Demystifying Sendmail Hal Pomeranz Deer Run Associates Demystifying Sendmail All material in this course Copyright © Hal Pomeranz and Deer Run Associates, 2005- 2006. All rights reserved. Hal Pomeranz * Founder/CEO * [email protected] Deer Run Associates * PO Box 50638 * Eugene, OR 97405 +1 541-683-8680 (voice) * +1 541-683-8681 (fax) http://www.deer-run.com/ 1 Demystifying Sendmail.......................................................................................................1 Welcome! ........................................................................................................................5 About This Course ..........................................................................................................8 Course Organization......................................................................................................10 An Important Message from Your Instructor................................................................ 12 Sendmail Basics ................................................................................................................13 Sendmail as an Email “Backbone”................................................................................14 The Argument for Multiple Layers ...............................................................................18 What is SMTP? .............................................................................................................21 MTA vs. MSP ...............................................................................................................23 How Does This Really Work? ......................................................................................29 Open Source vs. Vendor Provided Sendmail................................................................ 35 Creating Sendmail Configuration Files.........................................................................37 Exercises........................................................................................................................39 Answers to Exercises .................................................................................................... 44 Internal Relays...................................................................................................................50 The mailertable Feature ........................................................................................54 Other Configuration Directives.....................................................................................59 Masquerading................................................................................................................61 Exercises........................................................................................................................64 Answers to Exercises .................................................................................................... 67 External Mail Relays.........................................................................................................70 Operating System Security............................................................................................71 External Sendmail Posture ............................................................................................75 No Promiscuous Relays! ...............................................................................................76 Simple External Relay Configuration ...........................................................................79 Additional Configuration ..............................................................................................83 Limiting Sendmail Privileges........................................................................................86 Other Considerations.....................................................................................................91 MAIL_HUB Oddity......................................................................................................93 Exercises........................................................................................................................95 Answers to Exercises .................................................................................................. 101 Sendmail Administration.................................................................................................108 What Version of Sendmail is Installed?...................................................................... 109 The Sendmail Queue ...................................................................................................111 Stopping and Starting Sendmail..................................................................................119 Testing Sendmail.........................................................................................................125 Exercises......................................................................................................................137 Answsers to Exercises................................................................................................. 141 2 Performance Tuning........................................................................................................148 Don’t Tune If You Don't Have To ..............................................................................149 Increasing Throughput Through Parallelization ......................................................... 150 Typical Sendmail Performance Bottlenecks ............................................................... 152 Synchronous Writes ....................................................................................................153 Problems with "Deep" Queues....................................................................................155 DNS Issues ..................................................................................................................159 Tuning Timeout Values...............................................................................................161 Throttling Controls......................................................................................................163 Exercises......................................................................................................................165 Answers to Exercises .................................................................................................. 167 Spam and Virus Control..................................................................................................170 Before You Begin........................................................................................................171 Sendmail's Built-In Anti-Spam Features..................................................................... 177 The Sendmail Milter Interface ....................................................................................186 Now I'm Really Confused ...........................................................................................194 Exercises......................................................................................................................195 Answers to Exercises .................................................................................................. 197 Virtual Domains ..............................................................................................................200 What Are We Talking About? ....................................................................................201 "Parallel" Domains......................................................................................................202 Virtual Domains ..........................................................................................................204 Deployment Issues ......................................................................................................205 The virtusertable Feature .................................................................................208 Other Foreign Domains...............................................................................................212 Exercises......................................................................................................................213 Answers to Exercises .................................................................................................. 216 Aliases and Mailing Lists................................................................................................221 Aliases Overview ........................................................................................................222 Some Sample Aliases Entries......................................................................................224 When Alias Expansion Happens.................................................................................226 Setting Up a Machine to Handle Aliases .................................................................... 228 Dealing With Common Issues.....................................................................................230 About Mailing Lists ....................................................................................................236 Exercises......................................................................................................................239 Answers to Exercises .................................................................................................. 245 Wrap Up ..........................................................................................................................252