Building Internet Firewalls Second Edition

By Elizabeth D. Zwicky, Simon Cooper and D. Brent Chapman

This book is divided into five parts.

Part I, "Network Security", explores the problem of Internet security and focuses on firewalls as part of an effective strategy to address that problem.

• Chapter 1, "Why Internet Firewalls?", 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, "Internet Services", outlines the services users want and need from the Internet, and summarizes the security problems posed by those services.
• Chapter 3, "Security Strategies", outlines the basic security principles an organization needs to understand before it adopts a security policy and invests in specific security mechanisms.

Part II, "Building Firewalls", describes how to build firewalls.

• Chapter 4, "Packets and Protocols ", describes the basic network concepts firewalls work with.
• Chapter 5, "Firewall Technologies", explains the terms and technologies used in building firewalls.
• Chapter 6, "Firewall Architectures", describes the major architectures used in constructing firewalls, and the situations they are best suited to.
• Chapter 7, "Firewall Design", presents the process of designing a firewall.
• Chapter 8, "Packet Filtering" describes how packet filtering systems work, and discusses what you can and can't accomplish with them in building a firewall.
• Chapter 9, "Proxy Systems", describes how proxy clients and servers work, and how to use these systems in building a firewall.
• Chapter 10, "Bastion Hosts", presents a general overview of the process of designing and building the bastion hosts used in many firewall configurations.
• Chapter 11, "Unix and Linux Bastion Hosts", presents the details of designing and building a Unix or Linux bastion host.
• Chapter 12, "Windows NT and Windows 2000 Bastion Hosts ", presents the details of designing and building a Windows NT bastion host.

Part III, "Internet Services", describes how to configure services in the firewall environment.

• Chapter 13, "Internet Services and Firewalls", describes the general issues involved in selecting and configuring services in the firewall environment.
• Chapter 14, "Intermediary Protocols", discusses basic protocols that are used by multiple services.
• Chapter 15, "The World Wide Web", discusses the Web and related services.
• Chapter 16, "Electronic Mail and News", discusses services used for transferring electronic mail and Usenet news.
• Chapter 17, "File Transfer, File Sharing, and Printing", discusses the services used for moving files from one place to another.
• Chapter 18, "Remote Access to Hosts", discusses services that allow you to use one computer from another computer.
• Chapter 19, "Real-Time Conferencing Services", discusses services that allow people to interact with each other online.
• Chapter 20, "Naming and Directory Services", discusses the services used to distribute information about hosts and users.
• Chapter 21, "Authentication and Auditing Services", 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, "Administrative Services", discusses other services used to administer machines and networks.
• Chapter 23, "Databases and Games", discusses the remaining two major classes of popular Internet services, databases and games.
• Chapter 24, "Two Sample Firewalls", presents two sample configurations for basic firewalls.

Part IV, "Keeping Your Site Secure", 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, "Security Policies", 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, "Maintaining Firewalls", 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, "Responding to Security Incidents", describes what to do when a break-in occurs, or when you suspect that your security is being breached.

Part V, "Appendixes", consists of the following summary appendixes:

• Appendix A, "Resources", 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, "Tools", summarizes the best freely available firewall tools and how to get them.
• Appendix C, "Cryptography", contains background information on cryptography that is useful to anyone trying to decrypt the marketing materials for security products.

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DNS and BIND Fourth Edition

By Paul Albitz and Cricket Liu

The Domain Name System is a distributed database. This allows local control of the segments of the overall database, yet the data in each segment is available across the entire network through a client-server scheme. Robustness and adequate performance are achieved through replication and caching.

Programs called name servers constitute the server half of DNS's client-server mechanism. Name servers contain information about some segments of the database and make it available to clients, called resolvers. Resolvers are often just library routines that create queries and send them across a network to a name server.

The structure of the DNS database is very similar to the structure of the Unix filesystem, as shown in Figure 1-1. The whole database (or filesystem) is pictured as an inverted tree, with the root node at the top. Each node in the tree has a text label, which identifies the node relative to its parent. This is roughly analogous to a "relative pathname" in a filesystem, like bin. One label -- the null label, or "" -- is reserved for the root node. In text, the root node is written as a single dot ( .). In the Unix filesystem, the root is written as a slash ( / ).

