Building Internet Firewalls: Internet and Web security

542 Pages·2000·4.897 MB·English

by  Elizabeth D. Zwicky, Simon Cooper, D. Brent Chapman

Building Internet Firewalls

Elizabeth D. Zwicky, Simon Cooper & D. Brent Chapman
Second Edition, June 2000
ISBN: 1-56592-871-7, 890 pages

Completely revised and much expanded, the new edition of the highly respected and
bestselling Building Internet Firewalls now covers Unix, Linux, and Windows NT.
This practical and detailed guide explains in step-by-step fashion how to design and
install firewalls and configure Internet services to work with a firewall.
It covers a wide range of services and protocols and offers a complete list of
resources, including the location of many publicly available firewalls construction tools.

Release Team[oR] 2001
CONTENTS
Preface 1
Scope of This Book
Audience
Platforms
Products
Examples
Conventions Used in This Book
Comments and Questions
Acknowledgments for the Second Edition
Acknowledgments for the First Edition

I Network Security 8

1 Why Internet Firewalls? 9
1.1 What Are You Trying to Protect?
1.2 What Are You Trying to Protect Against?
1.3 Who Do You Trust?
1.4 How Can You Protect Your Site?
1.5 What Is an Internet Firewall?
1.6 Religious Arguments

2 Internet Services 27
2.1 Secure Services and Safe Services
2.2 The World Wide Web
2.3 Electronic Mail and News
2.4 File Transfer, File Sharing, and Printing
2.5 Remote Access
2.6 Real-Time Conferencing Services
2.7 Naming and Directory Services
2.8 Authentication and Auditing Services
2.9 Administrative Services
2.10 Databases
2.11 Games

3 Security Strategies 42
3.1 Least Privilege
3.2 Defense in Depth
3.3 Choke Point
3.4 Weakest Link
3.5 Fail-Safe Stance
3.6 Universal Participation
3.7 Diversity of Defense
3.8 Simplicity
3.9 Security Through Obscurity

II Building Firewalls 50

4 Packets and Protocols 51
4.1 What Does a Packet Look Like?
4.2 IP
4.3 Protocols Above IP
4.4 Protocols Below IP
4.5 Application Layer Protocols
4.6 IP Version 6
4.7 Non-IP Protocols
4.8 Attacks Based on Low-Level Protocol Details

5 Firewall Technologies 68
5.1 Some Firewall Definitions
5.2 Packet Filtering
5.3 Proxy Services
5.4 Network Address Translation
5.5 Virtual Private Networks

6 Firewall Architectures 81
6.1 Single-Box Architectures
6.2 Screened Host Architectures
6.3 Screened Subnet Architectures
6.4 Architectures with Multiple Screened Subnets
6.5 Variations on Firewall Architectures
6.6 Terminal Servers and Modem Pools
6.7 Internal Firewalls

7 Firewall Design 103
7.1 Define Your Needs
7.2 Evaluate the Available Products
7.3 Put Everything Together
8 Packet Filtering 108
8.1 What Can You Do with Packet Filtering?
8.2 Configuring a Packet Filtering Router
8.3 What Does the Router Do with Packets?
8.4 Packet Filtering Tips and Tricks
8.5 Conventions for Packet Filtering Rules
8.6 Filtering by Address
8.7 Filtering by Service
8.8 Choosing a Packet Filtering Router
8.9 Packet Filtering Implementations for General-Purpose Computers
8.10 Where to Do Packet Filtering
8.11 What Rules Should You Use?
8.12 Putting It All Together

9 Proxy Systems 146
9.1 Why Proxying?
9.2 How Proxying Works
9.3 Proxy Server Terminology
9.4 Proxying Without a Proxy Server
9.5 Using SOCKS for Proxying
9.6 Using the TIS Internet Firewall Toolkit for Proxying
9.7 Using Microsoft Proxy Server
9.8 What If You Can't Proxy?

