golang net/smtp SMTP AUTH LOGIN Auth Handler

smtp_login_auth.go

// MIT license (c) andelf 2013


import (
	"net/smtp"
	"errors"
)

type loginAuth struct {
  username, password string
}

func LoginAuth(username, password string) smtp.Auth {
	return &loginAuth{username, password}
}

func (a *loginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
	return "LOGIN", []byte{}, nil
}

func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
	if more {
		switch string(fromServer) {
		case "Username:":
			return []byte(a.username), nil
		case "Password:":
			return []byte(a.password), nil
		default:
			return nil, errors.New("Unkown fromServer")
		}
	}
	return nil, nil
}


// usage: 
// auth := LoginAuth("loginname", "password")
// err := smtp.SendMail(smtpServer + ":25", auth, fromAddress, toAddresses, []byte(message))
// or	
// client, err := smtp.Dial(smtpServer)
// client.Auth(LoginAuth("loginname", "password"))

Very useful, Thanks for your code, this works for me!

Also, there is login auth support in go-mail package https://github.com/go-gomail/gomail/blob/master/auth.go

Thank you so much!
Solution still working fine.

And if you don't want to choose between PLAIN and LOGIN:

func PlainOrLoginAuth(username, password, host string) smtp.Auth {
	return &plainOrLoginAuth{username: username, password: password, host: host}
}

func isLocalhost(name string) bool {
	return name == "localhost" || name == "127.0.0.1" || name == "::1"
}

type plainOrLoginAuth struct {
	username   string
	password   string
	host       string
	authMethod string
}

func (a *plainOrLoginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
	// Must have TLS, or else localhost server.
	// Note: If TLS is not true, then we can't trust ANYTHING in ServerInfo.
	// In particular, it doesn't matter if the server advertises PLAIN auth.
	// That might just be the attacker saying
	// "it's ok, you can trust me with your password."
	if !server.TLS && !isLocalhost(server.Name) {
		return "", nil, errors.New("unencrypted connection")
	}
	if server.Name != a.host {
		return "", nil, errors.New("wrong host name")
	}
	if !slices.Contains(server.Auth, "PLAIN") {
		a.authMethod = "LOGIN"
		return a.authMethod, nil, nil
	} else {
		a.authMethod = "PLAIN"
		resp := []byte("\x00" + a.username + "\x00" + a.password)
		return a.authMethod, resp, nil
	}
}

func (a *plainOrLoginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
	if !more {
		return nil, nil
	}

	if a.authMethod == "PLAIN" {
		// We've already sent everything.
		return nil, errors.New("unexpected server challenge")
	}

	switch {
	case bytes.Equal(fromServer, []byte("Username:")):
		return []byte(a.username), nil
	case bytes.Equal(fromServer, []byte("Password:")):
		return []byte(a.password), nil
	default:
		return nil, fmt.Errorf("unexpected server challenge: %s", fromServer)
	}
}

Thanks a lot!!!

Thank you for this!