This paper mainly uses Wireshark to analyze the process of receiving e-mail by using POP3 protocol, and uses telnet command for simple operation.
1. POP3 introduction
Post Office Protocol (English: Post Office Protocol, abbreviation: pop) is a member of TCP/IP protocol family and is defined by RFC 1939. This protocol is mainly used to support the remote management of e-mail on the server using the client. The latest version is POP3, whose full name is "Post Office Protocol - Version 3", and the POP3 protocol that provides SSL encryption is called POP3S.
Pop supports offline mail processing. The specific process is: the sender sends the mail to the server, the recipient client uses MUA to connect to the server, and downloads all unread emails. This offline access mode is a store and forward service, which sends mail from the mail server to the personal client, which can be a variety of devices such as PC or mobile phone. The old version of POP3 protocol will delete the mail on the mail server after the mail is downloaded. The improved POP3 protocol can be selected in the configuration. It can "only download mail and not delete it on the server", which is also the mainstream operation mode of POP3 at present.
2. Packet capturing environment
wireshark is still used in the packet capture tool. The protocol tested is POP3 and the default port is 110. Similarly, TLS/SSL encryption is not used for data analysis, but the results are not a problem.
After logging in, the test account automatically synchronizes the five existing emails in the inbox, and then I sent another email to the test account. Therefore, when capturing packets, there should be only five locally cached read emails on the MUA client and one unread email that has not been downloaded on the server. This is more in line with the actual situation.
Turn on wireshark to listen to the corresponding network card, set Filter as the IP of the mail server and set the protocol as pop, and you can basically grab the required packets. Then we can Follow the TCP stream corresponding to the pop data stream to see the following complete information.
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It is no surprise to see the three grip and four swing of TCP, which will not be repeated here.
3. POP3 packet analysis
First of all, we can see that after the client and the server establish a connection through TCP handshake for three times, the server sends a message to the client to inform the client that the connection with POP3 server is successfully established:
Next, the client sends a CAPA command:
wiki There is a list of commonly used POP3 commands, but there is no CAPA command. Check the following reference documents in the RFC document RFC2449 More detailed definitions are found in:
Section 3 describes the CAPA response using [ABNF]. When a
capability response describes an optional command, the
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
In other words, the CAPA command sent by the client is mainly used to obtain the commands that can be executed by the POP3 server, and then the POP3 server returns the corresponding + OK response and capability list follow
Next, the client starts transmitting the account and password for login. You can see that after successful login, the POP3 server returns a message indicating 6 messages and the total size of messages (25568 bytes), and then the client sends the UTF8 command to specify the encoding method, After receiving the confirmation message returned by the server, send the STAT command to request the server to send back the statistics about the mailbox (here is the total number of messages and total bytes).
Then the client continues to send the UIDL command, and the POP3 server returns the unique identifier of the mail. Each identifier of the POP3 session will be unique and always unique globally. That is, the identifier of the same e-mail is unchanged during each communication, which is helpful for MUA to compare the locally stored e-mail with the server-side e-mail, so as to know which e-mail has not been downloaded locally.
After obtaining the email identifier, the client will send the LIST command to query the number of emails and the size of each email. Here we can see that the total number and size of emails are consistent with those queried by the above STAT command.
As we said earlier, before capturing packets, there are 5 emails in the MUA and the 6th email is unread. Therefore, the client will send a RETR request to obtain the information of the 6th email:
The server first returns a message indicating OK, stating that the size of this email is 1199 bytes, and then starts to transmit the content of the whole email. The whole email content contains some standard email information (subject, body, sender, time, etc.) and some exclusive information of the corresponding email system.
The last email is in When the transmission ends, the client sends a QUIT request, and the server returns OK to end the transmission.
4. telnet operation
Like the previous SMTP, we can also use the telnet command to operate on port 110 of the server:
[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 .
The operation here is the same as before. In fact, we can also use the RETR command to view the contents of any email:
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/
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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
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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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.
Sending the NOOP command has no practical effect. It is mainly used to keep the data connection with the server without interruption.
NOOP +OK core mail
We can also use the DELE command to delete the specified mail. After deletion, use LIST to view the effect:
DELE 4 +OK core mail LIST +OK 5 19099 1 7959 2 1199 3 6445 5 2297 6 1199 .
You can also use the RSET command to undo the deletion and use the LIST command to view the effect
RSET +OK core mail LIST +OK 6 25568 1 7959 2 1199 3 6445 4 6469 5 2297 6 1199 .
Finally, use the QUIT command to disconnect and exit the system:
QUIT +OK core mail Connection closed by foreign host.