The Illustrated Network- P59 pps

USING POP3 TO ACCESS EMAIL The original host-to-host SMTP did not allow for attachments, limited messages to 1000 bytes, was a purely connection-oriented application, and never imagined

and even as executable code (much to the chagrin of network administrators) Base64

encoding converts a binary data stream to a sequence of “text” characters This usually results in the size of the binary fi le growing by about 33% in terms of bytes This is because 6 bits can indicate the numbers 0 through 63 But bytes are 8 bits, of course, at least where the Internet and TCP/IP are concerned.

An Example of a MIME Message

Consider a writer delivering a short story to an editor as an email attachment (been there, done that) What would the MIME headers that form the overall body of the email message look like? Well, they would resemble the following:

Content-Type: multipart/mixed;

boundary = " - = _NextPart_000_027HB582.0E7E0F6"

This is a message in MIME format

- = _NextPart 000_027HB582.0E7E0F6

Content-Type: text/plain

Table 21.3 MIME Content Types and Subtypes

Type Subtypes

text plain, richtext, tab-separated-values, html, sgml

image jpeg, gif, ief, tiff, g3fax, png

video mpeg, quicktime, vnd.vivo

audio basic, 32kadpcm, vnd.vivo

application octet-stream, postscript, rtf, pdf, zip, macwriteii, msword, remote-printing, EDI-X12,

EDIFACT, dec-dx, dca-rft, activemessage, applefi le, mac-binhex40, news-message-id, mews-transmission, wordperfect5.1, mathematica, pgp-encrypted, pgp-signature, pgp-keys, andrew-inset, slate, set-payment, set-registration, sgml, wita, lotus-wordpro, lotus-1-2-3, lotus-organizer, ms-excel, powerbuilder-6

multipart mixed, alternative, digest, parallel, appledouble, header-set, form-data, report,

voice-message, signed, encrypted

message rfc822, partial, external-body, news, http, delivery-status

Table 21.4 MIME Encoding Methods and Meanings

Method Meaning

7bit Ordinary ASCII as used in the United States

quoted-printable Adds a few special characters and coding to ASCII text

base64 Content is mapped into a “text” package (very common)

8bit Similar to 7bit, but can include 8-bit characters

binary True binary data

x-(name) Experimental encodings must have a name starting with “x”

Please take a look at the attached short story Thanks.

W

- = _NextPart_000_027HB582.0E7E0F6

Content-Type: application/msword;

name = "new story.doc"

Content-Transfer-Encoding: base64

Content-Disposition: attachment;

filename = "new story.doc"

(Lots of nonsense characters form the base64 table.)

- = _NextPart_000_027HB582.0E7E0F6

The lines in bold are the MIME headers.

USING POP3 TO ACCESS EMAIL

The original host-to-host SMTP did not allow for attachments, limited messages to

1000 bytes, was a purely connection-oriented application, and never imagined a world

of personal computers and intermittent email checking STMP was built for immediate email delivery to a specifi c host, sort of what we think of as instant messaging (IM) today Email today is often delivered to mailboxes on mail servers, not directly to the end user, that is, users who might only have dial-up Internet access.

FIGURE 21.8

A POP3 capture, highlighting how the email listing is sent to the user

Client–Server POP3 Connection (TCP 3-way handshake and close omitted)

⫹OK POP3 Inxserver (etc.)

⫹OK User name accepted, password please

⫹OK Mailbox open, 1 message

USER admin1

PASS admin11

DELE 1

QUIT

STAT

(SERVER) port 110

Inxserver

(CLIENT)

port 2447

Inxclient

⫹OK 1 1108

⫹OK Mailbox scan listing follows\r\n1 1108 (etc.)

⫹OK 1108 octets

⫹OK Message deleted

⫹OK Sayonara

LIST

RETR 1

FIGURE 21.9

A POP3 connection used to fetch email, showing a more schematic view than the capture

These intermittent Internet users log in and access their mailbox with POP3 (com-monly just called POP) POP3 does not send email: SMTP does that But POP3 retrieves the email, and the IMAP4 protocol maintains and controls access to the mailbox accounts.

POP3 uses TCP port 110, and users are authenticated by userID and password POP3 then places a lock on the mailbox to avoid access confl icts The POP3 server then enters transaction mode for user access to messages POP3 features include the abil-ity to view a list of email messages and their sizes and to selectively retrieve or delete messages, but many implementations simply dump all waiting mail to the client POP3 servers can be the same device as the SMTP mail server, but this is not a requirement Let’s add POP3 to our network We used the BSD hosts before, so let’s make lnxserver

(10.10.11.66) into our email server for the network We can then compose a fairly long (1108 bytes) message and send it to user admin1 Figure 21.8 shows the sequence of packets used to retrieve the message from host lnxclient (10.10.12.166).

