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Electronic mail – protocol evolution

Electronic mail – protocol evolution. E-mail standards. Three major components: user agents mail servers simple mail transfer protocol: SMTP , TCP port 25 User Agent a.k.a. “mail reader” composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger

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Electronic mail – protocol evolution

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  1. Electronic mail – protocol evolution

  2. E-mail standards

  3. Three major components: user agents mail servers simple mail transfer protocol: SMTP, TCP port 25 User Agent a.k.a. “mail reader” composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger outgoing, incoming messages stored on server user agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server outgoing message queue user mailbox Electronic Mail

  4. SMTP (RFC 821)

  5. Sample SMTP interaction: TCP port 25 S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu> S: 250 bob@hamburger.edu ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

  6. Mail Standard RFC822 • Published in 1982 • Lines no longer than 1000 char • Message body - plain US-ASCII text • Message header lines - plain US-ASCII text • Limit on message length

  7. RFC 822 format

  8. RFC 822 restrictions • no multiple objects in a single message • no multi-part message bodies • no non-textual bodies • no X.400 messages can be gatewayd • no multifont messages

  9. ASCII times are over! Now we want: • National language support • Possibility to send • pictures • audiofiles • other applications • video files • multimedia applications

  10. MIME - Multipurpose Internet Mail Extension RFC 2045-2048 obsolete RFC 1521, 1522,1590 • RFC 2045 Format of Internet Message Bodies • RFC 2046 Media Types • RFC 2047 Message Header Extension for Non-ASCII Text • RFC 2048 Registration Procedures To solve RFC822 restrictions without serious incompatibilities with it

  11. MIME

  12. MIME types and sub-types

  13. base64 encoding

  14. SMTP: protocol for exchanging email msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: differentfrom SMTP commands! body the “message”, 7-bit ASCII characters only Mail message format header blank line body

  15. MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data Message format: multimedia extensions MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data

  16. Multipart Type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=98766789 --98766789 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain Dear Bob, Please find a picture of a crepe. --98766789 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data --98766789--

  17. Multipart Type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=StartOfNextPart --StartOfNextPart Dear Bob, Please find a picture of a crepe. --StartOfNextPart Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data --StartOfNextPart Do you want the reciple?

  18. SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: Hotmail , Yahoo! Mail, etc. user agent user agent sender’s mail server SMTP Mail access protocols SMTP access protocol receiver’s mail server

  19. Try SMTP interaction for yourself: • telnet servername 25 • see 220 reply from server • enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send email without using email client (reader)

  20. Post Office Protocol (POP)

  21. authorization phase client commands: user: declare username pass: password server responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on POP3 protocol C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off

  22. IMAP

  23. Web Mail http://www.squirrelmail.org

  24. (Adjusted) Mail Architecture Off-Campus E-mail smtp smtp_internal Anti-virus Content Filter smtp_notify smtp_externel Director Antispam root mail parrot petrel alpha mx=10 “smtp_external” mx=20 “smtp_backup” mx=30 “smtp.ecs.” | “smtp” admsrvcs

  25. Mail from El Presidente Return-Path: <elvis@graceland.org> Delivered-To: steve@blighty.com Received: from fake-name.example.com (unknown [64.71.176.18]) by gp.word-to-the-wise.com (Postfix) with SMTP id 3DD7790000D for <steve@blighty.com>; Tue, 2 Dec 2003 12:55:36 -0800 (PST) From: El Presidente <president@whitehouse.com> To: Steve Atkins <steve@blighty.com> Subject: Fake Mail Message-Id: <20031202205536.3DD779@gp.word-to-the-wise.com> Date: Tue, 2 Dec 2003 12:55:36 -0800 (PST) Status: RO Content-Length: 15 Lines: 1 Some body text

  26. Sending spam (relay hijacking) Third-party mailserver (10.11.12.13) SMTP Spammer (64.71.176.18) SMTP POP3 Recipients MX

  27. Sending spam (relay hijacking) Received: from openrelay.com (mail.openrelay.com [10.11.12.13]) by gp.word-to-the-wise.com (Postfix) with SMTP id 3DD7790000D for <steve@blighty.com>; Tue, 2 Dec 2003 12:55:36 -0800 (PST) Received: from fake-spammer-helo (spammer.net [64.71.176.18]) by openrelay.com (Postfix) with SMTP id 3DD7790000D for <steve@blighty.com>; Tue, 2 Dec 2003 12:55:36 -0800 (PST) You can see the relay, and the original spammer

  28. Sending spam (direct to MX) SMTP POP3 Spammer (64.71.176.18) Recipients MX

  29. Sending spam (direct to MX) Received: from fake-spammer-helo (spammer.net [64.71.176.18]) by gp.word-to-the-wise.com (Postfix) with SMTP id 3DD7790000D for <steve@blighty.com>; Tue, 2 Dec 2003 12:55:36 -0800 (PST) You can see the spammer

