go smtp

最近看了几个 用 go 写的实现 gmail 发送邮件的程序，发现代码甚是简单。

然后仔细研究了一下 smtp 协议，已经相关技术。

1.go code

调研go 的 "net/smtp"包的两个函数就可以了。

func SendMail(addr string, a Auth, from string, to []string, msg []byte) error

SendMail connects to the server at addr, switches to TLS if possible, authenticates with the optional mechanism a if possible, and then sends an email from address from, to addresses to, with message msg.

func PlainAuth(identity, username, password, host string) Auth

PlainAuth returns an Auth that implements the PLAIN authentication mechanism as defined in RFC 4616. The returned Auth uses the given username and password to authenticate on TLS connections to host and act as identity. Usually identity will be left blank to act as username.

auth := smtp.PlainAuth(
　　"",
　　from@gmail.com,
　　mypassword,
　　"smtp.gmail.com",
)

err := smtp.SendMail(

　　"smtp.example.com:25",
　　auth,
　　"from@example.com",
　　mailList,
　　[]byte(sub+content))

当然 也有 更好的实现，有点类似socket,建立连接之后 客户端 持有 connection,  然后在 connection 上面操作。

可以查看：https://github.com/scorredoira/email

2.相关点

我花了一点是件简单看了一下 smtp协议，着重理解了一下 smtp 的auth 和 TLS.

(1) smtp 协议 类似 http 协议：http://www.rfc-editor.org/rfc/pdfrfc/rfc5321.txt.pdf

(2) TLS  it's a protocal to encrypt you trasmitted data .

refer to http://www.ruanyifeng.com/blog/2014/02/ssl_tls.html

(3) for auth

refer to : https://qmail.jms1.net/test-auth.shtml  <--- good artical

Once you are authenticated, you may continue with a normal SMTP conversation and the server should accept any message from you, whether you are relaying to an outside domain or not. Even if you don't authenticate, the server will still accept messages from you- it just won't relay (it will act the same as if you had never entered an AUTH command at all.)

How to auth using TLS in code:

auth := smtp.PlainAuth(     <------  just construct a object implementing the interface auth.
        "",
        from,
        password,
        "smtp.gmail.com",
    )

    conn, err := smtp.Dial("smtp.gmail.com:587")
    err = conn.StartTLS(&tls.Config{})
    err = conn.Auth(auth)
    err = conn.Mail(from@xxx)
    err = conn.Rcpt(sendto@xxx) //can add multiple receivers
    wc, err := conn.Data()
　　defer wc.Close()
    _, err = wc.Write(e.Bytes()) //write data


}

this code similar to <----  may be wrong,  I will confirm it by wireshark tomorrow.  <---- confirm by wireshark

(1) dial

S: 220 mail.example.com ESMTP Postfix (1.1.7)
C: EHLO example.com
S: 250-mail.example.com
S: 250-PIPELINING
S: 250-SIZE 10240000
S: 250-VRFY
S: 250-ETRN
S: 250-AUTH DIGEST-MD5 CRAM-MD5 GSSAPI PLAIN LOGIN
S: 250-AUTH=DIGEST-MD5 CRAM-MD5 GSSAPI PLAIN LOGIN
S: 250-XVERP
S: 250 8BITMIME
S: 250 STARTTLS

(2) startTLS

C: STARTTLS
S: 220 Go ahead
C: <starts TLS negotiation>
S: 220 Ready to start TLS ... TLS negotiation proceeds. Further commands protected by TLS layer 
...

(3) auto

SSL  transmission

(4) send data

SSL ...

(5) close
C: QUIT 
S: 221 Bye

(3) ,(4) and (5) are encrypted by TLS.