Internet Electronic Mail
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1 Internet Electronic Mail Antonio Carzaniga Faculty of Informatics University of Lugano March 9, 2010
2 Outline General concepts Transport protocol: SMTP Basic message format MIME format
3 A Postal Service for the Internet Antonio Carzaniga
4 A Postal Service for the Internet Alice
5 A Postal Service for the Internet Alice Bob
6 A Postal Service for the Internet Alice Alice Green via Buffi 6 Lugano, CH-6900 Bob Bob Blue via Lambertenghi 10A Lugano, CH-6900
7 A Postal Service for the Internet Alice Alice Green via Buffi 6 Lugano, CH-6900 Bob Bob Blue via Lambertenghi 10A Lugano, CH-6900 Bob Blue Re: Your lecture on DNS Alice Green Lugano, April 15, 2005 Dear Bob, I wanted to tell you that last week s lecture on DNS......
8 Features and Goals Antonio Carzaniga
9 Features and Goals Asynchronous communication Alice sends a message when it is convenient to her Bob reads Alice s message whenever he has time to do that
10 Features and Goals Asynchronous communication Alice sends a message when it is convenient to her Bob reads Alice s message whenever he has time to do that One-to-many communication Alice can send a message to Bob and Charlie a mailing list sends messages to several receivers
11 Features and Goals Asynchronous communication Alice sends a message when it is convenient to her Bob reads Alice s message whenever he has time to do that One-to-many communication Alice can send a message to Bob and Charlie a mailing list sends messages to several receivers Multi-media content images and all sorts of attachments as well as normal text
12 Limitations Antonio Carzaniga
13 Limitations No authentication Bob can not know for sure that the message he reads was actually written by Alice messages can be modified messages can be forged
14 Limitations No authentication Bob can not know for sure that the message he reads was actually written by Alice messages can be modified messages can be forged No confidentiality Alice can not make sure that only Bob will read the message the message can be read by others
15 Limitations No authentication Bob can not know for sure that the message he reads was actually written by Alice messages can be modified messages can be forged No confidentiality Alice can not make sure that only Bob will read the message the message can be read by others Little or no delivery guarantees Alice has no idea whether the messages was in fact receiver (much less read!) by Bob messages can be accidentally lost or intentionally blocked no reliable acknowledgement system
16 Architecture and Terminology Antonio Carzaniga
17 Architecture and Terminology Antonio Carzaniga
18 Architecture and Terminology Antonio Carzaniga
19 Architecture and Terminology user agent user agent
20 Architecture and Terminology user agent user agent mail server mail server
21 Architecture and Terminology user agent user agent mail server mail server
22 Architecture and Terminology user agent user agent transport protocol mail server mail server
23 Architecture and Terminology user agent user agent transport protocol mailbox mail server mail server
24 Architecture and Terminology user agent user agent access protocol transport protocol mailbox mail server mail server
25 Architecture and Terminology Antonio Carzaniga
26 Architecture and Terminology User agent allows a user to read, compose, reply to, send, and forward messages and also to save, classify, sort, search,...
27 Architecture and Terminology User agent allows a user to read, compose, reply to, send, and forward messages and also to save, classify, sort, search,... Mail servers accept messages for remote delivery store messages in a local persistent queue deliver messages to a remote (destination) server using the transport protocol accept messages for local delivery save messages in some local persistent mailbox allow user agents to access local mailboxes user agents can retrieve and/or delete messages this is done through an access protocol Antonio Carzaniga
28 SMTP Antonio Carzaniga
29 SMTP Simple Mail Transfer Protocol (defined in RFC 2821)
30 SMTP Simple Mail Transfer Protocol (defined in RFC 2821) Connection-oriented protocol
31 SMTP Simple Mail Transfer Protocol (defined in RFC 2821) Connection-oriented protocol It is simple indeed its simplicity is a reason for its success
32 SMTP Simple Mail Transfer Protocol (defined in RFC 2821) Connection-oriented protocol It is simple indeed its simplicity is a reason for its success It is an old protocol, compared to HTTP; the first RFCs date back to the early 80s it has some archaic charachteristics. E.g., it is restricted to 7-bit characters
33 SMTP Abstract Example usi.ch colorado.edu client server
34 SMTP Abstract Example usi.ch colorado.edu hello client server
35 SMTP Abstract Example usi.ch hello, this is usi.ch colorado.edu hello client server
36 SMTP Abstract Example usi.ch hello, this is usi.ch hello usi.ch, go ahead colorado.edu hello client server
37 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch client server
38 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch okay, I got the sender client server
39 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch okay, I got the sender client the message is for alw@cs.colorado.edu server
40 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch okay, I got the sender client the message is for alw@cs.colorado.edu okay, I got the receiver server
