dialer.go

// Portions of this code are derived from the go-mail/mail project.
// https://github.com/go-mail/mail (MIT License)

package mail

import (
	"crypto/tls"
	"fmt"
	"io"
	"net"
	"net/smtp"
	"strings"
	"time"
)

// Dialer connects to an SMTP server and sends emails.
type Dialer struct {
	Host     string
	Port     int
	Username string
	Password string
	Auth     smtp.Auth
	// SSL defines whether an SSL connection is used. It should be false in
	// most cases since the authentication mechanism should use the STARTTLS
	// extension instead.
	SSL bool
	// TLSConfig represents the TLS configuration used for the TLS (when the
	// STARTTLS extension is used) or SSL connection.
	TLSConfig *tls.Config
	// StartTLSPolicy represents the TLS security level required to communicate
	// with the SMTP server. Defaults to opportunistic STARTTLS.
	StartTLSPolicy StartTLSPolicy
	// LocalName is the hostname sent to the SMTP server with the HELO command.
	// By default, "localhost" is sent.
	LocalName string
	// Timeout to use for read/write operations. Defaults to 10 seconds, can
	// be set to 0 to disable timeouts.
	Timeout time.Duration
	// Whether we should retry mailing if the connection returned an error.
	RetryFailure bool
}

func newDialer(host string, port int, username, password string) *Dialer {
	return &Dialer{
		Host:         host,
		Port:         port,
		Username:     username,
		Password:     password,
		SSL:          port == 465,
		Timeout:      10 * time.Second,
		RetryFailure: true,
	}
}

// NetDialTimeout specifies the DialTimeout function to establish a connection
// to the SMTP server. This can be used to override dialing in the case that a
// proxy or other special behavior is needed.
var NetDialTimeout = net.DialTimeout

func (d *Dialer) dial() (sendCloser, error) {
	conn, err := NetDialTimeout("tcp", addr(d.Host, d.Port), d.Timeout)
	if err != nil {
		return nil, err
	}

	if d.SSL {
		conn = tlsClient(conn, d.tlsConfig())
	}

	c, err := smtpNewClient(conn, d.Host)
	if err != nil {
		return nil, err
	}

	if d.Timeout > 0 {
		conn.SetDeadline(time.Now().Add(d.Timeout))
	}

	if d.LocalName != "" {
		if err := c.Hello(d.LocalName); err != nil {
			return nil, err
		}
	}

	if !d.SSL && d.StartTLSPolicy != noStartTLS {
		ok, _ := c.Extension("STARTTLS")
		if !ok && d.StartTLSPolicy == mandatoryStartTLS {
			err := startTLSUnsupportedError{Policy: d.StartTLSPolicy}
			return nil, err
		}

		if ok {
			if err := c.StartTLS(d.tlsConfig()); err != nil {
				c.Close()
				return nil, err
			}
		}
	}

	if d.Auth == nil && d.Username != "" {
		if ok, auths := c.Extension("AUTH"); ok {
			if strings.Contains(auths, "CRAM-MD5") {
				d.Auth = smtp.CRAMMD5Auth(d.Username, d.Password)
			} else if strings.Contains(auths, "LOGIN") &&
				!strings.Contains(auths, "PLAIN") {
				d.Auth = &loginAuth{
					username: d.Username,
					password: d.Password,
					host:     d.Host,
				}
			} else {
				d.Auth = smtp.PlainAuth("", d.Username, d.Password, d.Host)
			}
		}
	}

	if d.Auth != nil {
		if err = c.Auth(d.Auth); err != nil {
			c.Close()
			return nil, err
		}
	}

	return &smtpSender{c, conn, d}, nil
}

func (d *Dialer) tlsConfig() *tls.Config {
	if d.TLSConfig == nil {
		return &tls.Config{ServerName: d.Host}
	}
	return d.TLSConfig
}

// StartTLSPolicy represents the TLS security level required to communicate
// with an SMTP server.
type StartTLSPolicy int

const (
	opportunisticStartTLS StartTLSPolicy = iota
	mandatoryStartTLS
	noStartTLS = -1
)

func (policy *StartTLSPolicy) String() string {
	switch *policy {
	case opportunisticStartTLS:
		return "OpportunisticStartTLS"
	case mandatoryStartTLS:
		return "MandatoryStartTLS"
	case noStartTLS:
		return "NoStartTLS"
	default:
		return fmt.Sprintf("StartTLSPolicy:%v", *policy)
	}
}

type startTLSUnsupportedError struct {
	Policy StartTLSPolicy
}

func (e startTLSUnsupportedError) Error() string {
	return "gomail: " + e.Policy.String() + " required, but SMTP server does not support STARTTLS"
}

func addr(host string, port int) string {
	return fmt.Sprintf("%s:%d", host, port)
}

func (d *Dialer) dialAndSend(m ...*smtpMessage) error {
	s, err := d.dial()
	if err != nil {
		return err
	}
	defer s.Close()

	for _, err := range sendSMTP(s, m...) {
		if err != nil {
			return err
		}
	}

	return nil
}

type smtpSender struct {
	smtpClient
	conn net.Conn
	d    *Dialer
}

func (c *smtpSender) retryError(err error) bool {
	if !c.d.RetryFailure {
		return false
	}

	if nerr, ok := err.(net.Error); ok && nerr.Timeout() {
		return true
	}

	return err == io.EOF
}

func (c *smtpSender) Send(from string, to []string, msg io.WriterTo) error {
	if c.d.Timeout > 0 {
		c.conn.SetDeadline(time.Now().Add(c.d.Timeout))
	}

	if err := c.Mail(from); err != nil {
		if c.retryError(err) {
			sc, derr := c.d.dial()
			if derr == nil {
				if s, ok := sc.(*smtpSender); ok {
					*c = *s
					return c.Send(from, to, msg)
				}
			}
		}

		return err
	}

	for _, addr := range to {
		if err := c.Rcpt(addr); err != nil {
			return err
		}
	}

	w, err := c.Data()
	if err != nil {
		return err
	}

	if _, err = msg.WriteTo(w); err != nil {
		w.Close()
		return err
	}

	return w.Close()
}

func (c *smtpSender) Close() error {
	return c.Quit()
}

// Stubbed out for tests.
var (
	tlsClient     = tls.Client
	smtpNewClient = func(conn net.Conn, host string) (smtpClient, error) {
		return smtp.NewClient(conn, host)
	}
)

type smtpClient interface {
	Hello(string) error
	Extension(string) (bool, string)
	StartTLS(*tls.Config) error
	Auth(smtp.Auth) error
	Mail(string) error
	Rcpt(string) error
	Data() (io.WriteCloser, error)
	Quit() error
	Close() error
}

type sendCloser interface {
	sender
	Close() error
}

type sender interface {
	Send(from string, to []string, msg io.WriterTo) error
}