The first implementation of the Domain Name System was called JEEVES, written by Paul Mockapetris himself. A later implementation was BIND, an acronym for Berkeley Internet Name Domain, which was written for Berkeley's 4.3 BSD Unix operating system by Kevin Dunlap. BIND is now maintained by the Internet Software Consortium.

BIND is the implementation we'll concentrate on in this book and is by far the most popular implementation of DNS today. It has been ported to most flavors of Unix and is shipped as a standard part of most vendors' Unix offerings. BIND has even been ported to Microsoft's Windows NT.

The fourth edition of this book deals with the new 9.1.0 and 8.2.3 versions of BIND as well as the older 4.9 versions. While 9.1.0 and 8.2.3 are the most recent versions as of this writing, they haven't made their way into many vendors' versions of Unix yet, partly because both versions have only recently been released and many vendors are wary of using such new software. We also occasionally mention other versions of BIND, especially 4.8.3, because many vendors continue to ship code based on this older software as part of their Unix products. Whenever a feature is available only in the 4.9, 8.2.3, or 9.1.0 version, or when there is a difference in the behavior of the versions, we try to point out which version does what.

We use nslookup, a name server utility program, very frequently in our examples. The version we use is the one shipped with the 8.2.3 BIND code. Older versions of nslookup provide much, but not quite all, of the functionality in the 8.2.3 nslookup. We've used commands common to most nslookup sin most of our examples; when this was not possible, we tried to note it.

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Network Troubleshooting Tools First Edition

by Joseph D. Sloan

This book is not a general introduction to network troubleshooting. Rather, it is about one aspect of troubleshooting -- information collection. This book is a tutorial introduction to tools and techniques for collecting information about computer networks. It should be particularly useful when dealing with network problems, but the tools and techniques it describes are not limited to troubleshooting. Many can and should be used on a regular basis regardless of whether you are having problems.

Some of the tools I have selected may be a bit surprising to many. I strongly believe that the best approach to troubleshooting is to be proactive, and the tools I discuss reflect this belief. Basically, if you don't understand how your network works before you have problems, you will find it very difficult to diagnose problems when they occur. Many of the tools described here should be used before you have problems. As such, these tools could just as easily be classified as network management or network performance analysis tools.

This book does not attempt to catalog every possible tool. There are simply too many tools already available, and the number is growing too rapidly. Rather, this book focuses on the tools that I believe are the most useful, a collection that should help in dealing with almost any problem you see. I have tried to include pointers to other relevant tools when there wasn't space to discuss them. In many cases, I have described more than one tool for a particular job. It is extremely rare for two tools to have exactly the same features. One tool may be more useful than another, depending on circumstances. And, because of the differences in operating systems, a specific tool may not be available on every system. It is worth knowing the alternatives.

The book is about freely available Unix tools. Many are open source tools covered by GNU- or BSD-style licenses. In selecting tools, my first concern has been availability. I have given the highest priority to the standard Unix utilities. Next in priority are tools available as packages or ports for FreeBSD or Linux. Tools requiring separate compilation or available only as binaries were given a lower priority since these may be available on fewer systems. In some cases, PC-only tools and commercial tools are noted but are not discussed in detail. The bulk of the book is specific to Ethernet and TCP/IP, but the general approach and many of the tools can be used with other technologies.

While this is a book about Unix tools, at the end of most of the chapters I have included a brief section for Microsoft Windows users. These sections are included since even small networks usually include a few computers running Windows. These sections are not, even in the wildest of fantasies, meant to be definitive. They are provided simply as starting points -- a quick overview of what is available.

Finally, this book describes a wide range of tools. Many of these tools are designed to do one thing and are often overlooked because of their simplicity. Others are extremely complex tools or sets of tools. I have not attempted to provide a comprehensive treatment for each tool discussed. Some of these tools can be extremely complex when used to their fullest. Some have manuals and other documentation that easily exceed the size of this book. Most have additional documentation that you will want to retrieve once you begin using them.

