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VPNs Illustrated:  Tunnels, VPNs, and IPsec: Tunnels, VPNs, and IPsec

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VPNs Illustrated: Tunnels, VPNs, and IPsec: Tunnels, VPNs, and IPsec


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  • Copyright 2006
  • Dimensions: 7-3/8 x 9-1/4
  • Pages: 480
  • Edition: 1st
  • Book
  • ISBN-10: 0-321-24544-X
  • ISBN-13: 978-0-321-24544-1

Virtual private networks (VPNs) based on the Internet instead of the traditional leased lines offer organizations of all sizes the promise of a low-cost, secure electronic network. However, using the Internet to carry sensitive information can present serious privacy and security problems. By explaining how VPNs actually work, networking expert Jon Snader shows software engineers and network administrators how to use tunneling, authentication, and encryption to create safe, effective VPNs for any environment.

Using an example-driven approach, VPNs Illustrated explores how tunnels and VPNs function by observing their behavior "on the wire." By learning to read and interpret various network traces, such as those produced by tcpdump, readers will be able to better understand and troubleshoot VPN and network behavior.

Specific topics covered include:

  • Block and stream symmetric ciphers, such as AES and RC4; and asymmetric ciphers, such as RSA and EIGamal
  • Message authentication codes, including HMACs
  • Tunneling technologies based on gtunnel
  • SSL protocol for building network-to-network VPNs
  • SSH protocols as drop-in replacements for telnet, ftp, and the BSD r-commands
  • Lightweight VPNs, including VTun, CIPE, tinc, and OpenVPN
  • IPsec, including its Authentication Header (AH) protocol, Encapsulating Security Payload (ESP), and IKE (the key management protocol)

Packed with details, the text can be used as a handbook describing the functions of the protocols and the message formats that they use. Source code is available for download, and an appendix covers publicly available software that can be used to build tunnels and analyze traffic flow.

VPNs Illustrated gives you the knowledge of tunneling and VPN technology you need to understand existing VPN implementations and successfully create your own.