POP3 employs a characteristic +OK and not a code when responding normally to a client The series of packets shown in Figure 21.8 is boiled down to its POP3 essentials

in Figure 21.9.

Note that the retrieval of the message (RETR) by the client and its deletion from the server (DELE) are separate steps You don’t have to delete email as you read it, of course The +OK Sayonara is also part of the POP3 protocol implementation.

HEADERS AND EMAIL

We’ve mentioned email headers already and supplied some details about MIME headers (header extensions) Email has its own proper set of headers as well, and an Internet email message is little more than a sequence of headers and their values, one after the other, from the start of the email message to the end Table 21.5 outlines the basic email header fi eld names and groups established by RFC 822.

Now we have everything in place to examine the headers created when sending a short email message through our email server (lnxserver) from a client host to another user We’ll use the admin account on lnxclient to send a message to the admin user on

Table 21.5 RFC 822 Email Header Fields and Characteristics

Field Group Field Name Appearance

Occurrences per Message Comment

Destination

Address Field

To: Usually present 1 Primary recipient list

Identifi cation

Fields

Message-ID: Usually present 1 Unique code applied when

sent In-Reply-To: Optional, normal

for replies

1 Provides method to

coordi-nate responses References: Optional 1 Other documents or

mes-sage IDs Informational

Fields

Subject: Usually present 1 Topic of the message Comments: Optional Unlimited Describe message Keywords: Optional Unlimited Useful search item Origination

Date

mail Originator

Fields

“originator”

“originator”

Reply-To: Optional 1 If absent, reply goes to

“from”

lnxserver (these are not necessarily the same users: they just share a mailbox name) Then we’ll fetch the message from the email server mailbox using the admin account, showing that we can fetch our email almost anywhere, even from the sending host.

We can use the same basic mail program as we did on the BSD hosts This time, we’ll use the –s fl ag to create a subject for the message The text is simple, and we end our message with a single dot as before.

[admin@lnxclient admin]$ mail –s "Here is another example"

admin@lnxserver.booklab.englab.juniper.net

This is text…

Cc: (enter)

Now we’ll use fetchmail to “fetch” the mail message with POP3 from the email server (lnxserver) and bring it back to lnxclient Note that when we run the program and have email we get a version of the familiar “you’ve got mail” prompt.

[admin@lnxclient admin]$ fetchmail

Enter password for admin@lnxserver.booklab.englab.juniper.net: (not shown)

You have new mail in /var/spool/mail/admin

Usually, our complete email application would display the information and the mes-sage But there’s nothing magical about that We can do the same with the command prompt, listing the mailbox content and displaying the email message with normal Unix commands.

[admin@lnxclient admin]$ ls –l /var/spool/mail/admin

-rw - 1 admin mail 3122 Jan 17 16:42 /var/spool/mail/admin

Table 21.5 (continued)

Field Group Field Name Appearance

Occurrences per Message Comment

Resent Fields Resent-Date:

Resent-From:

Resent-Sender:

Resent-To:

Resent-Cc:

Resent-Bcc:

Resent-Message-ID:

Each time message is resent, this block

is generated

Resent-Date:

and Resent-Sender: are mandatory;

all others optional

Special, used for forwarding an email message to others

Trace Fields Received:

Return-Path:

Inserted by email system

Unlimited Used to trace the

message through the email system

[admin@lnxclient admin]$ cat /var/spool/mail/admin

From admin@lnxserver.booklab.englab.juniper.net Wed Jan 16 13:04:50 2008 Return-Path: <admin@lnxclient.booklab.englab.juniper.net>

Received: from localhost (localhost.localdomain [127.0.0.1])

by lnxclient.booklab.englab.juniper.net (8.12.9/8.12.8) with ESMTP id

jBGL4onD026830

for <admin@localhost>; Wed, 16 Jan 2008 13:04:50 -0800

Received: from lnxserver.booklab.englab.juniper.net

by localhost with POP3 (fetchmail-6.2.0)

for admin@localhost (single-drop); Wed, 16 Jan 2008 13:04:50 -0800 (PST) Received: from lnxclient.booklab.englab.juniper.net ([10.10.12.166])

by lnxserver.booklab.englab.juniper.net (8.12.8/8.12.8) with ESMTP id

jBGL4HFa027257

for <admin@lnxserver.booklab.englab.juniper.net>; Wed, 16 Jan 2008

13:04:17 -0800 (PST)

