Auto detect authentication type

Signed-off-by: utkarshm <utkarshm@jfrog.com>

email.go

@@ -9,6 +9,7 @@ import (
 	"net/mail"
 	"net/textproto"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/toorop/go-dkim"
@@ -134,8 +135,22 @@ const (
 	AuthCRAMMD5
 	// AuthNone for SMTP servers without authentication
 	AuthNone
+	AuthAuto
 )
 
+func (at AuthType) String() string {
+	switch at {
+	case 0:
+		return "PLAIN"
+	case 1:
+		return "LOGIN"
+	case 2:
+		return "CRAM-MD5"
+	default:
+		return ""
+	}
+}
+
 // NewMSG creates a new email. It uses UTF-8 by default. All charsets: http://webcheatsheet.com/HTML/character_sets_list.php
 func NewMSG() *Email {
 	email := &Email{
@@ -152,7 +167,7 @@ func NewMSG() *Email {
 // NewSMTPClient returns the client for send email
 func NewSMTPClient() *SMTPServer {
 	server := &SMTPServer{
-		Authentication: AuthPlain,
+		Authentication: AuthAuto,
 		Encryption:     EncryptionNone,
 		ConnectTimeout: 10 * time.Second,
 		SendTimeout:    10 * time.Second,
@@ -717,7 +732,7 @@ func dial(host string, port string, encryption Encryption, config *tls.Config) (
 
 // smtpConnect connects to the smtp server and starts TLS and passes auth
 // if necessary
-func smtpConnect(host, port, helo string, a auth, at AuthType, encryption Encryption, config *tls.Config) (*smtpClient, error) {
+func smtpConnect(host, port, helo string, encryption Encryption, config *tls.Config) (*smtpClient, error) {
 	// connect to the mail server
 	c, err := dial(host, port, encryption, config)
 
@@ -746,42 +761,60 @@ func smtpConnect(host, port, helo string, a auth, at AuthType, encryption Encryp
 		}
 	}
 
-	// only pass authentication if defined
-	if at != AuthNone {
-		// pass the authentication if necessary
-		if a != nil {
-			if ok, _ := c.extension("AUTH"); ok {
-				if err = c.authenticate(a); err != nil {
-					c.close()
-					return nil, fmt.Errorf("Mail Error on Auth: %w", err)
-				}
-			}
-		}
-	}
-
 	return c, nil
 }
 
-// Connect returns the smtp client
-func (server *SMTPServer) Connect() (*SMTPClient, error) {
-
-	var a auth
-
-	switch server.Authentication {
-	case AuthPlain:
+func (server *SMTPServer) getAuth(a string) (auth, error) {
+	var afn auth
+	switch {
+	case strings.Contains(a, AuthPlain.String()):
 		if server.Username != "" || server.Password != "" {
-			a = plainAuthfn("", server.Username, server.Password, server.Host)
+			afn = plainAuthfn("", server.Username, server.Password, server.Host)
 		}
-	case AuthLogin:
+	case strings.Contains(a, AuthLogin.String()):
 		if server.Username != "" || server.Password != "" {
-			a = loginAuthfn("", server.Username, server.Password, server.Host)
+			afn = loginAuthfn("", server.Username, server.Password, server.Host)
 		}
-	case AuthCRAMMD5:
+	case strings.Contains(a, AuthCRAMMD5.String()):
 		if server.Username != "" || server.Password != "" {
-			a = cramMD5Authfn(server.Username, server.Password)
+			afn = cramMD5Authfn(server.Username, server.Password)
 		}
+	default:
+		return nil, fmt.Errorf("Mail Error on determining auth type, %s is not supported", a)
 	}
+	return afn, nil
+}
 
+func (server *SMTPServer) validateAuth(c *smtpClient) error {
+	var err error
+	var afn auth
+	switch {
+	case server.Authentication == AuthNone || server.Username == "":
+		return nil
+	case server.Authentication != AuthAuto:
+		afn, err = server.getAuth(server.Authentication.String())
+		if err != nil {
+			return err
+		}
+	}
+	if ok, a := c.extension("AUTH"); ok {
+		// Determine Auth type automatically from extension
+		if afn == nil {
+			afn, err = server.getAuth(a)
+			if err != nil {
+				return err
+			}
+		}
+		if err = c.authenticate(afn); err != nil {
+			c.close()
+			return fmt.Errorf("Mail Error on Auth: %w", err)
+		}
+	}
+	return nil
+}
+
+// Connect returns the smtp client
+func (server *SMTPServer) Connect() (*SMTPClient, error) {
 	var smtpConnectChannel chan error
 	var c *smtpClient
 	var err error
@@ -795,7 +828,7 @@ func (server *SMTPServer) Connect() (*SMTPClient, error) {
 	if server.ConnectTimeout != 0 {
 		smtpConnectChannel = make(chan error, 2)
 		go func() {
-			c, err = smtpConnect(server.Host, fmt.Sprintf("%d", server.Port), server.Helo, a, server.Authentication, server.Encryption, tlsConfig)
+			c, err = smtpConnect(server.Host, fmt.Sprintf("%d", server.Port), server.Helo, server.Encryption, tlsConfig)
 			// send the result
 			smtpConnectChannel <- err
 		}()
@@ -810,7 +843,7 @@ func (server *SMTPServer) Connect() (*SMTPClient, error) {
 		}
 	} else {
 		// no ConnectTimeout, just fire the connect
-		c, err = smtpConnect(server.Host, fmt.Sprintf("%d", server.Port), server.Helo, a, server.Authentication, server.Encryption, tlsConfig)
+		c, err = smtpConnect(server.Host, fmt.Sprintf("%d", server.Port), server.Helo, server.Encryption, tlsConfig)
 		if err != nil {
 			return nil, err
 		}
@@ -820,7 +853,7 @@ func (server *SMTPServer) Connect() (*SMTPClient, error) {
 		Client:      c,
 		KeepAlive:   server.KeepAlive,
 		SendTimeout: server.SendTimeout,
-	}, nil
+	}, server.validateAuth(c)
 }
 
 // Reset send RSET command to smtp client