10 Bastion Hosts 157
10.1 General Principles
10.2 Special Kinds of Bastion Hosts
10.3 Choosing a Machine
10.4 Choosing a Physical Location
10.5 Locating Bastion Hosts on the Network
10.6 Selecting Services Provided by a Bastion Host
10.7 Disabling User Accounts on Bastion Hosts
10.8 Building a Bastion Host
10.9 Securing the Machine
10.10 Disabling Nonrequired Services
10.11 Operating the Bastion Host
10.12 Protecting the Machine and Backups

11 Unix and Linux Bastion Hosts 176
11.1 Which Version of Unix?
11.2 Securing Unix
11.3 Disabling Nonrequired Services
11.4 Installing and Modifying Services
11.5 Reconfiguring for Production
11.6 Running a Security Audit

12 Windows NT and Windows 2000 Bastion Hosts 191
12.1 Approaches to Building Windows NT Bastion Hosts
12.2 Which Version of Windows NT?
12.3 Securing Windows NT
12.4 Disabling Nonrequired Services
12.5 Installing and Modifying Services

III Internet Services 203

13 Internet Services and Firewalls 204
13.1 Attacks Against Internet Services
13.2 Evaluating the Risks of a Service
13.3 Analyzing Other Protocols
13.4 What Makes a Good Firewalled Service?
13.5 Choosing Security-Critical Programs
13.6 Controlling Unsafe Configurations

14 Intermediary Protocols 223
14.1 Remote Procedure Call (RPC)
14.2 Distributed Component Object Model (DCOM)
14.3 NetBIOS over TCP/IP (NetBT)
14.4 Common Internet File System (CIFS) and Server Message Block (SMB)
14.5 Common Object Request Broker Architecture (CORBA) and Internet Inter-Orb Protocol (IIOP)
14.6 ToolTalk
14.7 Transport Layer Security (TLS) and Secure Socket Layer (SSL)
14.8 The Generic Security Services API (GSSAPI)
14.9 IPsec
14.10 Remote Access Service (RAS)
14.11 Point-to-Point Tunneling Protocol (PPTP)
14.12 Layer 2 Transport Protocol (L2TP)
15 The World Wide Web 245
15.1 HTTP Server Security
15.2 HTTP Client Security
15.3 HTTP
15.4 Mobile Code and Web-Related Languages
15.5 Cache Communication Protocols
15.6 Push Technologies
15.7 RealAudio and RealVideo
15.8 Gopher and WAIS

16 Electronic Mail and News 268
16.1 Electronic Mail
16.2 Simple Mail Transfer Protocol (SMTP)
16.3 Other Mail Transfer Protocols
16.4 Microsoft Exchange
16.5 Lotus Notes and Domino
16.6 Post Office Protocol (POP)
16.7 Internet Message Access Protocol (IMAP)
16.8 Microsoft Messaging API (MAPI)
16.9 Network News Transfer Protocol (NNTP)

17. File Transfer, File Sharing, and Printing 287
17.1 File Transfer Protocol (FTP)
17.2 Trivial File Transfer Protocol (TFTP)
17.3 Network File System (NFS)
17.4 File Sharing for Microsoft Networks
17.5 Summary of Recommendations for File Sharing
17.6 Printing Protocols
17.7 Related Protocols

18 Remote Access to Hosts 307
18.1 Terminal Access (Telnet)
18.2 Remote Command Execution
18.3 Remote Graphical Interfaces

19 Real-Time Conferencing Services 328
19.1 Internet Relay Chat (IRC)
19.2 ICQ
19.3 talk
19.4 Multimedia Protocols
19.5 NetMeeting
19.6 Multicast and the Multicast Backbone (MBONE)

20. Naming and Directory Services 341
20.1 Domain Name System (DNS)
20.2 Network Information Service (NIS)
20.3 NetBIOS for TCP/IP Name Service and Windows Internet Name Service
20.4 The Windows Browser
20.5 Lightweight Directory Access Protocol (LDAP)
20.6 Active Directory
20.7 Information Lookup Services

21 Authentication and Auditing Services 373
21.1 What Is Authentication?
21.2 Passwords
21.3 Authentication Mechanisms
21.4 Modular Authentication for Unix
21.5 Kerberos
21.6 NTLM Domains
21.7 Remote Authentication Dial-in User Service (RADIUS)
21.8 TACACS and Friends
21.9 Auth and identd

22 Administrative Services 397
22.1 System Management Protocols
22.2 Routing Protocols
22.3 Protocols for Booting and Boot-Time Configuration
22.4 ICMP and Network Diagnostics
22.5 Network Time Protocol (NTP)
22.6 File Synchronization
22.7 Mostly Harmless Protocols

23 Databases and Games 418
23.1 Databases
23.2 Games
24 Two Sample Firewalls 428
24.1 Screened Subnet Architecture
24.2 Merged Routers and Bastion Host Using General-Purpose Hardware

IV Keeping Your Site Secure 456

25 Security Policies 457
25.1 Your Security Policy
25.2 Putting Together a Security Policy
25.3 Getting Strategic and Policy Decisions Made
25.4 What If You Can't Get a Security Policy?

