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

Transcription

1 CIT 470: Advanced Network and System Administration CIT 470: Advanced Network and System Administration Slide #1 Topics 1. Mail Policies 2. Anatomy of a Mail Message 3. Components of an System 4. SMTP 5. IMAP & POP 6. Addresses 7. Aliases and Lists CIT 470: Advanced Network and System Administration Slide #2 1. Privacy Policy 2. Namespaces 3. Reliability 4. Scaling 5. Security Mail Policies CIT 470: Advanced Network and System Administration Slide #3 1

2 Privacy Policy Personal Use Policy Personal v. commercial use. When may employee 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 2

3 Header 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. as a conduit Brings viruses and trojans into organization. Leaks confidential information outward. ex (2005): Apple sues bloggers over releasing data about upcoming products. relaying Open relays used by spammers and scammers. Intercepting CIT 470: Advanced Network and System Administration Slide #8 Anatomy of a Mail Message Received: from mailfe2.nku.edu ([ ]) by brahms.utoledo.edu (8.11.9) with ESMTP id k3cjcm for <him@utoledo.edu>; Wed, 12 Apr :12: (EDT) Received: from mailfac1.nku.edu ([ ]) by mailfe2.nku.edu with MS SMTPSVC( ); Wed, 12 Apr :15: Subject: Test Date: Wed, 12 Apr :15: Message-ID: <F01E4499C4EC5842A@mailfac1.hh.nku.edu> From: You" <you@nku.edu> To: <him@utoledo.edu> Blank Body This is a test message. CIT 470: Advanced Network and System Administration Slide #9 3

4 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 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 4

5 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 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 5

6 Components of a Mail System UA Eudora DA mail.local Msg Store UA Outlook TA Sendmail TA Sendmail UA mutt UA mutt AA imapd 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 ; Wed, 12 Apr 2006 helo mydomain.com 250 brahms.nku.edu Hello mydomain.com, pleased to meet you mail from: me@mydomain.com me@mydomain.com... Sender ok rcpt to: friend@nku.edu friend@nku.edu... Recipient ok data 354 Enter mail, end with "." on a line by itself Subject: Test From: me@mydomain.com To: friend@nku.edu This is a test k3gicr Message accepted for delivery quit brahms.nku.edu closing connection CIT 470: Advanced Network and System Administration Slide #18 6

7 HELO hostname EHLO hostname MAIL FROM: addr RCPT TO: addr VRFY addr EXPN addr DATA QUIT RSET HELP SMTP Commands 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 7

8 IMAP Servers Cyrus UW IMAP Features Message store types Authentication Security (SSL) IMAP 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 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 8

9 Aliases Allow mail to be rerouted. Sysadmin: files (/etc/mail/aliases), local db, NIS, LDAP Personal: ~/.forward Alias destinations Local: address Remote: 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. interface. Web interface. Packages Mailman Majordomo Listserv List Archiving Mailman MHonArc CIT 470: Advanced Network and System Administration Slide #27 9

10 References 1. Bryan Cosales with Eric Allman, Sendmail, 3 rd edition, O Reilly, David H. Crocker, RFC 822: STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES, Aeleen Frisch, Essential System Administration, 3 rd edition, O Reilly, MIME, 5. Evi Nemeth et al, UNIX System Administration Handbook, 3 rd edition, Prentice Hall, Thomas A. Limoncelli and Christine Hogan, The Practice of System and Network Administration, Addison-Wesley, RedHat, Red Hat Enterprise Linux 4 System Administration Guide, Manual/sysadmin-guide/, Alan Schwartz, Managing Mailing Lists, O Reilly, CIT 470: Advanced Network and System Administration Slide #28 10