smtp_server.go

/**
Modified from Go-Guerrilla SMTPd for Scramble
Copyright (c) 2012 Flashmob, GuerrillaMail.com

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated
documentation files (the "Software"), to deal in the Software without restriction, including without limitation the
rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the
Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

package scramble

import (
	"bufio"
	"bytes"
	_ "code.google.com/p/go.crypto/ripemd160"
	"compress/zlib"
	"crypto/md5"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"github.com/sloonz/go-iconv"
	"github.com/sloonz/go-qprintable"
	"io"
	"io/ioutil"
	"log"
	"mime"
	"mime/multipart"
	"net"
	"net/mail"
	"regexp"
	"strconv"
	"strings"
	"time"
)

// Public interface: a stream of SMTPMessages

type SMTPMessage struct {
	time     int64
	mailFrom string
	rcptTo   []string

	data SMTPMessageData

	saveSuccess chan bool
}

type SMTPMessageData struct {
	messageID   *EmailAddress
	threadID    *EmailAddress
	ancestorIDs EmailAddresses

	from     *mail.Address
	toList   []*mail.Address
	ccList   []*mail.Address
	subject  string
	body     string
	textBody string
}

var SaveMailChan chan *SMTPMessage

// Private implementation

var emailRegex = regexp.MustCompile(`<(.+?)>`)
var mimeHeaderRegex = regexp.MustCompile(`=\?(.+?)\?([QBqp])\?(.+?)\?=`)
var charsetIllegalCharRegex = regexp.MustCompile(`[_:.\/\\]`)

type client struct {
	clientID int64
	state    int
	helo     string
	response string
	conn     net.Conn
	bufin    *bufio.Reader
	bufout   *bufio.Writer
	killTime int64
	errors   int

	// Email properties
	time       int64
	mailFrom   string
	rcptTo     []string
	subject    string
	data       string
	remoteAddr string
}

var serverName string
var listenAddress string
var maxSize int
var timeout time.Duration
var sem chan int

func configure() {
	// MX server name
	serverName = GetConfig().SMTPMxHost
	// SMTP port that nginx forwards to
	listenAddress = fmt.Sprintf("127.0.0.1:%d", GetConfig().SMTPPort)
	// max email size
	maxSize = GetConfig().MaxEmailSize
	// timeout for reads
	timeout = time.Duration(20)
	// currently active client list, 500 is maxClients
	sem = make(chan int, 500)
	// database writing workers
	SaveMailChan = make(chan *SMTPMessage, 5)
}

func StartSMTPServer() {
	configure()

	// Start listening for SMTP connections
	listener, err := net.Listen("tcp", listenAddress)
	if err != nil {
		log.Printf("Cannot listen on port, %v\n", err)
	} else {
		log.Printf("Listening on %s (SMTP)\n", listenAddress)
	}

	go handleClients(listener)
}

func handleClients(listener net.Listener) {
	var clientID int64
	for clientID = 1; ; clientID++ {
		conn, err := listener.Accept()
		if err != nil {
			log.Printf("Accept error: %s\n", err)
			continue
		}
		sem <- 1 // Wait for active queue to drain.
		go handleClient(&client{
			conn:       conn,
			remoteAddr: conn.RemoteAddr().String(),
			time:       time.Now().Unix(),
			bufin:      bufio.NewReader(conn),
			bufout:     bufio.NewWriter(conn),
			clientID:   clientID,
		})
	}
}

