Introduction to Unix

1. Introduction to Unix SMTP – Simple Mail Transfer Protocol

2. Overview • Some SPAM Statistics • Introduction to SMTP and Email • Message Breakdown • Sample Messages • Extensions (MIME) • MTA’s and Mailbox Protocols

3. 1st – What is SMTP? • The de facto standard for e-mail transmissions across the Internet • It is defined in RFC 821 • It is a relatively simple, text-based protocol • Not entirely secure thus vulnerable to SPAM

4. Email Spam Statistics 2006 • Email considered Spam 40% of all email • Daily Spam emails sent 12.4 billion • Daily Spam received per person 6 • Annual Spam received per person 2,200 • Spam cost to all non-corp Internet users $255 million • Spam cost to all U.S. Corporations in 2002 $8.9 billion • States with Anti-Spam Laws 26 • Email address changes due to Spam 16% • Estimated Spam increase by 2007 63% • Annual Spam in 1,000 employee company 2.1 million • Users who reply to Spam email 28% • Users who purchased from Spam email 8% • Corporate email that is considered Spam 15-20% • Wasted corporate time per Spam email 4-5 seconds

9. SMTP • Originated in 1982 (rfc0821, Jon Postel) • Goal: To transfer mail reliably and efficiently

10. SMTP • SMTP clients and servers have two main components • User Agents – Prepares the message, encloses it in an envelope. (Eudora for example) • Mail Transfer Agent(MTA) – Transfers the mail across the internet User Agent Mail Transfer Agents

11. SMTP • SMTP also allows the use of Relays allowing other MTAs to relay the mail

12. What is Mail? • Mail is a text file • Envelope – • sender address • receiver address • other information • Message – • Mail Header – defines the sender, the receiver, the subject of the message, and some other information • Mail Body – Contains the actual information in the message

13. Post Office Mailbox Return-Path: <Jwatson@cis.udel.edu>Delivered-To: jwatson@cis.udel.edu Received: by mail.eecis.udel.edu (Postfix, from userid 62) id 17FBD328DE; Wed, 5 Nov 2003 11:27:02 Received: from mail.acad.ece.udel.edu (devil-rays.acad.ece.udel.edu [128.4.60.10]) by mail.eecis.udel.edu (Postfix) with ESMTP id 5F41832893 for <Jwatson@cis.udel.edu>; Wed, 5 Nov 2003 11:27:01 Received: by mail.acad.ece.udel.edu (Postfix, from userid 62)id 47509456C; Wed, 5 Nov 2003 11:27:01 Received: from stimpy.eecis.udel.edu(stimpy.eecis.udel.edu [128.4.40.17])by mail.acad.ece.udel.edu (Postfix) with SMTP id 7C2943D79 for <Jwatson@cis.udel.edu>; Wed, 5 Nov 2003 11:26:34 Message-Id: <20031105162634.7C2943D79@mail.acad.ece.udel.edu>Date: Wed, 5 Nov 2003 11:26:34 From: Jwatson@cis.udel.eduTo: undisclosed-recipients: ;MIME-Version: 1.0This is a test. Post office and mail route Receivers Mailbox

14. How SMTP works • The Essentials • How about a Demo?

15. 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!

16. Status Codes • 211 System status, or system help reply . • 214 Help message. • 220 <domain> Service ready. • 221 <domain> Service closing transmission channel. • 250 Requested mail action okay, completed. • 251 User not local; will forward to <forward-path>. • 354 Start mail input; end with <CRLF>.<CRLF>. • 421 <domain> Service not available, closing transmission channel. [This may be a reply to any command if the service knows it must shut down]. • 450 Requested mail action not taken: mailbox unavailable. • 451 Requested action aborted: local error in processing • 452 Requested action not taken: insufficient system storage.

17. Status Codes • 500 Syntax error, command unrecognized. [This may include errors such as command line too long] • 501 Syntax error in parameters or arguments. • 502 Command not implemented. • 503 Bad sequence of commands. • 504 Command parameter not implemented. • 550 Requested action not taken: mailbox unavailable. • 551 User not local; please try <forward-path>. • 552 Requested mail action aborted: exceeded storage allocation. • 553 Requested action not taken: mailbox name not allowed. [E.g., mailbox syntax incorrect] • 554 Transaction failed.

18. Connection Establishment TCP Connection Establishment

19. Message Progress

20. Connection Termination TCP Connection Termination

21. Problems with SMTP • No inherent security • Authentication • Encryption • Only uses NVT (Network Virtual Terminal) 7-bit ASCII format

22. E-mails can be forged….. HELO mail.rose.edu MAIL FROM: carberry@rose.edu RCPT TO: wrichards@rose.edu DATA From: Dr. Art Zenner To: Professor Richards Subject: CIT 2243 Professor Richards, By department decree all students in your “Introduction to Unix” class are hereby to be granted automatic A’s. Thank you, Dr. Art Zenner . QUIT

23. Extensions to SMTP • MIME – Multipurpose Internet Mail Extensions • Transforms non-ASCII data to NVT (Network Virtual Terminal) ASCII data • Text • Application • Image • Audio • Video

24. MIME and Base64 Encoding • If the internet is the information highway, then the path for email is a narrow tunnel • Only very small vehicles can pass trough • Then how do you send a big truck through a small ravine? • You have to break it down to smaller pieces and transport the pieces through the ravine, and reassemble the truck

25. MIME and Base64 Encoding • The same happens when you send a file attachment via email. • This is known as encoding • the binary data (256 bits) is transformed to ASCII text (128 bits • allowing it to fit through the tunnel • On the recipient's end, the data is decoded and the original file is rebuilt.

26. Mail Transfer Agents (MTA) • MTAs do the actual mail transfers • MTAs are not meant to be directly accessed by users. • Other MTA’s are: • Postfix • Qmail • MS Exchange • CC:Mail • Lotus Notes • ….etc.

27. Problems with simple SMTP • The first one relates to message length. Some older implementations cannot handle messages exceeding 64KB. • Another problem relates to timeouts. If the Client and server have different timeouts, one of them may give up while the other is still busy, unexpectedly terminating the connection. • Infinite mail storms can be triggered. For example, If host 1 holds mailing list A and host 2 holds mailing list B and each list contains an entry for the other one, then a message sent to either list could generate a never ending amount of email traffic unless somebody checks for it.

28. ESMTP (RFC 2821) • To get around the problems with simple SMTP, extended SMTP has been defined in RFC 2821. • Clients wanting to use it should send an EHLO message instead of HELO initially. If this is rejected, then the server is a regular SMTP server, and the client should proceed in the usual way. If the EHLO is accepted, then new commands and parameters are allowed.

29. Next week….Sendmail