  30. Sending spam (proxy hijacking) Open proxy (192.168.1.1) HTTP Spammer (64.71.176.18) SMTP POP3 Recipients MX

  31. Sending spam (proxy hijacking) Received: from fake-spammer-helo (open-proxy.net [192.168.1.1]) by gp.word-to-the-wise.com (Postfix) with SMTP id 3DD7790000D for <steve@blighty.com>; Tue, 2 Dec 2003 12:55:36 -0800 (PST) You can see the open proxy

  32. Sending spam (trojans) Infected computer (192.168.1.1) IRC? Spammer (64.71.176.18) SMTP POP3 Recipients MX

  33. ~ Sender ID’s authorization proof Mapping email to postal mail- the envelope Mail From /Envelope From / Return Path Recipient To

  34. Email Authentication Proposals • Client SMTP Validation (CSV): • http://www.ietf.org/internet-drafts/draft-ietf-marid-csv-intro-01.txt • Bounce Address Tag Validation (BATV): • http://www.ietf.org/internet-drafts/draft-levine-mass-batv-00.txt • DomainKeys: • http://antispam.yahoo.com/domainkeys • Identified Internet Mail (IIM): • http://www.ietf.org/internet-drafts/draft-fenton-identified-mail-01.txt • Sender ID (SPF + PRA): • http://www.ietf.org/internet-drafts/draft-ietf-marid-pra-00.txt • http://www.ietf.org/internet-drafts/draft-ietf-marid-core-03.txt

  35. SPF: Sender Policy Framework Domains use public records (DNS) to direct requests for different services (web, email, etc.) to the machines that perform those services.All domains already publish email (MX) records to tell the world what machines receive mail for the domain. SPF works by domains publishing "reverse MX" records to tell the world what machines send mail from the domain. When receiving a message from a domain, the recipient can check those records to make sure mail is coming from where it should be coming from. With SPF, those "reverse MX" records are easy to publish: one line in DNS is all it takes.

  36. Client SMTP Validation (CSV): CSV considers two questions at the start of each SMTP session: o Does a domain's management authorize this MTA to be sending email? o Do independent accreditation services consider that domain's policies and practices sufficient for controlling email abuse?

  37. Identified Internet Mail (IIM): Identified Internet Mail (IIM) provides a means by which cryptographic signatures can be applied to email messages to demonstrate that the sender of the message was authorized to use a given email address. Message recipients can verify the signature and consult the sender's domain to determine whether the key that was used to sign the message was authorized by that domain for that address. This confirms that the message was sent by an party authorized to use the sender's email address.

  38. DomainKeys Under DomainKeys, a domain owner generates one or more private/public key-pairs that will be used to sign messages originating from that domain. The domain owner places the public-key in his domain namespace (i.e., in a DNS record associated with that domain), and makes the private-key available to the outbound email system. When an email is submitted by an authorized user of that domain, the email system uses the private-key to digitally sign the email associated with the sending domain. The signature is added as a "DomainKey-Signature:" header to theemail, and the message is transferred to its recipients in the usualway. When a message is received with a DomainKey signature header, the receiving system can verify the signature as follows: 1. Extract the signature and claimed sending domain from the email. 2. Fetch the public-key from the claimed sending domain namespace. 3. Use public-key to determine whether the signature of the email has been generated with the corresponding private-key, and thus whether the email was sent with the authority of the claimed sending domain. In the event that an email arrives without a signature or when the signature verification fails, the receiving system retrieves the policy of the claimed sending domain to ascertain the preferred disposition of such email. $ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM -----BEGIN PUBLIC KEY----- MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6l MIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7E XzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB -----END PUBLIC KEY----- This public-key data is placed in the DNS: _domainkey IN TXT "t=y; o=-; n=notes; r=emailAddress"

  39. DomainKeys Example DNS TXT query for: brisbane._domainkey.football.example.com DomainKey-Status: good DomainKey-Signature: a=rsa-sha1; s=brisbane; d=football.example.com; c=simple; q=dns; b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZ VoG4ZHRNiYzR; Received: from dsl-10.2.3.4.football.example.com [10.2.3.4] by submitserver.football.example.com with SUBMISSION; Fri, 11 Jul 2003 21:01:54 -0700 (PDT) From: "Joe SixPack" <joe@football.example.com> To: "Suzie Q" <suzie@shopping.example.net> Subject: Is dinner ready? Date: Fri, 11 Jul 2003 21:00:37 -0700 (PDT) Message-ID: <20030712040037.46341.5F8J@football.example.com> Hi. We lost the game. Are you hungry yet? Joe.

  40. IP based (Sender ID) Find outbound IPs, publish in DNS Receiver verifies mail from authorized IP Sender is not authenticated -- Last IP to touch mail is Forwarders & mail lists must change before technology can be fully used Digital Signature (DomainKeys) Generate public/private keys, publish public-key in DNS Sign mail with private-key Receiver verifies signature Original Sender is authenticated In transit modifications may invalidate signature Two authentication strategies compared 19

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