How Internet Works

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1 How Internet Works Everything you never wanted to know about but were afraid to ask... Dave O'Neill

So, you use email...... but do you know how it works?

2 So, you use but do you know how it works? If you don't care how it works, leave now, or you'll be bored silly.

3 How many of you... know what SMTP is? know what an MX is? can understand your headers? Know what R$* [ $+ ] : $- : $*> $* $#$3 $@ $4 $: $1 [$2] > $5 means?

4 The Whole Mess What a typical system looks like

5 The Whole Mess

6 Mail User Agents Reading mail Managing folders Composing mail Examples: Mutt Thunderbird Outlook

7 Mail Transfer Agents Transfer mail to another user or system Examples: Sendmail Postfix Qmail Exchange

8 Mail Delivery Agents Deliver mail to an individual mailbox Examples: procmail maildrop mail.local

9 Mail Retrieval Agents Retrieve mail on a remote machine Pretty much the opposite of an MTA Examples: fetchmail getmail

10 Okay, but... Much of the time, those four theoretical components don't exist separately Even when they do, the lines get a bit blurry Mail software usually incorporates two (or more) of MUA, MTA, MRA, MDA

11 Mozilla Thunderbird Is a MUA Also acts as an MRA, reading mail via IMAP and POP3 Also acts as a minimal MTA, sending to your SMTP server Most GUI mail clients have MRA functionality

12 Sendmail Is an MTA But it comes bundled with an MDA Most Sendmail people use a different MDA, like Procmail

13 MS Exchange Exchange provides an MTA, MRA, MDA, and MUA Many people think it does none of these well

14 The MUA Reads mail delivered by an MDA, or accessed via an MRA Sends via an MTA If it reads and writes , it's an MUA.

15 The MTA An MTA takes an incoming mail message and determines where and how to transfer it Internet is sent via SMTP over port 25

16 What's port 25? TCP port 25 is where SMTP is usually sent and received If you want to receive on the internet, you need to have a server listening on port 25 If you want to direct mail for a particular domain to a particular server, you publish an MX record in your DNS

17 What's an MX record? Mail exchange record Tells senders what server, or servers, accept mail for a domain Listed in order of priority lower number is preferred host Multiple MX hosts can be good for reliability

18 MX Information $ host t mx dmo.ca dmo.ca mail is handled by 10 mail.dmo.ca. dmo.ca mail is handled by 15 home.dmo.ca.

19 What's SMTP It's Simple Mail Transfer Protocol It's RFC 2821 It's not entirely simple anymore Here's a simple example anyway...

20 SMTP In Action telnet mail.dmo.ca 25 Trying Connected to colo.dmo.ca. Escape character is '^]'. 220 colo.dmo.ca ESMTP Sendmail (No UCE/UBE) logging access from: home.dmo.ca(ok) home.dmo.ca [ ] HELO dmo.ca 250 colo.dmo.ca Hello home.dmo.ca [ ], pleased to meet you MAIL Sender ok RCPT Recipient ok DATA 354 Enter mail, end with "." on a line by itself Subject: This is a minimal message From: Dave O'Neill <dmo@dmo.ca> To: Dave O'Neill <dmo@dmo.ca> Date: Sun, 05 Dec :57: Here's the body text ib61u9ds Message accepted for delivery

21 Other Transfer Protocols UUCP X.400 MAPI Evil Hacks Mail::Queue::DB

22 The MDA The MTA gives the MDA a message The MDA delivers it to a user mailbox And maybe it does other things...

23 Procmail An MDA that does other things Sort from mailing lists Client-side spam filtering Call other programs

24 Fun With Procmail MAILDIR=$HOME/mail LOGFILE=$HOME/log/procmail NEWDATE="`/bin/date +%Y %m`" THREADKILLER=/home/dmo/bin/killthread.pl ## Backup all messages first :0 c: archive/backup $NEWDATE # First, killfile an individual or two: :0 * ^From:.*annoying@somedomain.ca $THREADKILLER kill :0 * ^List Id:.*oclug.lists.oclug.on.ca { :0 hw $THREADKILLER check :0: oclug/list/$newdate }

25 Sieve RFC 3028 A filtering language like procmail, but not quite as confusing, destructive, or powerful Designed for GUI filter editors require "fileinto"; if header :contains "From" "annoying@somedomain.ca" { discard; } elsif header :contains "List Id:" "oclug.lists.oclug.on.ca" { fileinto "oclug/list"; }