41 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch okay, I got the sender client the message is for alw@cs.colorado.edu okay, I got the receiver here comes the message server
42 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch okay, I got the sender client the message is for alw@cs.colorado.edu okay, I got the receiver here comes the message okay, I m ready to copy server
43 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch okay, I got the sender client the message is for alw@cs.colorado.edu okay, I got the receiver here comes the message okay, I m ready to copy... server
44 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch okay, I got the sender client the message is for alw@cs.colorado.edu okay, I got the receiver here comes the message okay, I m ready to copy... okay, I got the message server
45 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch okay, I got the sender client the message is for alw@cs.colorado.edu okay, I got the receiver here comes the message okay, I m ready to copy... okay, I got the message bye now server
46 SMTP Abstract Example usi.ch colorado.edu hello hello, this is usi.ch hello usi.ch, go ahead I have a message from antonio.carzaniga@usi.ch okay, I got the sender client the message is for alw@cs.colorado.edu okay, I got the receiver here comes the message okay, I m ready to copy... okay, I got the message bye now okay, bye server
47 SMTP Concrete Example usi.ch colorado.edu client server
48 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu client server
49 SMTP Concrete Example usi.ch HELO usi.ch colorado.edu 220 colorado.edu client server
50 SMTP Concrete Example usi.ch HELO usi.ch colorado.edu 220 colorado.edu 250 ok client server
51 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: client server
52 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: 250 ok client server
53 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: 250 ok client RCPT TO: server
54 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: 250 ok client RCPT TO: 250 ok server
55 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: 250 ok client RCPT TO: 250 ok DATA server
56 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: 250 ok client RCPT TO: 250 ok DATA 354 End data with <CR><LF>.<CR><LF> server
57 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: 250 ok client RCPT TO: 250 ok DATA 354 End data with <CR><LF>.<CR><LF>... server
58 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: 250 ok client RCPT TO: 250 ok DATA 354 End data with <CR><LF>.<CR><LF> Message accepted server
59 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: 250 ok client RCPT TO: 250 ok DATA 354 End data with <CR><LF>.<CR><LF> Message accepted QUIT server
60 SMTP Concrete Example usi.ch colorado.edu 220 colorado.edu HELO usi.ch 250 ok MAIL FROM: 250 ok client RCPT TO: 250 ok DATA 354 End data with <CR><LF>.<CR><LF> Message accepted QUIT 221 Bye server
61 SMTP Concrete Example usi.ch HELO usi.ch colorado.edu 220 colorado.edu MAIL FROM: RCPT TO: DATA... QUIT 250 ok 250 ok 250 ok 354 End data with <CR><LF>.<CR><LF> 250 Message accepted 221 Bye envelope
62 SMTP Concrete Example message usi.ch HELO usi.ch colorado.edu 220 colorado.edu MAIL FROM: RCPT TO: DATA... QUIT 250 ok 250 ok 250 ok 354 End data with <CR><LF>.<CR><LF> 250 Message accepted 221 Bye envelope
63 Message Format Antonio Carzaniga
64 Message Format From: Date: Mon, 3 Apr :48: (MDT) To: carzanig@cs.colorado.edu Subject: how to send fake messages Hey Dude, I heard this story about forging messages. Do you know anything about that?...
65 Message Format From: Date: Mon, 3 Apr :48: (MDT) To: carzanig@cs.colorado.edu Subject: how to send fake messages header lines Hey Dude, I heard this story about forging messages. Do you know anything about that?...
66 Message Format From: Date: Mon, 3 Apr :48: (MDT) To: carzanig@cs.colorado.edu Subject: how to send fake messages Hey Dude, I heard this story about forging messages. Do you know anything about that?... header lines empty line
67 Message Format From: Date: Mon, 3 Apr :48: (MDT) To: carzanig@cs.colorado.edu Subject: how to send fake messages Hey Dude, I heard this story about forging messages. Do you know anything about that?... header lines empty line message body
68 Received: Headers SMTP is almost completely oblivious to the content of a message. One exception is the Received: header. Every receiving SMTP server must add a Received: header.
69 Received: Headers SMTP is almost completely oblivious to the content of a message. One exception is the Received: header. Every receiving SMTP server must add a Received: header. Received: from mroe.cs.colorado.edu (mroe-fs.cs.colorado.edu [ ]) by serl.cs.colorado.edu (Postfix) with ESMTP id 9AC463D07 for <carzanig@serl.cs.colorado.edu>; Mon, 3 Apr :39: Received: from max.colorado.edu (max.colorado.edu [ ]) by mroe.cs.colorado.edu (Postfix) with ESMTP id 541C8577A for <carzanig@cs.colorado.edu>; Mon, 3 Apr :43: Received: from cs.colorado.edu (host pool82107.interbusiness.it [ ]) by max.colorado.edu (8.13.6/8.13.6/Hesiod+SSL) with ESMTP id... for <carzanig@cs.colorado.edu>; Mon, 3 Apr :38:
70 Consider the following SMTP client directives Message vs. Envelope
71 Consider the following SMTP client directives Message vs. Envelope 1. MAIL FROM:
72 Consider the following SMTP client directives Message vs. Envelope 1. MAIL FROM: 2. RCPT TO:
73 Consider the following SMTP client directives Message vs. Envelope 1. MAIL FROM: 2. RCPT TO: 3. From: Barak H. Obama To: Deserters Subject: warning... You can run, but you can t hide!