func handleClient(client *client) {
	defer Recover()
	defer closeClient(client)
	// TODO: is it safe to show the clientID counter & sem?
	//  it is nice debug info
	greeting := "220 " + serverName +
		" SMTP Scramble-SMTPd #" + strconv.FormatInt(client.clientID, 10) +
		" (" + strconv.Itoa(len(sem)) + ") " + time.Now().Format(time.RFC1123Z)
	advertiseTls := "250-STARTTLS\r\n"
	for i := 0; i < 100; i++ {
		switch client.state {
		case 0: // GREET
			responseAdd(client, greeting)
			client.state = 1
		case 1: // READ COMMAND
			input, err := readSMTP(client)
			if err != nil {
				log.Printf("Read error: %v\n", err)
				if err == io.EOF {
					// client closed the connection already
					return
				}
				if neterr, ok := err.(net.Error); ok && neterr.Timeout() {
					// too slow, timeout
					return
				}
				break
			}
			input = strings.Trim(input, " \n\r")
			cmd := strings.ToUpper(input)
			switch {
			case strings.Index(cmd, "HELO") == 0:
				if len(input) > 5 {
					client.helo = input[5:]
				}
				responseAdd(client, "250 "+serverName+" Hello ")
			case strings.Index(cmd, "EHLO") == 0:
				if len(input) > 5 {
					client.helo = input[5:]
				}
				responseAdd(client, "250-"+serverName+
					" Sup "+client.helo+"["+client.remoteAddr+"]"+"\r\n"+
					"250-SIZE "+strconv.Itoa(maxSize)+"\r\n"+
					advertiseTls+"250 HELP")
			case strings.Index(cmd, "MAIL FROM:") == 0:
				email := extractEmail(input[10:])
				if email == "" {
					responseAdd(client, "550 Invalid address")
					killClient(client)
				} else {
					client.mailFrom = email
					responseAdd(client, "250 Accepted")
				}
			case strings.Index(cmd, "XCLIENT") == 0:
				// Nginx sends this
				// XCLIENT ADDR=212.96.64.216 NAME=[UNAVAILABLE]
				client.remoteAddr = input[13:]
				client.remoteAddr = client.remoteAddr[0:strings.Index(client.remoteAddr, " ")]
				log.Println("Remote client address: " + client.remoteAddr)
				responseAdd(client, "250 OK")
			case strings.Index(cmd, "RCPT TO:") == 0:
				email := extractEmail(input[8:])
				if email == "" {
					responseAdd(client, "550 Invalid address")
					killClient(client)
				} else {
					client.rcptTo = append(client.rcptTo, email)
					responseAdd(client, "250 Accepted")
				}
			case strings.Index(cmd, "NOOP") == 0:
				responseAdd(client, "250 OK")
			case strings.Index(cmd, "RSET") == 0:
				client.mailFrom = ""
				client.rcptTo = nil
				responseAdd(client, "250 OK")
			case strings.Index(cmd, "DATA") == 0:
				responseAdd(client, "354 Enter message, ending with \".\" on a line by itself")
				client.state = 2
			case (strings.Index(cmd, "STARTTLS") == 0):
				// with Nginx this shouldn't happen
				// responseAdd(client, "220 Ready to start TLS")
				log.Panic("STARTTLS requested but not supported. Nginx SMTP SSL support?")
			case strings.Index(cmd, "QUIT") == 0:
				responseAdd(client, "221 Bye")
				killClient(client)
			default:
				responseAdd(client, fmt.Sprintf("500 unrecognized command"))
				client.errors++
				if client.errors > 3 {
					responseAdd(client, fmt.Sprintf("500 Too many unrecognized commands"))
					killClient(client)
				}
			}
		case 2: // READ DATA
			var err error
			client.data, err = readSMTP(client)
			if err == nil {
				// DEBUG ONLY: log the full message so that we can diagnose SMTP problems
				log.Printf("YOU'VE GOT MAIL\n%s\n", client.data)

				// place on the channel so that one of the save mail workers can pick it up
				smtpMessage, err := createSMTPMessage(client)

				var success bool
				if err == nil {
					SaveMailChan <- smtpMessage
					// wait for the save to complete
					success = <-smtpMessage.saveSuccess
				} else {
					log.Printf("Could not parse SMTP message: %v", err)
					success = false
				}

				if success {
					responseAdd(client, "250 OK : queued")
				} else {
					responseAdd(client, "554 Error : transaction failed")
				}
			} else {
				log.Printf("DATA read error: %v\n", err)
			}
			client.state = 1
		}
		// Send a response back to the client
		err := responseWrite(client)
		if err != nil {
			if err == io.EOF {
				// client closed the connection already
				return
			}
			if neterr, ok := err.(net.Error); ok && neterr.Timeout() {
				// too slow, timeout
				return
			}
		}
		if client.killTime > 1 {
			return
		}
	}

}

func createSMTPMessage(client *client) (*SMTPMessage, error) {
	// check mailFrom and rcptTo, etc.
	if err := validateEmailData(client); err != nil {
		return nil, err
	}

