smtpd/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"
	"errors"
	"fmt"
	"log/slog"
  "log"
	"net"
	"sync"
	"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)
	EnableProxyProtocol bool // Enable proxy protocol support (default: false)

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

	ProtocolLogger *slog.Logger

	// mu guards doneChan and makes closing it and listener atomic from
	// perspective of Serve()
	mu sync.Mutex
	doneChan chan struct{}
	listener *net.Listener
	waitgrp sync.WaitGroup
	inShutdown atomicBool // true when server is in shutdown
}

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

const (
	// SMTP
	SMTP Protocol = "SMTP"

	// Extended 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
}

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,
		},
	}

	// Check if the underlying connection is already TLS.
	// This will happen if the Listerner provided Serve()
	// is from tls.Listen()

	var tlsConn *tls.Conn

	tlsConn, s.tls = c.(*tls.Conn)

	if s.tls {
		// run handshake otherwise it's done when we first
		// read/write and connection state will be invalid
		tlsConn.Handshake()
		state := tlsConn.ConnectionState()
		s.peer.TLS = &state
	}

	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 {
	if srv.shuttingDown() {
		return ErrServerClosed
	}

	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 {
	if srv.shuttingDown() {
		return ErrServerClosed
	}

	srv.configureDefaults()

	l = &onceCloseListener{Listener: l}
	defer l.Close()
	srv.listener = &l

	var limiter chan struct{}

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

	for {
		conn, e := l.Accept()
		if e != nil {
			select {
			case <-srv.getDoneChan():
				return ErrServerClosed
			default:
			}

			if ne, ok := e.(net.Error); ok && ne.Timeout() {
				time.Sleep(time.Second)
				continue
			}
			return e
		}

		session := srv.newSession(conn)

		srv.waitgrp.Add(1)
		go func() {
			defer srv.waitgrp.Done()
			if limiter != nil {
				select {
				case limiter <- struct{}{}:
					session.serve()
					<-limiter
				default:
					session.reject()
				}
			} else {
				session.serve()
			}
		}()
	}

}

// Shutdown instructs the server to shutdown, starting by closing the
// associated listener. If wait is true, it will wait for the shutdown
// to complete. If wait is false, Wait must be called afterwards.
func (srv *Server) Shutdown(wait bool) error {
	var lnerr error
	srv.inShutdown.setTrue()

	// First close the listener
	srv.mu.Lock()
	if srv.listener != nil {
		lnerr = (*srv.listener).Close();
	}
	srv.closeDoneChanLocked()
	srv.mu.Unlock()

	// Now wait for all client connections to close
	if wait {
		srv.Wait()
	}

	return lnerr
}

// Wait waits for all client connections to close and the server to finish
// shutting down.
func (srv *Server) Wait() error {
	if !srv.shuttingDown() {
		return errors.New("Server has not been Shutdown")
	}

	srv.waitgrp.Wait()
	return nil
}

// Address returns the listening address of the server
func (srv *Server) Address() net.Addr {
	return (*srv.listener).Addr();
}

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)
	}

}

// From net/http/server.go

func (s *Server) shuttingDown() bool {
	return s.inShutdown.isSet()
}

func (s *Server) getDoneChan() <-chan struct{} {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.getDoneChanLocked()
}

func (s *Server) getDoneChanLocked() chan struct{} {
	if s.doneChan == nil {
		s.doneChan = make(chan struct{})
	}
	return s.doneChan
}

func (s *Server) closeDoneChanLocked() {
	ch := s.getDoneChanLocked()
	select {
	case <-ch:
		// Already closed. Don't close again.
	default:
		// Safe to close here. We're the only closer, guarded
		// by s.mu.
		close(ch)
	}
}