26 Maintaining Firewalls 468
26.1 Housekeeping
26.2 Monitoring Your System
26.3 Keeping up to Date
26.4 How Long Does It Take?
26.5 When Should You Start Over?

27 Responding to Security Incidents 481
27.1 Responding to an Incident
27.2 What to Do After an Incident
27.3 Pursuing and Capturing the Intruder
27.4 Planning Your Response
27.5 Being Prepared

V Appendixes 500

A Resources 501
A.1 Web Pages
A.2 FTP Sites
A.3 Mailing Lists
A.4 Newsgroups
A.5 Response Teams
A.6 Other Organizations
A.7 Conferences
A.8 Papers
A.9 Books

B Tools 513
B.1 Authentication Tools
B.2 Analysis Tools
B.3 Packet Filtering Tools
B.4 Proxy Systems Tools
B.5 Daemons
B.6 Utilities

C Cryptography 520
C.1 What Are You Protecting and Why?
C.2 Key Components of Cryptographic Systems
C.3 Combined Cryptography
C.4 What Makes a Protocol Secure?
C.5 Information About Algorithms

Colophon 535
Introduction
In the five years since the first edition of this classic book was published, Internet use has exploded. The
commercial world has rushed headlong into doing business on the Web, often without integrating sound security
technologies and policies into their products and methods. The security risks - and the need to protect both
business and personal data - have never been greater. We've updated Building Internet Firewalls to address
these newer risks.
What kinds of security threats does the Internet pose? Some, like password attacks and the exploiting of known
security holes, have been around since the early days of networking. And others, like the distributed denial of
service attacks that crippled Yahoo, E-Bay, and other major e-commerce sites in early 2000, are in current
headlines.
Firewalls, critical components of today's computer networks, effectively protect a system from most Internet
security threats. They keep damage on one part of the network - such as eavesdropping, a worm program, or file
damage - from spreading to the rest of the network. Without firewalls, network security problems can rage out of
control, dragging more and more systems down.
Like the bestselling and highly respected first edition, Building Internet Firewalls, 2nd Edition, is a practical and
detailed step-by-step guide to designing and installing firewalls and configuring Internet services to work with a
firewall. Much expanded to include Linux and Windows coverage, the second edition describes:
•
Firewall technologies: packet filtering, proxying, network address translation, virtual private
networks
•
Architectures such as screening routers, dual-homed hosts, screened hosts, screened subnets,
perimeter networks, internal firewalls
•
Issues involved in a variety of new Internet services and protocols through a firewall
•
Email and News
•
Web services and scripting languages (e.g., HTTP, Java, JavaScript, ActiveX, RealAudio,
RealVideo)
•
File transfer and sharing services such as NFS, Samba
•
Remote access services such as Telnet, the BSD "r" commands, SSH, BackOrifice 2000
•
Real-time conferencing services such as ICQ and talk
•
Naming and directory services (e.g., DNS, NetBT, the Windows Browser)
•
Authentication and auditing services (e.g., PAM, Kerberos, RADIUS);
•
Administrative services (e.g., syslog, SNMP, SMS, RIP and other routing protocols, and ping and
other network diagnostics)
•
Intermediary protocols (e.g., RPC, SMB, CORBA, IIOP)
•
Database protocols (e.g., ODBC, JDBC, and protocols for Oracle, Sybase, and Microsoft SQL
Server)
The book's complete list of resources includes the location of many publicly available firewall construction tools.
Building Internet Firewalls
Preface
This book is a practical guide to building your own firewall. It provides step-by-step explanations of how to design
and install a firewall at your site and how to configure Internet services such as electronic mail, FTP, the World
Wide Web, and others to work with a firewall. Firewalls are complex, though, and we can't boil everything down
to simple rules. Too much depends on exactly what hardware, operating system, and networking you are using at
your site, and what you want your users to be able to do and not do. We've tried to give you enough rules,
examples, and resources here so you'll be able to do the rest on your own.
What is a firewall, and what does it do for you? A firewall is a way to restrict access between the Internet and
your internal network. You typically install a firewall at the point of maximum leverage, the point where your
network connects to the Internet. The existence of a firewall at your site can greatly reduce the odds that outside
attackers will penetrate your internal systems and networks. The firewall can also keep your own users from
compromising your systems by sending dangerous information - unencrypted passwords and sensitive data - to
the outside world.
The attacks on Internet-connected systems we are seeing today are more serious and more technically complex
than those in the past. To keep these attacks from compromising our systems, we need all the help we can get.
Firewalls are a highly effective way of protecting sites from these attacks. For that reason, we strongly
recommend you include a firewall in your site's overall Internet security plan. However, a firewall should be only
one component in that plan. It's also vital that you establish a security policy, that you implement strong host
security, and that you consider the use of authentication and encryption devices that work with the firewalls you
install. This book will touch on each of these topics while maintaining its focus on firewalls.

