Simple Mail Transfer Protocol (SMTP)

1. Simple Mail Transfer Protocol(SMTP) Kanika Thapar CISC 856 TCP/IP and Upper Layer Protocols 11/8/2007 (Some slides provided by Ezra Kissel, some figures taken from Forouzan’s book)

2. Overview • Introduction • Sendmail • How SMTP works? • SMTP data transfer • Examples • Limitations & extensions • Retrieving mail (Mailbox protocols)

3. Introduction

4. Collects mail and delivers to user agent Transfers mails from one host to another Introduction… Transfers mails from one host to another Collects mail and delivers to user agent Composes, reads, replies to ,forwards and handles mailboxes

5. Message transfer… SMTP is a push protocol

6. Sendmail • Mail Transfer Agent [MTA] : is a computer program or software agent that transfers electronic mail messages from one computer to another. • Sendmail is a MTA • Supports several mail transfers including SMTP • Pro’s • Can perform header rewriting, mail routing • Extensive support available • Con’s • Not secure • Code is bulky [compared to other MTA’s such as qmail]

7. SMTP server can also assume the role of a “relay” SMTP mail gateways are used to transport mail prepared by a protocol other than SMTP Relays and Gateways Non-SMTP supported SMTP supported

8. How SMTP works? Command format: Keyword: argument(s) Response format: 3-digit status code [textual information] Link Layer PCI Link Layer PCI IP-PCI IP-PCI TCP-PCI TCP-PCI TCP-PCI SMTP command/response SMTP command/response SMTP command/response

9. SMTP A-PDU’s The Basics The Extras

10. How SMTP works : Status Codes The Server responds with a 3 digit code that may be followed by text info • 2## - Success • 3## - Command can be accepted with more information • 4## - Command was rejected, but error condition is temporary • 5## - Command rejected, Bad User!

11. Traditional mail vs email E-mail envelope and body Traditional mail envelope and body

12. Connection Establishment SYN SYN+ACK Ephemeral port Ephemeral port Port 25 Port 25 SYN

13. Ephemeral port Port 25

14. Connection Termination Ephemeral port Port 25 FIN ACK RESET

15. Simulating MTA client using telnet % telnet mail.adelphia.net 25Trying 68.168.78.100...Connected to mail.adelphia.net (68.168.78.100). ================== ConnectionEstablishment================ 220 mta13.adelphia.net SMTP server ready Thur, 8 Nov 2007 ..HELO mail.adelphia.net250 mta13.adelphia.net =====================Envelope========================== MAIL FROM: forouzanb@adelphia.net250 Sender <forouzanb@adelphia.net> OkRCPT TO: thapar@udel.edu 250 Recipient <thapar@udel.edu> Ok

16. Simulating MTA client using telnet =================== Header and Body =================== DATA354 Ok Send data ending with <CRLF>.<CRLF>From: ForouzanTO: ThaparThis is a test messageto show SMTP in action.. 250 Message received: adelphia.net@mail.adelphia.net ============= Connection Termination==================== QUIT 221 mta13.adelphia.net SMTP server closing connection Connection closed by foreign host.

17. Limitations in SMTP • Only uses NVT 7 bit ASCII format • How to represent other data types? • Susceptible to misuse (Spamming, faking sender address)

18. Solution: SMTP extensions • MIME – Multipurpose Internet Mail Extensions Transforms non-ASCII data to NVT (Network Virtual Terminal) ASCII data

19. MIME headers

20. MIME headers (cont’d) • Content-Type – Type of data used in the Body • Text: plain, unformatted text; HTML • Multipart: Body contains different data types • Message: Body contains a whole, part, or pointer to a message • Image: Message contains a static image (JPEG, GIF) • Video: Message contains an animated image (MPEG) • Audio: Message contains a basic sound sample (8kHz) • Application: Message is of data type not previously defined • Content-Transfer-Encoding – How to encode the message • 7 bit – no encoding needed • 8 bit – Non-ASCII, short lines • Binary – Non-ASCII, unlimited length lines • Base64 – 6 bit blocks encoded into 8-bit ASCII • Quoted-printable – send non-ASCII characters as 3 ASCII characters

21. Base64 Encoding • Divides binary data into 24 bit blocks • Each block is then divided into 6 bit chunks • Each 6-bit section is interpreted as one character 11001100 10000001 00111001 110011 001000 000100 111001 (51) (8) (4) (57) (z) (I) (E) (5) 01111010 01001001 01000101 00110101

22. Base64 Encoding table

23. Multipart, Encoded MIME Message From: Kanika Thapar <thapar@udel.edu> To: thapar@udel.edu Subject: attachment test MIME-Version: 1.0 Content-Type: MULTIPART/MIXED; boundary="MIMEStream=_0+92061_793033260215529_597673089" --MIMEStream=_0+92061_793033260215529_597673089 Content-Type: TEXT/PLAIN; charset="US-ASCII"; format=flowed There is an image attached... --MIMEStream=_0+92061_793033260215529_597673089 Content-Type: IMAGE/jpeg; name="test.jpg" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="test.jpg" /9j/4AAQSkZJRgABAgEASABIAAD/4QNxRXhpZgAATU0AKgAAAAgABwESAAMA AAABAAEAAAEaAAUAAAABAAAAYgEbAAUAAAABAAAAagEoAAMAAAABAAIAAAEx … osv/0I5nPvr7sVdirsVf/9GO5z76+6hxQ1il2Kv/0o5nPvr7eKtYq7FX/9k= --MIMEStream=_0+92061_793033260215529_597673089--

24. Mail Access Protocols POP3 & IMAP4 POP3 IMAP4 SMTP SMTP

25. POP3

26. Post Office Protocol v3 • Allows the user to obtain a list of their Emails • Users can retrieve their emails • Users can either delete or keep the email on their system • Minimizes server resources

27. Internet Mail Access Protocol v4 • User can check the email header before downloading • Can search the email for a specific string of characters before downloading • User can download parts of an email • User can create, delete, or rename mailboxes on a server

28. Quoted-Printable Encoding • Used when the data has a small non-ASCII portion • Non-ASCII characters are sent as 3 characters • First is ‘=‘, second and third are the hex representation of the byte • =##, ## is the hex representation of the byte 01001100 10011101 00111001 (=) (9) (D) 00111101 00111001 01000100

29. ASCII table