Transcript mail from

CIT 470: Advanced Network and
System Administration
E-mail
CIT 470: Advanced Network and System Administration
Slide #1
Topics
1.
2.
3.
4.
5.
6.
7.
Mail Policies
Anatomy of a Mail Message
Components of an E-mail System
SMTP
IMAP & POP
E-mail Addresses
Aliases and Lists
CIT 470: Advanced Network and System Administration
Slide #2
Mail Policies
1.
2.
3.
4.
5.
Privacy Policy
Namespaces
Reliability
Scaling
Security
CIT 470: Advanced Network and System Administration
Slide #3
Privacy Policy
Personal Use Policy
– Personal v. commercial use.
– When may employee e-mail be read?
• By whom
• Under what circumstances
– Automatic monitoring
Retention Policy
– Legal requirements.
CIT 470: Advanced Network and System Administration
Slide #4
Namespaces
Avoid first.last format addresses.
– There will be duplicates: John.Smith.
– Use middle initials?
– Append numbers?
Create unique organization-wide namespace.
– Use directory to lookup addresses.
CIT 470: Advanced Network and System Administration
Slide #5
Reliability
Customers expect same reliability as power.
– Failures generate many support calls.
Reliability measures
–
–
–
–
Redundant servers.
Backup MX hosts.
RAID arrays.
Multiple NICs, power supplies, processors, etc.
CIT 470: Advanced Network and System Administration
Slide #6
Scalability
Types of scability
– To address growth in avg messages/day.
– To address spikes in mail traffic.
Number of messages grows
– faster than linearly with number of users.
– with time, even if user base is constant.
– due to spam too.
Size of messages grows
– due to technology: more + larger attachments.
CIT 470: Advanced Network and System Administration
Slide #7
Security
Mail server as a target
– Complexity of mail leads to vulnerabilities.
– Mail is an asset attackers want to take.
E-mail as a conduit
– Brings viruses and trojans into organization.
– Leaks confidential information outward.
– ex (2005): Apple sues bloggers over releasing data about
upcoming products.
E-mail relaying
– Open relays used by spammers and scammers.
Intercepting e-mail
CIT 470: Advanced Network and System Administration
Slide #8
Header
Anatomy of a Mail Message
Received: from mailfe2.nku.edu ([192.122.237.68])
by brahms.utoledo.edu (8.11.9) with ESMTP id k3CJCM
for <[email protected]>; Wed, 12 Apr 2006 15:12:22 0400 (EDT)
Received: from mailfac1.nku.edu ([172.28.102.15]) by
mailfe2.nku.edu with MS SMTPSVC(6.0.3790.211); Wed,
12 Apr 2006 15:15:18 -0400
Subject: Test
Date: Wed, 12 Apr 2006 15:15:18 -0400
Message-ID: <[email protected]>
From: “You" <[email protected]>
To: <[email protected]>
Blank
Body This is a test message.
CIT 470: Advanced Network and System Administration
Slide #9
Header
Header Format
– Header-name: Header-data
Common headers
–
–
–
–
–
From:
To:, CC:, Reply-To:
Date:
Message-ID:
Subject:
Multiple headers
– Received: for each mail server handling message.
CIT 470: Advanced Network and System Administration
Slide #10
Body
• Separated from header by blank line.
• Contains 7-bit ASCII text by default.
• Any non-ASCII text must be encoded:
– uuencode
– MIME
CIT 470: Advanced Network and System Administration
Slide #11
MIME
Multipurpose Internet Mail Extensions
– Text in non-ASCII character sets.
– Non-text attachments.
– Multi-part message bodies.
Identified by Content-Type: header.
–
–
–
–
–
text/plain: regular e-mail
text/html: HTML markup
multipart/mixed: text/plain + attachments
image/jpeg: JPEG image attachment
Many other formats
CIT 470: Advanced Network and System Administration
Slide #12
Base64
Binary to text encoding scheme
– Each character represents 6 bits.
– Uses 64 characters from 7-bit ASCII: A-Za-z0-9+=
Encodes in 3-byte chunks
3 bytes = 24 bits = 4 base-64 characters
M
a
n
77
97
110
010011|01 0110|0001 01|101110
19
22
5
46
T
W
F
u
CIT 470: Advanced Network and System Administration
Slide #13
Multipart MIME Message Example
MIME-version: 1.0
Content-type: multipart/mixed; boundary="frontier“
This is a multi-part message in MIME format.
--frontier
Content-type: text/plain
This is the body of the message.
--frontier
Content-type: application/octet-stream
Content-transfer-encoding: base64
PGh0bWw+CiAgPGhlYWQ+CiAgPC9oZWFkPgogIDxib2R5PgogICA
gPHA+VGhpcyBpcyB0aGUg
Ym9keSBvZiB0aGUgbWVzc2FnZS48L3A+CiAgPC9ib2R5Pgo8L2h0
bWw+Cg==
--frontier--
CIT 470: Advanced Network and System Administration
Slide #14
Envelope
Headers aren’t the full story
– Recipient isn’t necessarily on To: or CC:
– Sender isn’t necessarily given on From: header.
Envelope specifies sender/receiver
–
–
–
–
Specified via SMTP commands.
Envelope recipient used for BCC:
Envelope recipient used by mail lists.