	// parse the smtp body (which contains from, to, subject, body)
	smtpData, err := parseSMTPData(client.data)
	if err != nil {
		return nil, err
	}

	// return a fully parsed, received email
	return &SMTPMessage{
		time:     client.time,
		mailFrom: client.mailFrom,
		rcptTo:   client.rcptTo,

		data: *smtpData,

		saveSuccess: make(chan bool),
	}, nil
}

func parseSMTPData(smtpData string) (*SMTPMessageData, error) {
	// parse the mail data to get the headers & body
	parsed, err := mail.ReadMessage(strings.NewReader(smtpData))
	if err != nil {
		return nil, err
	}

	// parse message id
	messageIDStr := strings.Trim(parsed.Header.Get("Message-ID"), "<>")
	messageID, ok := ParseEmailAddressSafe(messageIDStr)
	if !ok {
		messageID = GenerateMessageID()
	}

	// [<thread_id>?,...,<grandparent_message_id>,<parent_message_id>]
	// Should start with the thread id, but it may have been truncated away.
	// See: http://www.jwz.org/doc/threading.html
	ancestorIDs := sanitizeAncestorIDs(parsed.Header)

	// If from another Scramble server, get threadID
	threadIDStr := strings.Trim(parsed.Header.Get("X-Scramble-Thread-ID"), "<>")
	threadID, ok := ParseEmailAddressSafe(threadIDStr)
	if !ok {
		threadID = computeThreadID(messageID, ancestorIDs)
	}

	data := new(SMTPMessageData)
	data.messageID = messageID
	data.threadID = threadID
	data.ancestorIDs = ancestorIDs

	// parse from, to and cc
	data.from, err = mail.ParseAddress(parsed.Header.Get("From"))
	if err != nil {
		return nil, err
	}
	data.toList, err = parsed.Header.AddressList("To")
	if err != nil {
		return nil, err
	}
	data.ccList, err = parsed.Header.AddressList("CC")
	if err != nil && err != mail.ErrHeaderNotPresent {
		return nil, err
	}

	// parse subject
	data.subject = mimeHeaderDecode(parsed.Header.Get("Subject"))

	// parse the body
	// TODO: multipart support
	bodyBytes, err := ioutil.ReadAll(parsed.Body)
	if err != nil {
		return nil, err
	}
	data.body = string(bodyBytes)

	// get the body as plain text. parse multipart mime if needed
	contentType := parsed.Header.Get("Content-Type")
	contentEncoding := parsed.Header.Get("Content-Transfer-Encoding")
	decodedBody := decodeContent(data.body, contentEncoding)
	data.textBody, err = readPlainText(decodedBody, contentType)
	if err != nil {
		return nil, err
	}
	return data, nil
}

func decodeContent(str string, contentEncoding string) string {
	if strings.EqualFold(contentEncoding, "base64") {
		return fromBase64(str)
	} else if strings.EqualFold(contentEncoding, "quoted-printable") {
		return fromQuotedP(str)
	} else {
		return str
	}
}

// Extract plain text from mime body. Maybe be multipart.
// Note: most email is multipart (html + plain text) and some is *nested*
//  multipart---where one of the parts itself has a mimetype of "multipart/..."
// Hence, this function is recursive. It concatenates and returns all the plain text.
func readPlainText(body string, contentType string) (string, error) {
	var mimeType string
	var mimeParams map[string]string
	var err error
	if contentType == "" {
		// no content-type header? assume plain text
		contentType = "text/plain"
	}
	mimeType, mimeParams, err = mime.ParseMediaType(contentType)
	if err != nil {
		return "", err
	}
	if mimeType == "text/plain" {
		// plain text? read the entire thing
		charset := mimeParams["charset"]
		return convertToUTF8(body, charset), nil
	} else if strings.HasPrefix(mimeType, "multipart/") {
		// multipart mime? extract just the parts that are plain text
		multiBoundary := mimeParams["boundary"]
		return readPlainTextFromMultipart(body, multiBoundary)
	}
	// neither plain text nor multipart? then there's no plain text here...
	return "", nil
}