My goal is to make you aware of the tools and to provide you with enough information that you can decide which ones may be the most useful to you and in what context so that you can get started using the tools. Each chapter centers on a collection of related tasks or problems and tools useful for dealing with these tasks. The discussion is limited to features that are relevant to the problem being discussed. Consequently, the same tool may be discussed in several places throughout the book.

Please be warned: the suitability or behavior of these tools on your system cannot be guaranteed. While the material in this book is presented in good faith, neither the author nor O'Reilly & Associates makes any explicit or implied warranty as to the behavior or suitability of these tools. We strongly urge you to assess and evaluate these tool as appropriate for your circumstances.

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Network Troubleshooting Tools (O'Reilly System Administration)

Essential SNMP First Edition

by Douglas R. Mauro and Kevin J. Schmidt

The Simple Network Management Protocol (SNMP) is an Internet-standard protocol for managing devices on IP networks. Many kinds of devices support SNMP, including routers, switches, servers, workstations, printers, modem racks, and uninterruptible power supplies (UPSs). The ways you can use SNMP range from the mundane to the exotic: it's fairly simple to use SNMP to monitor the health of your routers, servers, and other pieces of network hardware, but you can also use it to control your network devices and even send pages or take other automatic action if problems arise. The information you can monitor ranges from relatively simple and standardized items, like the amount of traffic flowing into or out of an interface, to more esoteric hardware- and vendor-specific items, like the air temperature inside a router.

Given that there are already a number of books about SNMP in print, why write another one? Although there are many books on SNMP, there's a lack of books aimed at the practicing network or system administrator. Many books cover how to implement SNMP or discuss the protocol at a fairly abstract level, but none really answers the network administrator's most basic questions: How can I best put SNMP to work on my network? How can I make managing my network easier?

We provide a brief overview of the SNMP protocol in Chapter 2, "A Closer Look at SNMP" then spend a few chapters discussing issues such as hardware requirements and the sorts of tools that are available for use with SNMP. However, the bulk of this book is devoted to discussing, with real examples, how to use SNMP for system and network administration tasks.

Most newcomers to SNMP ask some or all of the following questions:

• What exactly is SNMP?
• How can I, as a system or network administrator, benefit from SNMP?
• What is a MIB?
• What is an OID?
• What is a community string?
• What is a trap?
• I've heard that SNMP is insecure. Is this true?
• Do any of my devices support SNMP? If so, how can I tell if they are configured properly?
• How do I go about gathering SNMP information from a device?
• I have a limited budget for purchasing network-management software. What sort of free/open source software is available?
• Is there an SNMP Perl module that I can use to write cool scripts?
  

This book answers all these questions and more. Our goal is to demystify SNMP and make it more accessible to a wider range of users.

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Managing NFS and NIS Second Edition

by Hal Stern, Mike Eisler and Ricardo Labiaga

This book is of interest to system administrators and network managers who are installing or planning new NFS and NIS networks, or debugging and tuning existing networks and servers. It is also aimed at the network user who is interested in the mechanics that hold the network together.

We'll assume that you are familiar with the basics of Unix system administration and TCP/IP networking. Terms that are commonly misused or particular to a discussion will be defined as needed. Where appropriate, an explanation of a low-level phenomenon, such as Ethernet congestion will be provided if it is important to a more general discussion such as NFS performance on a congested network. Models for these phenomena will be drawn from everyday examples rather than their more rigorous mathematical and statistical roots.

This book focuses on the way NFS and NIS work, and how to use them to solve common problems in a distributed computing environment. Because Sun Microsystems developed and continues to innovate NFS and NIS, this book uses Sun's Solaris operating system as the frame of reference. Thus if you are administering NFS on non-Solaris systems, you should use this book in conjunction with your vendor's documentation, since utilities and their options will vary by implementation and release. This book explains what the configuration files and utilities do, and how their options affect performance and system administration issues. By walking through the steps comprising a complex operation or by detailing each step in the debugging process, we hope to shed light on techniques for effective management of distributed computing environments. There are very few absolute constraints or thresholds that are universally applicable, so we refrain from stating them. This book should help you to determine the fair utilization and performance constraints for your network.