Envelope facilities used by spammers too.
CIT 470: Advanced Network and System Administration
Slide #15
Components of a Mail System
UA
DA
Eudora
mail.local
UA
TA
TA
Outlook
Sendmail
Sendmail
Msg
Store
AA
UA
UA
mutt
imapd
mutt
CIT 470: Advanced Network and System Administration
Slide #16
MTAs
• Mail Transport Agents
– Receive mail from MUAs.
– Route mail across internet.
• MTA Protocol: SMTP
• MTA Examples
– sendmail
– postfix
– qmail
CIT 470: Advanced Network and System Administration
Slide #17
SMTP
220 brahms.nku.edu ESMTP Sendmail 8.13.3; Wed, 12 Apr 2006
helo mydomain.com
250 brahms.nku.edu Hello mydomain.com, pleased to meet you
mail from: [email protected]
250 2.1.0 [email protected]... Sender ok
rcpt to: [email protected]
250 2.1.5 [email protected]... Recipient ok
data
354 Enter mail, end with "." on a line by itself
Subject: Test
From: [email protected]
To: [email protected]
This is a test.
.
250 2.0.0 k3GIcr001606 Message accepted for delivery
quit
221 2.0.0 brahms.nku.edu closing connection
CIT 470: Advanced Network and System Administration
Slide #18
SMTP Commands
HELO
EHLO
MAIL
RCPT
VRFY
EXPN
DATA
QUIT
RSET
HELP
hostname
hostname
FROM: addr
TO: addr
addr
addr
CIT 470: Advanced Network and System Administration
Slide #19
Message Store
• Communication
– Receives data from MDA (mail.local, procmail)
– Provides data to MAA (IMAP, POP, NFS, web)
• Types of stores
– Files (all messages for a user in one file)
– Directories (directory per user)
– Databases
CIT 470: Advanced Network and System Administration
Slide #20
Mail Access Agents
• Older systems directly accessed mail files.
• Modern systems use network
– POP: Post Office Protocol
• Simple download protocol for offline reading.
– IMAP: Internet Mail Access Protocol
•
•
•
•
Online and offline modes of reading.
Partial message fetch (headers, attachments, etc.)
Message state stored on server, not client.
Multiple mailbox and multiple client support.
CIT 470: Advanced Network and System Administration
Slide #21
IMAP
• IMAP Servers
– Cyrus
– UW
• IMAP Features
– Message store types
– Authentication
– Security (SSL)
CIT 470: Advanced Network and System Administration
Slide #22
Mail User Agents
• Text clients
– mail
– mutt
– pine
• GUI clients
– Eudora
– Mozilla Thunderbird
– MS Outlook
• Web clients
– Run on remote web server.
CIT 470: Advanced Network and System Administration
Slide #23
Mail Addressing
• Relative Addresses
– mcvax!uunet!ucbvax!hao!boulder!air!evi
• Absolute Addresses
– user@domain
• MX Records
– Mail clients use MX records, not A records.
– Lowest preference # = highest priority.
– Permits failover if server down.
CIT 470: Advanced Network and System Administration
Slide #24
Aliases
• Allow mail to be rerouted.
– Sysadmin: files (/etc/mail/aliases), local db, NIS, LDAP
– Personal: ~/.forward
• Alias destinations
–
–
–
–
Local: address
Remote: address@domain
File: :include:pathname
Program: |pathname
• Required aliases
– postmaster, abuse, root
CIT 470: Advanced Network and System Administration
Slide #25
Mailing List Aliases
Mailing List Aliases
mylist: :include:/etc/mail/include/mylist
owner-mylist: mylist-request
mylist-request: me
owner-owner: postmaster
Purpose
owner: Messages appear to be from owner. Receives
bounces, list management mail.
request: Indirection ensures owner’s real address doesn’t
appear on Return-Path.
owner-owner: Receives errors from messages destined for
owner-* aliases.
CIT 470: Advanced Network and System Administration
Slide #26
Mailing List Software
• Automate list management.
– E-mail interface.
– Web interface.
• Packages
– Mailman
– Majordomo
– Listserv
• List Archiving
– Mailman
– MHonArc
CIT 470: Advanced Network and System Administration
Slide #27
References
1.
2.
3.
4.
5.
6.
7.
8.
Bryan Cosales with Eric Allman, Sendmail, 3rd edition, O’Reilly,
2002.
David H. Crocker, RFC 822: STANDARD FOR THE FORMAT OF
ARPA INTERNET TEXT MESSAGES,
http://www.ietf.org/rfc/rfc0822.txt, 1982.
Aeleen Frisch, Essential System Administration, 3rd edition, O’Reilly,
2002.
MIME, http://en.wikipedia.org/wiki/MIME
Evi Nemeth et al, UNIX System Administration Handbook, 3rd
edition, Prentice Hall, 2001.
Thomas A. Limoncelli and Christine Hogan, The Practice of System
and Network Administration, Addison-Wesley, 2002.
RedHat, Red Hat Enterprise Linux 4 System Administration Guide,
http://www.redhat.com/docs/manuals/enterprise/RHEL-4Manual/sysadmin-guide/, 2005.
Alan Schwartz, Managing Mailing Lists, O’Reilly, 1998.
CIT 470: Advanced Network and System Administration
Slide #28