TCP Example of SMTP Authentication

smtp-auth-tcp-example.go

// reference: https://tools.ietf.org/html/rfc5321
package main

import (
	"bufio"
	"encoding/base64"
	"fmt"
	"log"
	"net"
	"os"
	"os/signal"
	"strings"
	"syscall"
)

func main() {
	closed := make(chan struct{})

	user := "USERNAME"
	pass := "PASSWORD"
	host := "SMTP_SERVER"
	//port := "587"
	port := "25"
	conn, err := net.Dial("tcp", host+":"+port)
	if err != nil {
		log.Panic(err)
	}
	defer conn.Close()

	go func() {
		signals := make(chan os.Signal, 1)
		signal.Notify(signals, syscall.SIGINT, syscall.SIGKILL, syscall.SIGTERM)
		<-signals
		log.Println("Initializing shutdown...")
		close(closed)
	}()

	var code string
	var message string
	var more []string

	code = printAndGetCode(conn)
	fmt.Printf("code: %s\n", code)

	conn.Write([]byte("EHLO " + host + "\r\n"))
	code = printAndGetCode(conn)

	//334 VXNlcm5hbWU6
	//334 UGFzc3dvcmQ6

	conn.Write([]byte("AUTH LOGIN\r\n"))

	code, message, more = parseResp(conn)
	printMoreMsg(more)
	if code != "334" && message != "VXNlcm5hbWU6" {
		log.Printf("Error(1): %s: %s\n", code, message)
	}

	conn.Write([]byte(base64.StdEncoding.EncodeToString([]byte(user)) + "\r\n"))
	// Expected: 334 UGFzc3dvcmQ6
	code, message, more = parseResp(conn)
	printMoreMsg(more)
	if code != "334" && message != "UGFzc3dvcmQ6" {
		log.Printf("Error(2): %s: %s\n", code, message)
	}

	conn.Write([]byte(base64.StdEncoding.EncodeToString([]byte(pass)) + "\r\n"))
	code, message, more = parseResp(conn)
	if code != "235" && strings.Index(message, "Authentication successful") < 0 {
		log.Printf("Error(3): %s: %s\n", code, message)
	} else {
		log.Printf("OK: %s#%s\n", code, message)
	}

	<-closed
	fmt.Println("Done")
}

func readAndPrint(conn net.Conn) {
	buf := make([]byte, 1024)
	for {
		l, err := conn.Read(buf)
		if err != nil {
			fmt.Printf("Error: %v\n", err)
		}

		fmt.Printf("Len: %d\n", l)
		if l > 0 {
			fmt.Printf("Data: %s\n", string(buf[0:l]))
		}
		return
	}
}

func parseResp(conn net.Conn) (code string, message string, more []string) {
	count := 0
	br := bufio.NewReader(conn)
	for {
		resp, _ := br.ReadString('\n')
		if len(resp) > 4 {
			code = resp[0:3]
			if resp[3:4] == "-" {
				more = append(more, strings.TrimSpace(resp))
			} else {
				message = strings.TrimSpace(resp[4:])
				return
			}
		}
		count++
		if count > 20 {
			fmt.Println("WARN: force break")
			return
		}
	}
}

func printAndGetCode(conn net.Conn) string {
	code, message, more := parseResp(conn)
	if len(more) > 0 {
		for _, m := range more {
			fmt.Println(m)
		}
	}
	fmt.Printf("%s %s", code, message)
	return code
}

func printMoreMsg(more []string) {
	if len(more) > 0 {
		for _, m := range more {
			fmt.Println(m)
		}
	}
}