smtpd.go

// Package smtpd implements an SMTP server with support for STARTTLS, authentication (PLAIN/LOGIN), XCLIENT and optional restrictions on the different stages of the SMTP session.
package smtpd

import (
	"bufio"
	"crypto/tls"
	"fmt"
	"log"
	"net"
	"time"
)

// Server defines the parameters for running the SMTP server
type Server struct {
	Hostname       string // Server hostname. (default: "localhost.localdomain")
	WelcomeMessage string // Initial server banner. (default: "<hostname> ESMTP ready.")

	ReadTimeout  time.Duration // Socket timeout for read operations. (default: 60s)
	WriteTimeout time.Duration // Socket timeout for write operations. (default: 60s)
	DataTimeout  time.Duration // Socket timeout for DATA command (default: 5m)

	MaxConnections int // Max concurrent connections, use -1 to disable. (default: 100)
	MaxMessageSize int // Max message size in bytes. (default: 10240000)
	MaxRecipients  int // Max RCPT TO calls for each envelope. (default: 100)

	// New e-mails are handed off to this function.
	// Can be left empty for a NOOP server.
	// If an error is returned, it will be reported in the SMTP session.
	Handler func(peer Peer, env Envelope) error

	// Enable various checks during the SMTP session.
	// Can be left empty for no restrictions.
	// If an error is returned, it will be reported in the SMTP session.
	// Use the Error struct for access to error codes.
	ConnectionChecker func(peer Peer) error              // Called upon new connection.
	HeloChecker       func(peer Peer, name string) error // Called after HELO/EHLO.
	SenderChecker     func(peer Peer, addr string) error // Called after MAIL FROM.
	RecipientChecker  func(peer Peer, addr string) error // Called after each RCPT TO.

	// Enable PLAIN/LOGIN authentication, only available after STARTTLS.
	// Can be left empty for no authentication support.
	Authenticator func(peer Peer, username, password string) error

	EnableXCLIENT bool // Enable XCLIENT support (default: false)

	TLSConfig *tls.Config // Enable STARTTLS support.
	ForceTLS  bool        // Force STARTTLS usage.
}

// Protocol represents the protocol used in the SMTP session
type Protocol string

const (
	SMTP  Protocol = "SMTP"
	ESMTP          = "ESMTP"
)

// Peer represents the client connecting to the server
type Peer struct {
	HeloName   string               // Server name used in HELO/EHLO command
	Username   string               // Username from authentication, if authenticated
	Password   string               // Password from authentication, if authenticated
	Protocol   Protocol             // Protocol used, SMTP or ESMTP
	ServerName string               // A copy of Server.Hostname
	Addr       net.Addr             // Network address
	TLS        *tls.ConnectionState // TLS Connection details, if on TLS
}

// Error represents an Error reported in the SMTP session.
type Error struct {
	Code    int    // The integer error code
	Message string // The error message
}

// Error returns a string representation of the SMTP error
func (e Error) Error() string { return fmt.Sprintf("%d %s", e.Code, e.Message) }

type session struct {
	server *Server

	peer     Peer
	envelope *Envelope

	conn net.Conn

	reader  *bufio.Reader
	writer  *bufio.Writer
	scanner *bufio.Scanner

	tls bool
}

func (srv *Server) newSession(c net.Conn) (s *session) {

	s = &session{
		server: srv,
		conn:   c,
		reader: bufio.NewReader(c),
		writer: bufio.NewWriter(c),
		peer: Peer{
			Addr:       c.RemoteAddr(),
			ServerName: srv.Hostname,
		},
	}

	s.scanner = bufio.NewScanner(s.reader)

	return

}

// ListenAndServe starts the SMTP server and listens on the address provided
func (srv *Server) ListenAndServe(addr string) error {

	srv.configureDefaults()

	l, err := net.Listen("tcp", addr)
	if err != nil {
		return err
	}

	return srv.Serve(l)
}

// Serve starts the SMTP server and listens on the Listener provided
func (srv *Server) Serve(l net.Listener) error {

	srv.configureDefaults()

	defer l.Close()

