esmtp = true
 123                         conn.doEHLO()
 124                 case "STARTTLS":
 125                         conn.doSTARTTLS()
 126                 case "AUTH":
 127                         conn.doAUTH()
 128                 case "MAIL":
 129                         conn.doMAIL()
 130                 case "RCPT":
 131                         conn.doRCPT()
 132                 case "DATA":
 133                         conn.doDATA()
 134                 case "RSET":
 135                         conn.doRSET()
 136                 case "VRFY":
 137                         conn.writeReply(252, "I'll do my best")
 138                 case "EXPN":
 139                         conn.writeReply(550, "access denied")
 140                 case "NOOP":
 141                         conn.reply(ReplyOK)
 142                 case "HELP":
 143                         conn.writeReply(250, "https://tools.ietf.org/html/rfc5321")
 144                 default:
 145                         conn.writeReply(500, "unrecognized command")
 146                 }
 147         }
 148 }
 149
 150 func (conn *connection) reply(reply ReplyLine) error {
 151         return conn.writeReply(reply.Code, reply.Message)
 152 }
 153
 154 func (conn *connection) writeReply(code int, msg string) error {
 155         conn.log.Info("writeReply", zap.Int("code", code))
 156         var err error
 157         if len(msg) > 0 {
 158                 err = conn.tp.PrintfLine("%d %s", code, msg)
 159         } else {
 160                 err = conn.tp.PrintfLine("%d", code)
 161         }
 162         if err != nil {
 163                 conn.log.Error("writeReply",
 164                         zap.Int("code", code),
 165                         zap.Error(err))
 166         }
 167         return err
 168 }
 169
 170 // parsePath parses out either a forward-, reverse-, or return-path from the
 171 // current connection line. Returns a (valid-path, ReplyOK) if it was
 172 // successfully parsed.
 173 func (conn *connection) parsePath(command string) (string, ReplyLine) {
 174         if len(conn.line) < len(command) {
 175                 return "", ReplyBadSyntax
 176         }
 177         if strings.ToUpper(command) != strings.ToUpper(conn.line[:len(command)]) {
 178                 return "", ReplyLine{500, "unrecognized command"}
 179         }
 180         params := conn.line[len(command):]
 181         idx := strings.Index(params, ">")
 182         if idx == -1 {
 183                 return "", ReplyBadSyntax
 184         }
 185         return strings.ToLower(params[:idx+1]), ReplyOK
 186 }
 187
 188 func (conn *connection) doEHLO() {
 189         conn.resetBuffers()
 190
 191         var cmd string
 192         _, err := fmt.Sscanf(conn.line, "%s %s", &cmd, &conn.ehlo)
 193         if err != nil {
 194                 conn.reply(ReplyBadSyntax)
 195                 return
 196         }
 197
 198         if cmd == "HELO" {
 199                 conn.writeReply(250, fmt.Sprintf("Hello %s [%s]", conn.ehlo, conn.remoteAddr))
 200         } else {
 201                 conn.tp.PrintfLine("250-Hello %s [%s]", conn.ehlo, conn.remoteAddr)
 202                 if conn.server.TLSConfig() != nil && conn.tls == nil {
 203                         conn.tp.PrintfLine("250-STARTTLS")
 204                 }
 205                 if conn.tls != nil {
 206                         conn.tp.PrintfLine("250-AUTH PLAIN")
 207                 }
 208                 conn.tp.PrintfLine("250 SIZE %d", 40960000)
 209         }
 210
 211         conn.log.Info("doEHLO()", zap.String("ehlo", conn.ehlo))
 212
 213         conn.state = stateInitial
 214 }
 215
 216 func (conn *connection) doSTARTTLS() {
 217         if conn.state != stateInitial {
 218                 conn.reply(ReplyBadSequence)
 219                 return
 220         }
 221
 222         tlsConfig := conn.server.TLSConfig()
 223         if !conn.esmtp || tlsConfig == nil {
 224                 conn.writeReply(500, "unrecognized command")
 225                 return
 226         }
 227
 228         conn.log.Info("doSTARTTLS()")
 229         conn.writeReply(220, "initiate TLS connection")
 230
 231         tlsConn := tls.Server(conn.nc, tlsConfig)
 232         if err := tlsConn.Handshake(); err != nil {
 233                 conn.log.Error("failed to do TLS handshake", zap.Error(err))
 234                 return
 235         }
 236
 237         conn.nc = tlsConn
 238         conn.tp = textproto.NewConn(tlsConn)
 239         conn.state = stateNew
 240
 241         connState := tlsConn.ConnectionState()
 242         conn.tls = &connState
 243
 244         conn.log.Info("TLS connection done", zap.String("state", conn.getTransportString()))
 245 }
 246
 247 func (conn *connection) doAUTH() {
 248         if conn.state != stateInitial || conn.tls == nil {
 249                 conn.reply(ReplyBadSequence)
 250                 return
 251         }
 252
 253         if conn.authc != "" {
 254                 conn.writeReply(503, "already authenticated")
 255                 return
 256         }
 257
 258         var cmd, authType, authString string
 259         n, err := fmt.Sscanf(conn.line, "%s %s %s", &cmd, &authType, &authString)
 260         if n < 2 {
 261                 conn.reply(ReplyBadSyntax)
 262                 return
 263         }
 264
 265         if authType != "PLAIN" {
 266                 conn.writeReply(504, "unrecognized auth type")
 267                 return
 268         }
 269
 270         // If only 2 tokens were scanned, then an initial response was not provided.