// Extract plain text from multipart mime email.
func readPlainTextFromMultipart(body string, multiBoundary string) (string, error) {
	multiReader := multipart.NewReader(strings.NewReader(body), multiBoundary)
	var plainTexts []string
	for {
		part, err := multiReader.NextPart()
		if err == io.EOF {
			break
		} else if err != nil {
			return "", err
		}
		partContentType := part.Header.Get("Content-Type")
		partContentEncoding := part.Header.Get("Content-Transfer-Encoding")
		partBodyBytes, err := ioutil.ReadAll(part)
		if err != nil {
			return "", err
		}
		partBody := string(partBodyBytes)
		partDecodedBody := decodeContent(partBody, partContentEncoding)
		partPlainText, err := readPlainText(partDecodedBody, partContentType)
		if err != nil {
			return "", err
		}
		plainTexts = append(plainTexts, partPlainText)
	}
	return strings.Join(plainTexts, ""), nil
}

func responseAdd(client *client, line string) {
	client.response = line + "\r\n"
}
func closeClient(client *client) {
	client.conn.Close()
	<-sem // Done; enable next client to run.
}
func killClient(client *client) {
	client.killTime = time.Now().Unix()
}

func readSMTP(client *client) (input string, err error) {
	var reply string
	// Command state terminator by default
	suffix := "\r\n"
	if client.state == 2 {
		// DATA state
		suffix = "\r\n.\r\n"
	}
	for err == nil {
		client.conn.SetDeadline(time.Now().Add(timeout * time.Second))
		reply, err = client.bufin.ReadString('\n')
		if reply != "" {
			input = input + reply
			if len(input) > maxSize {
				err = errors.New("Maximum DATA size exceeded (" + strconv.Itoa(maxSize) + ")")
				return input, err
			}
			if client.state == 2 {
				// Extract the subject while we are at it.
				scanSubject(client, reply)
			}
		}
		if err != nil {
			break
		}
		if strings.HasSuffix(input, suffix) {
			break
		}
	}
	return input, err
}

// Scan the data part for a Subject line. Can be a multi-line
func scanSubject(client *client, reply string) {
	if client.subject == "" && (len(reply) > 8) {
		test := strings.ToUpper(reply[0:9])
		if i := strings.Index(test, "SUBJECT: "); i == 0 {
			// first line with \r\n
			client.subject = reply[9:]
		}
	} else if strings.HasSuffix(client.subject, "\r\n") {
		// chop off the \r\n
		client.subject = client.subject[0 : len(client.subject)-2]
		if (strings.HasPrefix(reply, " ")) || (strings.HasPrefix(reply, "\t")) {
			// subject is multi-line
			client.subject = client.subject + reply[1:]
		}
	}
}

func responseWrite(client *client) (err error) {
	var size int
	client.conn.SetDeadline(time.Now().Add(timeout * time.Second))
	size, err = client.bufout.WriteString(client.response)
	client.bufout.Flush()
	client.response = client.response[size:]
	return err
}

func validateEmailData(client *client) error {
	if client.mailFrom == "" {
		return errors.New("missing MAIL FROM")
	}
	if len(client.rcptTo) == 0 {
		return errors.New("missing RCPT TO")
	}
	for _, addr := range client.rcptTo {
		// TODO check the hosts of the rcptTo addresses.
		addr = addr // noop :P
	}
	return nil
}

func extractEmail(str string) string {
	var email string
	if matched := emailRegex.FindStringSubmatch(str); len(matched) > 1 {
		email = strings.Trim(matched[1], " ")
	} else {
		email = strings.Trim(str, " ")
	}
	if ok := validateAddressSafe(email); !ok {
		return ""
	}

	return email
}

// Decode strings in MIME header format (RFC 2047)
// eg. =?ISO-2022-JP?B?GyRCIVo9dztSOWJAOCVBJWMbKEI=?=
func mimeHeaderDecode(str string) string {
	// Find and decode all RFC-2047-encoded substrings
	return mimeHeaderRegex.ReplaceAllStringFunc(str, func(encoded string) string {
		matched := mimeHeaderRegex.FindStringSubmatch(encoded)
		if len(matched) <= 2 {
			panic("regex was already matched, and now it doesn't match?")
		}