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Table of Contents

Preface xiii

Part 1. Background 1

Chapter 1. Introduction 3

1.1 Purpose 3

1.2 Readers 4

1.3 Typographical Conventions 4

1.4 Source Code and Third-Party Programs 5

1.5 Testbed 5

1.6 Road Map 6

1.7 Summary 8

Chapter 2. TCP/IP Overview 9

2.1 Introduction 9

2.2 Layering 9

2.3 Encapsulation 11

2.4 Addressing 13

2.5 IP 20

2.6 UDP 22

2.7 TCP 24

2.8 ICMP 31

2.9 NAT and Private IP Addresses 35

2.10 PPP 40

2.11 IPv6 43

2.12 Routing 47

2.13 Summary 54

Exercises 55

Chapter 3. Cryptography Overview 57

3.1 Introduction 57

3.2 Symmetric Ciphers 58

3.3 Asymmetric Ciphers 69

3.4 Cryptographic Hash Functions, MACs, and HMACs 75

3.5 Digital Signatures 80

3.6 Certificates 83

3.7 Summary 87

Exercises 88

Chapter 4. Tunnels 89

4.1 Introduction 89

4.2 IP-in-IP Tunnels 92

4.3 PPPoE 95

4.4 GRE 100

4.5 PPTP 104

4.6 L2TP 109

4.7 MPLS 135

4.8 gtunnel 145

4.9 Summary 151

Exercises 152

Part 2. Tunnels and VPNs 153

Chapter 5. Virtual Private Networks 155

5.1 Introduction 155

5.2 PPTP 157

5.3 L2TP 158

5.4 Other VPNs 162

5.5 Summary 163

Exercises 163

Chapter 6. Secure Sockets Layer 165

6.1 Introduction 165

6.2 Cipher Suites 166

6.3 The SSL Protocol 167

6.4 SSL on the Wire 171

6.5 OpenSSL 191

6.6 The stunnel Program 196

6.7 SSL Security 204

6.8 Summary 205

Exercises 205

Chapter 7. SSH 207

7.1 Introduction 207

7.2 The SSHv1 Protocol 208

7.3 The SSHv2 Protocol 232

7.4 Building VPNs with SSH 260

7.5 Summary 266

Exercises 266

Chapter 8. Lightweight VPNs 267

8.1 Introduction 267

8.2 VTun 267

8.3 CIPE 272

8.4 Tinc 283

8.5 OpenVPN 292

8.6 Summary 302

Exercises 302

Part 3. IPSec 305

Chapter 9. IPsec 307

9.1 Introduction 307

9.2 An Overview of IPsec 308

9.3 Road Map for Part 3 308

9.4 Summary 309

Exercises 310

Chapter 10. IPsec Architecture 311

10.1 Introduction 311

10.2 Protocols 312

10.3 IPsec Modes 313

10.4 Security Associations 316

10.5 Combining Security Associations 318

10.6 Policies 320

10.7 IPsec Processing 321

10.8 Summary 323

Exercises 324

Chapter 11. AH 325

11.1 Introduction 325

11.2 The AH Header 326

11.3 Sequence Numbers 328

11.4 AH Processing 330

11.5 Transport Mode 331

11.6 Tunnel Mode 333

11.7 AH with IPv6 336

11.8 Summary 338

Exercises 338

Chapter 12. ESP 341

12.1 Introduction 341

12.2 The ESP Header 342

12.3 ESP Processing 344

12.4 Transport Mode 345

12.5 Tunnel Mode 348

12.6 ESP with IPv6 353

12.7 Summary 354

Exercises 355

Chapter 13. IKE 357

13.1 Introduction 357

13.2 ISAKMP 358

13.3 IKE 375

13.4 An Example Negotiation 388

13.5 Summary 393

Exercises 394

Chapter 14. IPsec Futures 397

14.1 Introduction 397

14.2 IPsec Architecture 398

14.3 AH 401

14.4 ESP 403

14.5 IKE 404

14.6 NAT Traversal 409

14.7 Summary 416

Exercises 416

Appendix A Source Code 419

A.1 Introduction 419

A.2 Cryptographic Routines 419

A.3 Library Code 423

Appendix B Miscellaneous Software 425

B.1 Netcat 425

B.2 tcpdump and Other Packet Sniffers 426

B.3 ssldump 429

B.4 PPP 431

Bibliography 435

Index 445


Untitled Document Introduction There is a revolution going on in enterprise networking. Until very recently, enterprises that needed to link computers in geographically dispersed locations had to build their own wide area networks (WANs). Usually this meant renting expensive and, by today's Internet standard, relatively slow frame relay circuits. A typical 56 Kb/s circuit could cost several hundred, or even over a thousand, dollars a month.

Today, the typical home computer user enjoys a broadband Internet connection having a 5 Mb/s download speed for a cost of about $40 per month. Commercial variants of this service, offering higher speeds and other amenities, are available for between $100 and $200 per month. Obviously, this significant increase in speed and decrease in cost represent a tremendous opportunity for enterprises, but they do introduce new problems.

The Internet is an open environment and, compared to leased lines, dreadfully insecure. Increases in bandwidth and decreases in cost are worthless if they mean that an enterprise's vital data can be intercepted by competitors, or that financial transactions are subject to manipulation by outsiders. This book discusses ways to overcome these problems by recreating the security of leased lines in a public medium such as the Internet.

The fundamental mechanism that allows us to have secure communications in the Internet is the notion of a tunnel. As we'll see, tunnels are a way of overlaying a logical or virtual network on top of a physical network. Once we have such a tunnel, we can secure it by encrypting and authenticating the network traffic that flows through it, thus recreating the security of private leased lines.

Of course, this simple description hides a substantial set of details and problems. We'll see that it's actually quite difficult to endow such tunnels with robust security. Much of the book is concerned with exploring solutions to these problems,and seeing why the successful solutions work and where the unsuccessful ones fail.

Source Code and Errata Availability Source code discussed in the text and other supporting material are available on my Web site athttp://home.netcom.com/~jsnader. The networking libraries and skeletons from Effective TCP/IPProgramming, which I mention and use occasionally in the text, are also available on the Web site.

My readers, it turns out, are much better at finding mistakes than I am. Although I go over the text carefully, checking that every i is dotted and every t crossed, errors still manage to evade me. Fortunately, most of these are caught by the careful and fastidious professionals at Addison-Wesley. Still, some errors will no doubt escape into the final published text. As these are discovered--usually by careful readers--I add them to an errata list for the book. This list is always available at my Web site.

Colophon As with my previous book, I produced camera-ready copy for this text using James Clark's splendid Groff typesetting suite (now maintained by Ted Harding and Werner Lemberg) and Rich Stevens' modified ms macros.I used the gpic, gtbl, and geqn preprocessors for the figures, tables, and mathematical notation, respectively. Some of the figures use gpic macros from Rich Stevens and Gary Wright. Indexing tools from Jon Bentley and Brian Kernighan were a huge help in the production of the index. I included the source code for the programming examples directly from their source files with Dave Hanson's loom utility. The text is set in the Palatino typeface.

As always, I welcome readers' comments, suggestions, and corrections. Please feel free to email me at the address below.

Jon C. Snader
Tampa, Florida
October 2005


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