image

本文主要使用Wireshark对邮件客户端使用POP3协议收取邮件的过程进行抓包分析并使用telnet命令进行简单操作。

1、POP3简介

邮局协议（英语：Post Office Protocol，缩写：POP）属于TCP/IP协议族中的一员，由RFC 1939定义。此协议主要用于支持使用客户端远程管理在服务器上的电子邮件。最新版本为POP3，全名“Post Office Protocol - Version 3”，而提供了SSL加密的POP3协议被称为POP3S。

POP支持离线邮件处理。其具体过程是：发件人将邮件发送到服务器上，收件人客户端使用MUA以连接服务器，并下载所有未阅读的电子邮件。这种离线访问模式是一种存储转发服务，将邮件从邮件服务器端送到个人客户端上，可以是PC或者手机等多种设备。旧版的POP3协议在邮件被下载后，会删除掉邮件服务器上的邮件。改进的POP3协议可以在配置中选择可以“只下载邮件，服务器端并不删除”，也是目前POP3的主流操作方式。

2、抓包环境

抓包工具还是使用的wireshark，测试的协议是POP3，默认端口110，同样地为了方便分析数据没有使用TLS/SSL加密，但是结果都问题不大。

测试的账号登录之后自动同步了收件箱中已有的5封邮件，然后我又给测试账号发了一封邮件，因此抓包的时候MUA客户端上应该是只有本地已缓存的5封已读邮件和服务器端尚未下载的1封未读邮件，这样比较符合实际的情况。

开启wireshark监听对应的网卡，设定Filter为邮件服务器的IP并且设定协议为pop基本就能抓取到需要的数据包，然后我们再Follow这条POP数据流对应的TCP stream就可以看到下面的完整信息。

image

毫无意外地可以看到TCP的三握四挥，这里不作赘述。

3、POP3数据包分析

首先我们可以看到在客户端和服务端TCP三次握手建立连接后，服务端发送报文给客户端告知顺利和POP3服务器建立连接：

image

接下来客户端发送了一个CAPA命令：

image

wiki里面有列出常用的POP3命令，但是却并没有CAPA这条命令，顺着下面的参考文档查了一下，在RFC的文档RFC2449中找到了比较详细的定义：

Section 3 describes the CAPA response using [ABNF]. When a
capability response describes an optional command, the <capa-tag>
SHOULD be identical to the command keyword. CAPA response tags are
case-insensitive.

    CAPA

    Arguments:
        none

    Restrictions:
        none

    Discussion:
        An -ERR response indicates the capability command is not
        implemented and the client will have to probe for
        capabilities as before.

        An +OK response is followed by a list of capabilities, one
        per line.  Each capability name MAY be followed by a single
        space and a space-separated list of parameters.  Each
        capability line is limited to 512 octets (including the
        CRLF).  The capability list is terminated by a line
        containing a termination octet (".") and a CRLF pair.

     Possible Responses:
         +OK -ERR

也就是说客户端发送CAPA命令主要是用于获取POP3服务端可以执行的命令，然后POP3服务器果然就返回了对应的+OK response和Capability list follows

image

接下来客户端开始传输账号密码用于登录，可以看到在登录成功之后，POP3服务端返回了消息提示有6封邮件（6 messages）和邮件的总大小（25568 bytes），然后客户端发送UTF8命令指定编码方式，收到服务端返回的确认消息后再发送STAT命令来请求服务器发回关于邮箱的统计资料（此处为邮件总数和总字节数）。

image

紧接着客户端继续发送UIDL命令，POP3服务器端返回邮件的唯一标识符，POP3会话的每个标识符都将是唯一的，并且是全局始终唯一。即同一封邮件在每次通信的时候的标识符都是不变的，这样就有利于MUA将本地存储的邮件和服务器端的邮件进行对比从而知道哪些邮件还没有被下载到本地。

获取邮件标识符后，客户端会发送LIST命令查询邮件数量和每个邮件的大小，这里我们可以看到邮件的总数量和总大小是和上面的STAT命令查询的一致的。

image

前面我们说过在抓包之前MUA中是存着5封邮件并且第6封邮件是未读的，因此这时客户端就会发送RETR请求获取第6封邮件的信息：

image

服务器首先返回一条信息表示OK，并且说明这封邮件的大小是1199个字节，然后开始传输整封邮件的内容，整个邮件内容包含了标准的一些邮件信息（主题、正文、收发件人、时间等）和一些对应的邮件系统的专属信息。