		// Find the specified encoding and charset
		charset := matched[1]
		encodingChar := strings.ToUpper(matched[2])
		encodedPayload := matched[3]
		var encoding string
		switch encodingChar {
		case "B":
			encoding = "base64"
		case "Q":
			encoding = "quoted-printable"
		}

		// Decode encoded->bytes using encoded (eg "quoted-printable")
		payload := decodeContent(encodedPayload, encoding)

		// Decode bytes->string using charset (eg "utf-8")
		return convertToUTF8(payload, charset)
	})
}

// Decode, for example, "ISO-2022-JP" to UTF-8
func convertToUTF8(str string, charset string) string {
	charset = fixCharset(charset)

	if charset == "utf-8" {
		return str
	}

	// eg. charset can be "ISO-2022-JP"
	convstr, err := iconv.Conv(str, "UTF-8", charset)
	if err == nil {
		return convstr
	}
	return str
}

func fromBase64(data string) string {
	buf := bytes.NewBufferString(data)
	decoder := base64.NewDecoder(base64.StdEncoding, buf)
	res, _ := ioutil.ReadAll(decoder)
	return string(res)
}

func fromQuotedP(data string) string {
	buf := bytes.NewBufferString(data)
	decoder := qprintable.NewDecoder(qprintable.BinaryEncoding, buf)
	res, _ := ioutil.ReadAll(decoder)
	return string(res)
}

func compress(s string) string {
	var b bytes.Buffer
	w, _ := zlib.NewWriterLevel(&b, zlib.BestSpeed) // flate.BestCompression
	w.Write([]byte(s))
	w.Close()
	return b.String()
}

func fixCharset(charset string) string {
	fixedCharset := strings.ToLower(charset)
	if fixedCharset == "" {
		fixedCharset = "utf-8"
	}

	fixedCharset = charsetIllegalCharRegex.ReplaceAllString(charset, "-")
	// Fix charset
	// borrowed from http://squirrelmail.svn.sourceforge.net/viewvc/squirrelmail/trunk/squirrelmail/include/languages.php?revision=13765&view=markup
	// OE ks_c_5601_1987 > cp949
	fixedCharset = strings.Replace(fixedCharset, "ks-c-5601-1987", "cp949", -1)
	// Moz x-euc-tw > euc-tw
	fixedCharset = strings.Replace(fixedCharset, "x-euc", "euc", -1)
	// Moz x-windows-949 > cp949
	fixedCharset = strings.Replace(fixedCharset, "x-windows_", "cp", -1)
	// windows-125x and cp125x charsets
	fixedCharset = strings.Replace(fixedCharset, "windows-", "cp", -1)
	// ibm > cp
	fixedCharset = strings.Replace(fixedCharset, "ibm", "cp", -1)
	// iso-8859-8-i -> iso-8859-8
	fixedCharset = strings.Replace(fixedCharset, "iso-8859-8-i", "iso-8859-8", -1)
	return fixedCharset
}

func md5hex(str string) string {
	h := md5.New()
	h.Write([]byte(str))
	sum := h.Sum([]byte{})
	return hex.EncodeToString(sum)
}

func sanitizeAncestorIDs(headers mail.Header) EmailAddresses {
	inReplyToStr := headers.Get("In-Reply-To")
	referencesStr := headers.Get("References")
	inReplyTo := ParseAngledEmailAddressesSmart(inReplyToStr)
	references := ParseAngledEmailAddressesSmart(referencesStr)
	if len(inReplyTo) > 0 && (len(references) == 0 || references[len(references)-1] != inReplyTo[0]) {
		references = append(references, inReplyTo[0])
	}
	return references
}

// Generally the ancestorIDs[0], we do a lookup in our db
//  to see if we can find an older ancestor.
func computeThreadID(messageID *EmailAddress, ancestorIDs EmailAddresses) *EmailAddress {
	if len(ancestorIDs) > 0 {
		var ancestors []interface{}
		for _, messageID := range ancestorIDs {
			ancestors = append(ancestors, messageID.String())
		}
		threadIDs := LoadThreadIDsForMessageIDs(ancestors)
		// This algo isn't perfect, but might be good enough.
		for _, threadID := range threadIDs {
			if threadID != "" {
				return ParseEmailAddress(threadID)
			}
		}
	}
	return messageID
}