26 The MRA Retrieves mail from another server Gives it to an MTA, MDA, or MUA Common protocols are POP3 and IMAP

27 POP3 Post Office Protocol, version 3 Defined in RFC 1939 Limited to one remote folder per account

28 IMAP Internet Mail Access Protocol Latest version defined in RFC 3501 Allows multiple remote folders, shared folders, updating of flags on messages, etc More feature-ful than POP3. Also more resource-intensive

29 Reading Headers Know where your mail comes from Know where your spam comes from Figure out which address you used to subscribe to that list

30 Let's see that message Received: from localhost (bowmore [ ]) by bowmore.i.dmo.ca ( Beta0/ Beta0/Debian 1) with ESMTP id i79jfkjn for Mon, 9 Aug :41: Received: from mail.dmo.ca by localhost with IMAP (fetchmail 6.2.4) for dmo@localhost (single drop); Mon, 09 Aug :41: (EDT) Received: from tux.oclug.on.ca (tux.oclug.on.ca [ ]) by mail.dmo.ca ( / /Debian 3) with ESMTP id i79jtlse for <dmo@dmo.ca>; Mon, 9 Aug :29: Received: from localhost (localhost [ ]) by tux.oclug.on.ca (Postfix) with ESMTP id 25D7D47837 for <oclug@lists.oclug.on.ca>; Mon, 9 Aug :29: (EDT) Received: from colo.roaringpenguin.com (colo.roaringpenguin.com [nnn.nnn.nnn.nnn]) by tux.oclug.on.ca (Postfix) with ESMTP id 5B for <oclug@lists.oclug.on.ca>; Mon, 9 Aug :29: (EDT)

31 Headers, continued Received: from (hydrogen.roaringpenguin.com [nnn.nnn.nnn.nnn]) by colo.roaringpenguin.com (8.13.1/8.13.0) with ESMTP id i79jts2i for Mon, 9 Aug :29: Received: from oxygen.roaringpenguin.com (oxygen.roaringpenguin.com [ ]) by hydrogen.roaringpenguin.com ( / ) with ESMTP id i79jtscl for <oclug@lists.oclug.on.ca>; Mon, 9 Aug :29:

Email Really Sucks No security Designed for plain

32 Really Sucks No security Designed for plain ASCII text Unicast and everyone used to be so trustworthy...

Email Security SMTP used to have no notion of authentication SMTP, IMAP,

33 Security SMTP used to have no notion of authentication SMTP, IMAP, and POP3 protocols are all cleartext Mail relays can see/modify anything

34 SSL SMTP, IMAP, and POP3 can all work over SSL, assuming your client and server support it At least then nobody in the middle can snoop your data or your passwords If you're on a public network, you should probably be using this

35 SMTP AUTH SMTP servers used to let anyone send to anyone Now, most relays only allow you to send to addresses they handle mail for send from addresses within their networks What if you're remote, and need to send mail? SMTP has an AUTH extension now

36 Encryption Keep your data secret from all but the intended recipient Prove authenticity of sender's identity Beyond the scope of this talk

37 MIME Multipurpose Internet Mail Extensions Lets you send arbitrary binary data via Simple idea, complicated implementation

38 Aliases and Lists is mainly a one-to-one medium Discussion groups are a hack added on through aliases and listservs An alias is an MTA feature to resend inbound mail to one or more addresses A listserv is a program that expands upon this to provide other features

Spam / UCE Unwanted email advertisements Filtering is now a

39 Spam / UCE Unwanted advertisements Filtering is now a necessity Filtering is a big enough topic to require its own talk

40 Client-Side Filtering Typically performed by the MUA or MDA Hides or deletes unwanted messages You've already wasted bandwidth, but at least you don't waste your time viewing them

41 Server-Side Filtering Typically performed by MTA or MDA Lets you reject messages before the end user has to download it May even be able to reject before SMTP conversation is over (RBLs, inline filtering milter plugins, etc)

42 Discussion and Arguments

CIT 470: Advanced Network and System Administration. Topics. Mail Policies.

CIT 470: Advanced Network and System Administration E-mail CIT 470: Advanced Network and System Administration Slide #1 Topics 1. Mail Policies 2. Anatomy of a Mail Message 3. Components of an E-mail System