 271         if n == 2 && conn.line[len(conn.line)-1] != ' ' {
 272                 conn.reply(ReplyBadSyntax)
 273                 return
 274         }
 275
 276         conn.log.Info("doAUTH()")
 277
 278         if authString == "" {
 279                 conn.writeReply(334, " ")
 280
 281                 authString, err = conn.tp.ReadLine()
 282                 if err != nil {
 283                         conn.log.Error("failed to read auth line", zap.Error(err))
 284                         conn.reply(ReplyBadSyntax)
 285                         return
 286                 }
 287         }
 288
 289         authBytes, err := base64.StdEncoding.DecodeString(authString)
 290         if err != nil {
 291                 conn.reply(ReplyBadSyntax)
 292                 return
 293         }
 294
 295         authParts := strings.Split(string(authBytes), "\x00")
 296         if len(authParts) != 3 {
 297                 conn.log.Error("bad auth line syntax")
 298                 conn.reply(ReplyBadSyntax)
 299                 return
 300         }
 301
 302         if !conn.server.Authenticate(authParts[0], authParts[1], authParts[2]) {
 303                 conn.log.Error("failed to authenticate", zap.String("authc", authParts[1]))
 304                 conn.writeReply(535, "invalid credentials")
 305                 return
 306         }
 307
 308         conn.log.Info("authenticated", zap.String("authz", authParts[0]), zap.String("authc", authParts[1]))
 309         conn.authc = authParts[1]
 310         conn.reply(ReplyOK)
 311 }
 312
 313 func (conn *connection) doMAIL() {
 314         if conn.state != stateInitial {
 315                 conn.reply(ReplyBadSequence)
 316                 return
 317         }
 318
 319         mailFrom, reply := conn.parsePath("MAIL FROM:")
 320         if reply != ReplyOK {
 321                 conn.reply(reply)
 322                 return
 323         }
 324
 325         var err error
 326         conn.mailFrom, err = mail.ParseAddress(mailFrom)
 327         if err != nil || conn.mailFrom == nil {
 328                 conn.reply(ReplyBadSyntax)
 329                 return
 330         }
 331
 332         if conn.server.VerifyAddress(*conn.mailFrom) == ReplyOK {
 333                 if DomainForAddress(*conn.mailFrom) != DomainForAddressString(conn.authc) {
 334                         conn.writeReply(550, "not authenticated")
 335                         return
 336                 }
 337                 conn.delivery = deliverOutbound
 338         } else {
 339                 conn.delivery = deliverInbound
 340         }
 341
 342         conn.log.Info("doMAIL()", zap.String("address", conn.mailFrom.Address))
 343
 344         conn.state = stateMail
 345         conn.reply(ReplyOK)
 346 }
 347
 348 func (conn *connection) doRCPT() {
 349         if conn.state != stateMail && conn.state != stateRecipient {
 350                 conn.reply(ReplyBadSequence)
 351                 return
 352         }
 353
 354         rcptTo, reply := conn.parsePath("RCPT TO:")
 355         if reply != ReplyOK {
 356                 conn.reply(reply)
 357                 return
 358         }
 359
 360         address, err := mail.ParseAddress(rcptTo)
 361         if err != nil {
 362                 conn.reply(ReplyBadSyntax)
 363                 return
 364         }
 365
 366         if reply := conn.server.VerifyAddress(*address); reply != ReplyOK && conn.delivery == deliverInbound {
 367                 conn.log.Warn("invalid address",
 368                         zap.String("address", address.Address),
 369                         zap.Stringer("reply", reply))
 370                 conn.reply(reply)
 371                 return
 372         }
 373
 374         conn.log.Info("doRCPT()",
 375                 zap.String("address", address.Address),
 376                 zap.String("delivery", conn.delivery.String()))