最后邮件以.结束传输，客户端发送QUIT请求，服务端返回OK结束本次传输。

4、telnet操作

和之前的SMTP一样，我们也可以使用telnet命令对服务器的110端口进行操作：

[root@www coremail]# telnet localhost 110
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
+OK Welcome to coremail Mail Pop3 Server (126coms[c92b4e18679ada4069d0bde6e2528ad1s])
CAPA
+OK Capability list follows
TOP
USER
PIPELINING
UIDL
LANG
UTF8
SASL PLAIN
STLS
.
USER test01@coremail.cn
+OK core mail
PASS password
+OK 6 message(s) [25568 byte(s)]
UTF8
+OK UTF-8 OK
STAT
+OK 6 25568
UIDL
+OK 6 25568
1 1tbiAQACE10Y3LsAAAAAsy
2 1tbiAQACE10Y3LsAAAACsw
3 1tbiAQACE10Y3LsAAAADsx
4 1tbiAQACE10Y3LsAAAAEs2
5 1tbiAQACE10Y3LsAAAAFs3
6 1tbiAQACE10Y3LsAAAAGs0
.
LIST
+OK 6 25568
1 7959
2 1199
3 6445
4 6469
5 2297
6 1199
.

到这里的操作是和之前一样的，实际上我们还可以使用RETR命令查看任意一封邮件的内容：

RETR 5
+OK 2297 octets
Received: from TINYDESKTOP (unknown [10.228.12.149])
        by www.example.com (Coremail) with SMTP id AQAAfwBXc7qLVrZeWAAAAA--.16S2;
        Sat, 09 May 2020 15:06:51 +0800 (CST)
From: "Microsoft Outlook" <test01@coremail.cn>
Sender: test01@coremail.cn
To: <test01@coremail.cn>
Subject: =?utf-8?B?TWljcm9zb2Z0IE91dGxvb2sg5rWL6K+V5raI5oGv?=
MIME-Version: 1.0
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        jiiSDUUUU
Date: Sat, 9 May 2020 15:06:51 +0800 (CST)
X-CM-SenderInfo: hwhv3imr6f02phpdxzgofq/

This is a multipart message in MIME format.

------=_NextPart_000_0000_01D62613.778669A0
Content-Type: text/plain;
        charset="utf-8"
Content-Transfer-Encoding: base64

6L+Z5piv5Zyo5rWL6K+V5L2g55qE5biQ5oi36K6+572u5pe2IE1pY3Jvc29mdCBPdXRsb29rIOiH
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.
RETR 6
+OK 1199 octets
Received: by ajax-webmail-www.example.com (Coremail) ; Sat, 9 May 2020
 15:35:01 +0800 (GMT+08:00)
X-Originating-IP: [10.228.12.149]
Date: Sat, 9 May 2020 15:35:01 +0800 (GMT+08:00)
X-CM-HeaderCharset: UTF-8
From: test02@coremail.cn
To: test01@coremail.cn
Subject: awesome pop
X-Priority: 3
X-Mailer: Coremail Webmail Server Version XT5.0.8a build 20190308(983496cf)
 Copyright (c) 2002-2020 www.mailtech.cn 126com
Content-Type: multipart/alternative;
        boundary="----=_Part_5_1271853342.1589009701828"
MIME-Version: 1.0
Message-ID: <4d02717c.1.171f85bdbc6.Coremail.test02@coremail.cn>
X-Coremail-Locale: zh_CN
X-CM-TRANSID:AQAAfwAXM7olXbZeXAAAAA--.0W
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Content-Type: text/plain; charset=UTF-8
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This is for pop test!
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Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 7bit

This is for pop test!
------=_Part_5_1271853342.1589009701828--
.
RETR 4
+OK 6469 octets
Received: from TINY-DESKTOP (unknown [10.228.12.149])
        by www.example.com (Coremail) with SMTP id AQAAfwCngFa5lNFcYQAAAA--.16S2;
        Tue, 07 May 2019 22:22:50 +0800 (CST)
Date: Thu, 7 May 2020 14:22:46 +0800
From: test02 <test02@coremail.cn>
To: =?utf-8?Q?test01=40coremail.cn?= <test01@coremail.cn>
Message-ID: <305443E1-9258-4260-AA18-1A5CDFBD60EE@coremail.cn>
Subject: smtp test
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eCAhaW1wb3J0YW50OyB3aWR0aDogMTAwJTsiPgogICAgICAgICAgICAgICAgICAgICAgICAgICAg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.

发送NOOP命令并无实际作用，主要是用于和服务器保持数据连接不要中断。

NOOP
+OK core mail

我们还可以使用DELE命令来删除指定的邮件，删除后使用LIST查看效果：

DELE 4
+OK core mail
LIST
+OK 5 19099
1 7959
2 1199
3 6445
5 2297
6 1199
.

还可以使用RSET命令来进行撤销删除的操作并且使用LIST命令查看效果

RSET
+OK core mail
LIST
+OK 6 25568
1 7959
2 1199
3 6445
4 6469
5 2297
6 1199
.

最后使用QUIT命令断开连接退出系统：

QUIT
+OK core mail
Connection closed by foreign host.