SMTP协议原始命令码和工作原理
http://www.linuxforum.net Jephe Wu (2001-04-21 18:16:42) 1.SMTP是工作在两种情况下：一是电子邮件从客户机传输到服务器；二是从某一个服务器传输到另一个 服务器 2.SMTP是个请求/响应协议，命令和响应都是基于ASCII文本，并以CR和LF符结束。响应包括一个表示返 回状态的三位数字代码 3.SMTP在TCP协议25号端口监听连接请求 4.连接和发送过程： a.建立TCP连接 b.客户端发送HELO命令以标识发件人自己的身份，然后客户端发送MAIL命令 服务器端正希望以OK作为响应，表明准备接收 c.客户端发送RCPT命令，以标识该电子邮件的计划接收人，可以有多个RCPT行 服务器端则表示是否愿意为收件人接受邮件 d.协商结束，发送邮件，用命令DATA发送 e. 以.表示结束输入内容一起发送出去 f.结束此次发送，用QUIT命令退出。 5.另外两个命令： VRFY---用于验证给定的用户邮箱是否存在，以及接收关于该用户的详细信息。 EXPN---用于扩充邮件列表。 6.邮件路由过程： SMTP服务器基于‘域名服务DNS中计划收件人的域名来路由电子邮件。SMTP服务器基于DNS中的MX记录 来路由电子邮件，MX记录注册了域名和相关的SMTP中继主机，属于该域的电子邮件都应向该主机发送。 若SMTP服务器mail.abc.com收到一封信要发到shuser@sh.abc.com: a.Sendmail请求DNS给出主机sh.abc.com的CNAME记录，如有，假若CNAME到shmail.abc.com，则再次 请求shmail.abc.com的CNAME记录，直到没有为止 b.假定被CNAME到shmail.abc.com,然后sendmail请求@abc.com域的DNS给出shmail.abc.com的MX记录， shmail MX 5 shmail.abc.com 10 shmail2.abc.com c. Sendmail最后请求DNS给出shmail.abc.com的A记录，即IP地址，若返回值为1.2.3.4 d. Sendmail与1.2.3.4连接，传送这封给shuser@sh.abc.com的信到1.2.3.4这台服务器的SMTP后台程序 7.SMTP基本命令集： 命令 描述 ------------------------------ HELO 向服务器标识用户身份 发送者能欺骗，说谎，但一般情况下服务器都能检测到。 MAIL 初始化邮件传输 mail from: RCPT 标识单个的邮件接收人；常在MAIL命令后面 可有多个rcpt to: DATA 在单个或多个RCPT命令后，表示所有的邮件接收人已标识，并初始化数据传输，以.结束。 VRFY 用于验证指定的用户/邮箱是否存在；由于安全方面的原因，服务器常禁止此命令 EXPN 验证给定的邮箱列表是否存在，扩充邮箱列表，也常被禁用 HELP 查询服务器支持什么命令 NOOP 无操作，服务器应响应OK QUIT 结束会话 RSET 重置会话，当前传输被取消 -------------------------------- 8. MAIL FROM命令中指定的地址是称作 envelope from地址，不需要和发送者自己的地址是一致的。 RCPT TO 与之等同，指明的接收者地址称为envelope to地址，而与实际的to：行是什么无关。 9.为什么没有RCPT CC和RCPT BCC:? 所有的接收者协商都通过RCPT TO命令来实现，如果是BCC，则协商发送后在对方接收时被删掉信封接收者 10.邮件被分为信封部分，信头部分和信体部分 envelope from, envelope to 与message from:, message to:完全不相干。 evnelope是由服务器主机间SMTP后台提供的，而message from/to是由用户提供的。有无冒号也是区别。 11. 怎样由信封部分检查是否一封信是否是伪造的？ a. received行的关联性。 现在的SMTP邮件传输系统，在信封部分除了两端的内部主机处理的之个，考虑两个公司防火墙之间 的部分，若两台防火墙机器分别为Ａ和Ｂ，但接收者检查信封received：行时发现经过了C.则是伪造的。 b. received：行中的主机和IP地址对是否对应如： Receibed: from galangal.org (turmeric.com [104.128.23.115] by mail .bieberdorf.edu.... c. 被人手动添加在最后面的received行： Received: from galangal.org ([104.128.23.115]) by mail .bieberdorf.edu (8.8.5) Received: from lemongrass.org by galangal.org (8.7.3) Received: from graprao.com by lemongrass.org (8.6.4) Jephe Wu
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