74 Consider the following SMTP client directives Message vs. Envelope 1. MAIL FROM: 2. RCPT TO: 3. From: Barak H. Obama To: Deserters Subject: warning... You can run, but you can t hide! Anything wrong with this exchange?
75 Message vs. Envelope The MAIL FROM: and RCPT TO: SMTP messages specify envelope addresses
76 Message vs. Envelope The MAIL FROM: and RCPT TO: SMTP messages specify envelope addresses From: and To: (and Cc:) headers within a message define message addresses
77 Message vs. Envelope The MAIL FROM: and RCPT TO: SMTP messages specify envelope addresses From: and To: (and Cc:) headers within a message define message addresses There are many situations in which it is perfectly legitimate to have envelope addresses that don t match up with the message addresses
78 Message vs. Envelope The MAIL FROM: and RCPT TO: SMTP messages specify envelope addresses From: and To: (and Cc:) headers within a message define message addresses There are many situations in which it is perfectly legitimate to have envelope addresses that don t match up with the message addresses a message from a mailing list
79 Message vs. Envelope The MAIL FROM: and RCPT TO: SMTP messages specify envelope addresses From: and To: (and Cc:) headers within a message define message addresses There are many situations in which it is perfectly legitimate to have envelope addresses that don t match up with the message addresses a message from a mailing list a blind copy
80 Message vs. Envelope The MAIL FROM: and RCPT TO: SMTP messages specify envelope addresses From: and To: (and Cc:) headers within a message define message addresses There are many situations in which it is perfectly legitimate to have envelope addresses that don t match up with the message addresses a message from a mailing list a blind copy a message to multiple receivers (To: and/or Cc:)
81 Message vs. Envelope The MAIL FROM: and RCPT TO: SMTP messages specify envelope addresses From: and To: (and Cc:) headers within a message define message addresses There are many situations in which it is perfectly legitimate to have envelope addresses that don t match up with the message addresses a message from a mailing list a blind copy a message to multiple receivers (To: and/or Cc:) a forwarded (or re-sent) message
82 Limitations of the Message Format The standard message format has some serious limitations
83 Limitations of the Message Format The standard message format has some serious limitations 7-bit (text) content
84 Limitations of the Message Format The standard message format has some serious limitations 7-bit (text) content only text
85 Limitations of the Message Format The standard message format has some serious limitations 7-bit (text) content only text essentially good exclusively for the English language
86 Limitations of the Message Format The standard message format has some serious limitations 7-bit (text) content only text essentially good exclusively for the English language monolithic data
87 Limitations of the Message Format The standard message format has some serious limitations 7-bit (text) content only text essentially good exclusively for the English language monolithic data The Multipurpose Internet Mail Extensions (MIME) specification (RFC 2045 and RFC 2046) defines extensions of the basic message format that support all of the above
88 MIME Supports multimedia content
89 MIME Supports multimedia content Supports different encodings for text (different from ASCII)
90 MIME Supports multimedia content Supports different encodings for text (different from ASCII) Supports messages consisting of multiple parts E.g., a message containing some text and an image a message containing a binary attachment (e.g., an executable program, a document, etc.) a message containing another message a message containing some Italian text plus another message containing German text a message containing another message, conataining another message,...