	var limiter chan struct{}

	if srv.MaxConnections > 0 {
		limiter = make(chan struct{}, srv.MaxConnections)
	} else {
		limiter = nil
	}

	for {

		conn, e := l.Accept()
		if e != nil {
			if ne, ok := e.(net.Error); ok && ne.Temporary() {
				time.Sleep(time.Second)
				continue
			}
			return e
		}

		session := srv.newSession(conn)

		if limiter != nil {
			go func() {
				select {
				case limiter <- struct{}{}:
					session.serve()
					<-limiter
				default:
					session.reject()
				}
			}()
		} else {
			go session.serve()
		}

	}

}

func (srv *Server) configureDefaults() {

	if srv.MaxMessageSize == 0 {
		srv.MaxMessageSize = 10240000
	}

	if srv.MaxConnections == 0 {
		srv.MaxConnections = 100
	}

	if srv.MaxRecipients == 0 {
		srv.MaxRecipients = 100
	}

	if srv.ReadTimeout == 0 {
		srv.ReadTimeout = time.Second * 60
	}

	if srv.WriteTimeout == 0 {
		srv.WriteTimeout = time.Second * 60
	}

	if srv.DataTimeout == 0 {
		srv.DataTimeout = time.Minute * 5
	}

	if srv.ForceTLS && srv.TLSConfig == nil {
		log.Fatal("Cannot use ForceTLS with no TLSConfig")
	}

	if srv.Hostname == "" {
		srv.Hostname = "localhost.localdomain"
	}

	if srv.WelcomeMessage == "" {
		srv.WelcomeMessage = fmt.Sprintf("%s ESMTP ready.", srv.Hostname)
	}

}

func (session *session) serve() {

	defer session.close()

	session.welcome()

	for {

		for session.scanner.Scan() {
			session.handle(session.scanner.Text())
		}

		err := session.scanner.Err()

		if err == bufio.ErrTooLong {

			session.reply(500, "Line too long")

			// Advance reader to the next newline

			session.reader.ReadString('\n')
			session.scanner = bufio.NewScanner(session.reader)

			// Reset and have the client start over.

			session.reset()

			continue
		}

		break
	}

}

func (session *session) reject() {
	session.reply(421, "Too busy. Try again later.")
	session.close()
}

func (session *session) reset() {
	session.envelope = nil
}

func (session *session) welcome() {

	if session.server.ConnectionChecker != nil {
		err := session.server.ConnectionChecker(session.peer)
		if err != nil {
			session.error(err)
			session.close()
			return
		}
	}

	session.reply(220, session.server.WelcomeMessage)

}

func (session *session) reply(code int, message string) {
	fmt.Fprintf(session.writer, "%d %s\r\n", code, message)
	session.flush()
}

func (session *session) flush() {
	session.conn.SetWriteDeadline(time.Now().Add(session.server.WriteTimeout))
	session.writer.Flush()
	session.conn.SetReadDeadline(time.Now().Add(session.server.ReadTimeout))
}

func (session *session) error(err error) {
	if smtpdError, ok := err.(Error); ok {
		session.reply(smtpdError.Code, smtpdError.Message)
	} else {
		session.reply(502, fmt.Sprintf("%s", err))
	}
}

func (session *session) extensions() []string {

	extensions := []string{
		fmt.Sprintf("SIZE %d", session.server.MaxMessageSize),
		"8BITMIME",
		"PIPELINING",
	}

	if session.server.EnableXCLIENT {
		extensions = append(extensions, "XCLIENT")
	}

	if session.server.TLSConfig != nil && !session.tls {
		extensions = append(extensions, "STARTTLS")
	}

	if session.server.Authenticator != nil && session.tls {
		extensions = append(extensions, "AUTH PLAIN LOGIN")
	}

	return extensions

}

func (session *session) deliver() error {
	if session.server.Handler != nil {
		return session.server.Handler(session.peer, *session.envelope)
	}
	return nil
}

func (session *session) close() {
	session.writer.Flush()
	time.Sleep(200 * time.Millisecond)
	session.conn.Close()
}