 377
 378         conn.rcptTo = append(conn.rcptTo, *address)
 379
 380         conn.state = stateRecipient
 381         conn.reply(ReplyOK)
 382 }
 383
 384 func (conn *connection) doDATA() {
 385         if conn.state != stateRecipient {
 386                 conn.reply(ReplyBadSequence)
 387                 return
 388         }
 389
 390         conn.writeReply(354, "Start mail input; end with <CRLF>.<CRLF>")
 391         conn.log.Info("doDATA()")
 392
 393         data, err := conn.tp.ReadDotBytes()
 394         if err != nil {
 395                 conn.log.Error("failed to ReadDotBytes()",
 396                         zap.Error(err),
 397                         zap.String("bytes", fmt.Sprintf("%x", data)))
 398                 conn.writeReply(552, "transaction failed")
 399                 return
 400         }
 401
 402         received := time.Now()
 403         env := Envelope{
 404                 RemoteAddr: conn.remoteAddr,
 405                 EHLO:       conn.ehlo,
 406                 MailFrom:   *conn.mailFrom,
 407                 RcptTo:     conn.rcptTo,
 408                 Received:   received,
 409                 ID:         conn.envelopeID(received),
 410                 Data:       data,
 411         }
 412
 413         conn.handleSendAs(&env)
 414
 415         conn.log.Info("received message",
 416                 zap.Int("bytes", len(data)),
 417                 zap.Time("date", received),
 418                 zap.String("id", env.ID),
 419                 zap.String("delivery", conn.delivery.String()))
 420
 421         trace := conn.getReceivedInfo(env)
 422
 423         env.Data = append(trace, env.Data...)
 424
 425         if conn.delivery == deliverInbound {
 426                 if reply := conn.server.OnMessageDelivered(env); reply != nil {
 427                         conn.log.Warn("message was rejected", zap.String("id", env.ID))
 428                         conn.reply(*reply)
 429                         return
 430                 }
 431         } else if conn.delivery == deliverOutbound {
 432                 conn.server.RelayMessage(env)
 433         }
 434
 435         conn.state = stateInitial
 436         conn.resetBuffers()
 437         conn.reply(ReplyOK)
 438 }
 439
 440 func (conn *connection) handleSendAs(env *Envelope) {
 441         if conn.delivery != deliverOutbound {
 442                 return
 443         }
 444
 445         // Find the separator between the message header and body.
 446         headerIdx := bytes.Index(env.Data, []byte("\n\n"))
 447         if headerIdx == -1 {
 448                 conn.log.Error("send-as: could not find headers index")
 449                 return
 450         }
 451
 452         var buf bytes.Buffer
 453
 454         headers := bytes.SplitAfter(env.Data[:headerIdx], []byte("\n"))
 455
 456         var fromIdx, subjectIdx int
 457         for i, header := range headers {
 458                 if bytes.HasPrefix(header, []byte("From:")) {
 459                         fromIdx = i
 460                         continue
 461                 }
 462                 if bytes.HasPrefix(header, []byte("Subject:")) {
 463                         subjectIdx = i
 464                         continue
 465                 }
 466         }
 467
 468         if subjectIdx == -1 {
 469                 conn.log.Error("send-as: could not find Subject header")
 470                 return
 471         }
 472         if fromIdx == -1 {
 473                 conn.log.Error("send-as: could not find From header")
 474                 return
 475         }
 476
 477         sendAs := SendAsSubject.FindSubmatchIndex(headers[subjectIdx])
 478         if sendAs == nil {
 479                 // No send-as modification.
 480                 return
 481         }
 482
 483         // Submatch 0 is the whole sendas magic. Submatch 1 is the address prefix.