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Building Internet Firewalls
Scope of This Book
This book is divided into five parts.
Part I
explores the problem of Internet security and focuses on firewalls as part of an effective strategy to address that
problem.

Chapter 1
introduces the major risks associated with using the Internet today; discusses what to protect, and what to
protect against; discusses various security models; and introduces firewalls in the context of what they can
and can't do for your site's security.

Chapter 2
outlines the services users want and need from the Internet, and summarizes the security problems posed
by those services.

Chapter 3
outlines the basic security principles an organization needs to understand before it adopts a security policy
and invests in specific security mechanisms.

Part II
describes how to build firewalls.

Chapter 4
describes the basic network concepts firewalls work with.

Chapter 5
explains the terms and technologies used in building firewalls.

Chapter 6
describes the major architectures used in constructing firewalls, and the situations they are best suited to.

Chapter 7
presents the process of designing a firewall.

Chapter 8
describes how packet filtering systems work, and discusses what you can and can't accomplish with them
in building a firewall.

Chapter 9
describes how proxy clients and servers work, and how to use these systems in building a firewall.

Chapter 10
presents a general overview of the process of designing and building the bastion hosts used in many
firewall configurations.

Chapter 11
presents the details of designing and building a Unix or Linux bastion host.

Chapter 12
presents the details of designing and building a Windows NT bastion host.

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Building Internet Firewalls
Part III
describes how to configure services in the firewall environment.

Chapter 13
describes the general issues involved in selecting and configuring services in the firewall environment.

Chapter 14
discusses basic protocols that are used by multiple services.

Chapter 15
discusses the Web and related services.

Chapter 16
discusses services used for transferring electronic mail and Usenet news.

Chapter 17
discusses the services used for moving files from one place to another.

Chapter 18
discusses services that allow you to use one computer from another computer.

Chapter 19
discusses services that allow people to interact with each other online.

Chapter 20
discusses the services used to distribute information about hosts and users.

Chapter 21
discusses services used to identify users before they get access to resources, to keep track of what sort of
access they should have, and to keep records of who accessed what and when.

Chapter 22
discusses other services used to administer machines and networks.

Chapter 23
discusses the remaining two major classes of popular Internet services, databases and games.

Chapter 24
presents two sample configurations for basic firewalls.

Part IV
describes how to establish a security policy for your site, maintain your firewall, and handle the security problems
that may occur with even the most effective firewalls.

Chapter 25
discusses the importance of having a clear and well-understood security policy for your site, and what that
policy should and should not contain. It also discusses ways of getting management and users to accept
the policy.

Chapter 26
describes how to maintain security at your firewall over time and how to keep yourself aware of new
Internet security threats and technologies.

Chapter 27
describes what to do when a break-in occurs, or when you suspect that your security is being breached.

Part V
consists of the following summary appendixes:

Appendix A contains a list of places you can go for further information and help with Internet security:
World Wide Web pages, FTP sites, mailing lists, newsgroups, response teams, books, papers, and
conferences.

Appendix B
summarizes the best freely available firewall tools and how to get them.

Appendix C
contains background information on cryptography that is useful to anyone trying to decrypt the marketing
materials for security products.
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