Received: from lnxclient.booklab.englab.juniper.net (localhost.localdomain [127.0.0.1])

by lnxclient.booklab.englab.juniper.net (8.12.8/8.12.8) with ESMTP id

jBGL4HnD026820

for <admin@lnxserver.booklab.englab.juniper.net>; Wed, 16 Jan 2008

13:04:17 -0800

Received: (from admin@localhost)

by lnxclient.booklab.englab.juniper.net (8.12.8/8.12.8/Submit) id jBGL4HHf026818

for admin@lnxserver.booklab.englab.juniper.net; Wed, 16 Jan 2008

13:04:17 -0800

Date: Wed, 16 Jan 2008 13:04:17 -0800

From: admin@lnxclient.booklab.englab.juniper.net

Message-Id: <200801172104.jBGL4HHf-26818 @lnxclient.booklab.englab.juniper.net>

To: admin@lnxserver.booklab.englab.juniper.net

Subject: Here is another example

X-IMAPbase: 1134766876 8

Status: o

X-UID: 8

X-Keywords:

This is text…

The important fi elds are highlighted Most of the other headers were added when the email was created, of course Most useful is the series of Received: headers, which

allows us to trace the message back to its origin It might seem odd that there are fi ve

receiver headers along the trace for a message that has gone from client to email server and then back to client But the application adds a localhost step at each end, at the sender (admin@localhost) and receiver (from localhost) to the message trace The complete path of the message recorded in the headers (from “bottom to top”) is:

1 The mail application receives the composed message from the local user.

2 The local mailbox receives the message using ESMTP.

3 The email server receives the message using ESMTP.

4 The other client retrieves the message from the email server using POP3

(fetchmail).

5 The local host transfers the message to the local mailbox using ESMTP.

6 The use of these protocols is highlighted in the headers.

HOME OFFICE EMAIL

Let’s end our email discussion by showing that Windows uses the same protocols and headers to send and receive email over the Internet This time, we’ll send a mes-sage from lnxclient on the Illustrated Network to my home offi ce host (which uses Outlook).

Almost all email applications have an option to view the complete headers In Out-look, it’s just “Message Header” in the singular, but the following is the result of viewing the message headers in Outlook Only the headers are displayed, not the message text itself.

Microsoft Mail Internet Headers Version 2.0

Received: from beta.jnpr.net ([172.24.18.109]) by positron.jnpr.net with Microsoft SMTPSVC(5.0.2195.6713);

Thu, 17 Jan 2008 07:37:14 -0700

Received: from merlot.juniper.net ([172.17.27.10]) by beta.jnpr.net over TLS secured channel with Microsoft SMTPSVC(6.0.3790.1830);

Thu, 17 Jan 2008 07:37:13 -0700

Received: from lnxclient.englab.juniper.net (lnxclient.englab.juniper.net [10.10.12.166])

by merlot.juniper.net (8.11.3/8.11.3) with ESMTP id k9JEbDH15244

for <walterg@juniper.net>; Thu, 17 Jan 2008 07:37:13 -0700 (PDT)

(envelope-from admin@lnxclient.englab.juniper.net)

Received: from lnxclient.englab.juniper.net (localhost.localdomain

[127.0.0.1])

by lnxclient.englab.juniper.net (8.12.8/8.12.8) with ESMTP id

k9JEacUg026193

for <walterg@juniper.net>; Thu, 17 Jan 2008 07:36:58 -0700

Received: (from admin@localhost)

by lnxclient.englab.juniper.net (8.12.8/8.12.8/Submit) id k9JEaSlp026191 for walterg@juniper.net; Thu, 17 Jan 2008 07:36:28 -0700

Date: Thu, 17 Jan 2008 07:36:28 -0700

From: admin@lnxclient.englab.juniper.net

Message-Id: <200801171436.k9JEaSlp026191@lnxclient.englab.juniper.net> To: walterg@juniper.net

Subject: here is an email example

Return-Path: admin@lnxclient.englab.juniper.net

X- OriginalArrivalTime: 17 Jan 2008 14:37:13.0230 (UTC) FILETIME=[10F80AE0: 01C6F38C]

QUESTIONS FOR READERS

Figure 21.10 shows some of the concepts discussed in this chapter and can be used to answer the following questions.

FIGURE 21.10

POP3 session capture

1 Which port does POP3 use?

2 Which password is provided by the user?

3 Was the email message deleted after it was retrieved?

4 How long was the message?

5 How many other messages are in the user’s mailbox?