 484         sendAsUser := headers[subjectIdx][sendAs[2]:sendAs[3]]
 485         sendAsAddress := string(sendAsUser) + "@" + DomainForAddressString(conn.authc)
 486
 487         for i, header := range headers {
 488                 if i == subjectIdx {
 489                         buf.Write(header[:sendAs[0]])
 490                         buf.Write(header[sendAs[1]:])
 491                 } else if i == fromIdx {
 492                         addressStart := bytes.LastIndexByte(header, byte('<'))
 493                         buf.Write(header[:addressStart+1])
 494                         buf.WriteString(sendAsAddress)
 495                         buf.WriteString(">\n")
 496                 } else {
 497                         buf.Write(header)
 498                 }
 499         }
 500
 501         buf.Write(env.Data[headerIdx:])
 502
 503         env.Data = buf.Bytes()
 504         env.MailFrom.Address = sendAsAddress
 505 }
 506
 507 func (conn *connection) envelopeID(t time.Time) string {
 508         var idBytes [4]byte
 509         rand.Read(idBytes[:])
 510         return fmt.Sprintf("m.%d.%x", t.UnixNano(), idBytes)
 511 }
 512
 513 func (conn *connection) getReceivedInfo(envelope Envelope) []byte {
 514         rhost, _, err := net.SplitHostPort(conn.remoteAddr.String())
 515         if err != nil {
 516                 rhost = conn.remoteAddr.String()
 517         }
 518
 519         rhosts, err := net.LookupAddr(rhost)
 520         if err == nil {
 521                 rhost = fmt.Sprintf("%s [%s]", rhosts[0], rhost)
 522         }
 523
 524         base := fmt.Sprintf("Received: from %s (%s)\r\n        ", conn.ehlo, rhost)
 525
 526         with := "SMTP"
 527         if conn.esmtp {
 528                 with = "E" + with
 529         }
 530         if conn.tls != nil {
 531                 with += "S"
 532         }
 533         base += fmt.Sprintf("by %s (mailpopbox) with %s id %s\r\n        ", conn.server.Name(), with, envelope.ID)
 534
 535         if len(envelope.RcptTo) > 0 {
 536                 base += fmt.Sprintf("for <%s>\r\n        ", envelope.RcptTo[0].Address)
 537         }
 538
 539         transport := conn.getTransportString()
 540         date := envelope.Received.Format(time.RFC1123Z) // Same as RFC 5322 § 3.3
 541         base += fmt.Sprintf("(using %s);\r\n        %s\r\n", transport, date)
 542
 543         return []byte(base)
 544 }
 545
 546 func (conn *connection) getTransportString() string {
 547         if conn.tls == nil {
 548                 return "PLAINTEXT"
 549         }
 550
 551         ciphers := map[uint16]string{
 552                 tls.TLS_RSA_WITH_RC4_128_SHA:                "TLS_RSA_WITH_RC4_128_SHA",
 553                 tls.TLS_RSA_WITH_3DES_EDE_CBC_SHA:           "TLS_RSA_WITH_3DES_EDE_CBC_SHA",
 554                 tls.TLS_RSA_WITH_AES_128_CBC_SHA:            "TLS_RSA_WITH_AES_128_CBC_SHA",
 555                 tls.TLS_RSA_WITH_AES_256_CBC_SHA:            "TLS_RSA_WITH_AES_256_CBC_SHA",
 556                 tls.TLS_RSA_WITH_AES_128_GCM_SHA256:         "TLS_RSA_WITH_AES_128_GCM_SHA256",
 557                 tls.TLS_RSA_WITH_AES_256_GCM_SHA384:         "TLS_RSA_WITH_AES_256_GCM_SHA384",
 558                 tls.TLS_ECDHE_ECDSA_WITH_RC4_128_SHA:        "TLS_ECDHE_ECDSA_WITH_RC4_128_SHA",
 559                 tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA:    "TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA",
 560                 tls.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA:    "TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA",
 561                 tls.TLS_ECDHE_RSA_WITH_RC4_128_SHA:          "TLS_ECDHE_RSA_WITH_RC4_128_SHA",
 562                 tls.TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA:     "TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA",
 563                 tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA:      "TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA",
 564                 tls.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA:      "TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA",
 565                 tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256:   "TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256",
 566                 tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256: "TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256",
 567                 tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384:   "TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384",
 568                 tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384: "TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384",
 569         }
 570         versions := map[uint16]string{
 571                 tls.VersionSSL30: "SSLv3.0",
 572                 tls.VersionTLS10: "TLSv1.0",
 573                 tls.VersionTLS11: "TLSv1.1",
 574                 tls.VersionTLS12: "TLSv1.2",
 575         }
 576
 577         state := conn.tls
 578
 579         version := versions[state.Version]
 580         cipher := ciphers[state.CipherSuite]
 581
 582         if version == "" {
 583                 version = fmt.Sprintf("%x", state.Version)
 584         }
 585         if cipher == "" {
 586                 cipher = fmt.Sprintf("%x", state.CipherSuite)
 587         }
 588
 589         name := ""
 590         if state.ServerName != "" {
 591                 name = fmt.Sprintf(" name=%s", state.ServerName)
 592         }
 593
 594         return fmt.Sprintf("%s cipher=%s%s", version, cipher, name)
 595 }
 596
 597 func (conn *connection) doRSET() {
 598         conn.log.Info("doRSET()")
 599         conn.state = stateInitial
 600         conn.resetBuffers()
 601         conn.reply(ReplyOK)
 602 }
 603
 604 func (conn *connection) resetBuffers() {
 605         conn.delivery = deliverUnknown
 606         conn.sendAs = nil
 607         conn.mailFrom = nil
 608         conn.rcptTo = make([]mail.Address, 0)
 609 }