91 MIME Headers The primary mechanism used by MIME extensions consists of added MIME headers MIME-Version: 1.0 signals a user agent that this message uses MIME extensions, version 1.0
92 MIME Headers The primary mechanism used by MIME extensions consists of added MIME headers MIME-Version: 1.0 signals a user agent that this message uses MIME extensions, version 1.0 Content-Type:... specifies the content of the message. Valid types include:
93 MIME Headers The primary mechanism used by MIME extensions consists of added MIME headers MIME-Version: 1.0 signals a user agent that this message uses MIME extensions, version 1.0 Content-Type:... specifies the content of the message. Valid types include: text/plain this is a normal ASCII message
94 MIME Headers The primary mechanism used by MIME extensions consists of added MIME headers MIME-Version: 1.0 signals a user agent that this message uses MIME extensions, version 1.0 Content-Type:... specifies the content of the message. Valid types include: text/plain this is a normal ASCII message text/html this is an HTML-formatted message
95 MIME Headers The primary mechanism used by MIME extensions consists of added MIME headers MIME-Version: 1.0 signals a user agent that this message uses MIME extensions, version 1.0 Content-Type:... specifies the content of the message. Valid types include: text/plain this is a normal ASCII message text/html this is an HTML-formatted message image/jpeg this message contains (only) an image file
96 MIME Headers The primary mechanism used by MIME extensions consists of added MIME headers MIME-Version: 1.0 signals a user agent that this message uses MIME extensions, version 1.0 Content-Type:... specifies the content of the message. Valid types include: text/plain this is a normal ASCII message text/html this is an HTML-formatted message image/jpeg this message contains (only) an image file multipart/mixed this message consists of multiple parts
97 MIME Headers The primary mechanism used by MIME extensions consists of added MIME headers MIME-Version: 1.0 signals a user agent that this message uses MIME extensions, version 1.0 Content-Type:... specifies the content of the message. Valid types include: text/plain this is a normal ASCII message text/html this is an HTML-formatted message image/jpeg this message contains (only) an image file multipart/mixed this message consists of multiple parts...
98 MIME Encoding An Internet mail message must contain only 7-bit characters, therefore any content that does not fit the 7-bit (ASCII) character set must be encoded
99 MIME Encoding An Internet mail message must contain only 7-bit characters, therefore any content that does not fit the 7-bit (ASCII) character set must be encoded Content-Transfer-Encoding: defines the encoding for the message content (or a part thereof). Common values are:
100 MIME Encoding An Internet mail message must contain only 7-bit characters, therefore any content that does not fit the 7-bit (ASCII) character set must be encoded Content-Transfer-Encoding: defines the encoding for the message content (or a part thereof). Common values are: base64
101 MIME Encoding An Internet mail message must contain only 7-bit characters, therefore any content that does not fit the 7-bit (ASCII) character set must be encoded Content-Transfer-Encoding: defines the encoding for the message content (or a part thereof). Common values are: base64 Quoted-Printable
102 MIME Multipart Messages Several functionalities of the MIME extensions depend on the ability to carry multiple parts within the same message e.g., to implement attachments Antonio Carzaniga
103 MIME Multipart Messages Several functionalities of the MIME extensions depend on the ability to carry multiple parts within the same message e.g., to implement attachments Content-Type: multipart/mixed; boundary="---_=_nextpart_001_01c539df.6607a632" Antonio Carzaniga
104 MIME Multipart Messages Several functionalities of the MIME extensions depend on the ability to carry multiple parts within the same message e.g., to implement attachments Content-Type: multipart/mixed; boundary="---_=_nextpart_001_01c539df.6607a632" The message consists of a list of parts (e.g., the main message text and an attached document) 1. parts are separated by a boundary line 2. parts are introduced (right after the separator line) by a set of specific headers that define that part 3. the list is terminated by a terminator line Antonio Carzaniga
105 MIME Multipart Messages Several functionalities of the MIME extensions depend on the ability to carry multiple parts within the same message e.g., to implement attachments Content-Type: multipart/mixed; boundary="---_=_nextpart_001_01c539df.6607a632" The message consists of a list of parts (e.g., the main message text and an attached document) 1. parts are separated by a boundary line 2. parts are introduced (right after the separator line) by a set of specific headers that define that part 3. the list is terminated by a terminator line This format is naturally recursive Antonio Carzaniga
106 MIME Example MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="gj7ppttfjl" Content-Transfer-Encoding: 7bit Date: Fri, 15 Apr :24: From: Antonio Carzaniga To: Antonio Carzaniga Subject: Immagini e testo -gj7ppttfjl Content-Type: text/plain; charset=iso Content-Description: message body text Content-Transfer-Encoding: quoted-printable questo =E8 un esempio di un messaggio che usa il formato MIME. -A -gj7ppttfjl Content-Type: image/png Content-Disposition: inline; filename="anto.png" Content-Transfer-Encoding: base64 ivborw0kggoaaaansuheugaaamgaaadicaiaaaaiojnjaaaacxbiwxmaaaixaaacmqf3bqbzaaaa B3RJTUUH1AwdCiYGBdI1HQAAIABJREFUeNqEu1mMJteVJnaWe2P5l/wz88+tcqusnRT3fRMpiqOW Wt0De9w9bczAGAM2jPHAfvOLDcNvfvODAW8YeOClDbeN7ullZnpa6kUttZqiKIoiWawiWaxibVlZ uvtu+a+x3hvo8uoq7ceaxsrtrcaixuldud928t/7l/6hodzazynu8kxnbqldbbab7xa71zxonzt5... Ts2sSh8efiVxP3z3GtgR5/9Wz/8DNJKaidrd/8MAAAAASUVORK5CYII= -gj7ppttfjl-
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