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SSH: The Secure Shell - The Definitive Guide

by Daniel J. Barrett and Richard E. Silverman

Privacy is a basic human right, but on today's computer networks, privacy isn't guaranteed. Much of the data that travels on the Internet or local networks is transmitted as plain text, and may be captured and viewed by anybody with a little technical know-how. The email you send, the files you transmit between computers, even the passwords you type may be readable by others. Imagine the damage that can be done if an untrusted third party -- a competitor, the CIA, your in-laws -- intercepted your most sensitive communications in transit.

Network security is big business as companies scramble to protect their information assets behind firewalls, establish virtual private networks (VPNs), and encrypt files and transmissions. But hidden away from all the bustle, there is a small, unassuming, yet robust solution many big companies have missed. It's reliable, reasonably easy to use, cheap, and available for most of today's operating systems.

It's SSH, the Secure Shell.

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TCP/IP Network Administration Third Edition

by Craig Hunt

The first edition of TCP/IP Network Administration was written in 1992. In the decade since, many things have changed, yet some things remain the same. TCP/IP is still the preeminent communications protocol for linking together diverse computer systems. It remains the basis of interoperable data communications and global computer networking. The underlying Internet Protocol (IP), Transmission Control Protocol, and User Datagram Protocol (UDP) are remarkably unchanged. But change has come in the way TCP/IP is used and how it is managed.
A clear symbol of this change is the fact that my mother-in-law has a TCP/IP network connection in her home that she uses to exchange electronic mail, compressed graphics, and hypertext documents with other senior citizens. She thinks of this as "just being on the Internet," but the truth is that her small system contains a functioning TCP/IP protocol stack, manages a dynamically assigned IP address, and handles data types that did not even exist a decade ago.

In 1991, TCP/IP was a tool of sophisticated users. Network administrators managed a limited number of systems and could count on the users for a certain level of technical knowledge. No more. In 2002, the need for highly trained network administrators is greater than ever because the user base is larger, more diverse, and less capable of handling technical problems on its own. This book provides the information needed to become an effective TCP/IP network administrator.

TCP/IP Network Administration was the first book of practical information for the professional TCP/IP network administrator, and it is still the best. Since the first edition was published there has been an explosion of books about TCP/IP and the Internet. Still, too few books concentrate on what a system administrator really needs to know about TCP/IP administration. Most books are either scholarly texts written from the point of view of the protocol designer, or instructions on how to use TCP/IP applications. All of those books lack the practical, detailed network information needed by the Unix system administrator. This book strives to focus on TCP/IP and Unix and to find the right balance of theory and practice.

I am proud of the earlier editions of TCP/IP Network Administration. In this edition, I have done everything I can to maintain the essential character of the book while making it better. Dynamic address assignment based on Dynamic Host Configuration Protocol (DHCP) is covered. The Domain Name System material has been updated to cover BIND 8 and, to a lesser extent, BIND 9. The email configuration is based on current version of sendmail 8, and the operating system examples are from the current versions of Solaris and Linux. The routing protocol coverage includes Routing Information Protocol version 2 (RIPv2), Open Shortest Path First (OSPF), and Border Gateway Protocol (BGP). I have also added a chapter on Apache web server configuration, new material on xinetd, and information about building a firewall with iptables. Despite the additional topics, the book has been kept to a reasonable length.

TCP/IP is a set of communications protocols that define how different types of computers talk to each other. TCP/IP Network Administration is a book about building your own network based on TCP/IP. It is both a tutorial covering the "why" and "how" of TCP/IP networking, and a reference manual for the details about specific network programs.

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Building Internet Firewalls First Edition

By D. Brent Chapman and Elizabeth D. Zwicky

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 your site 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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Sendmail Desktop Reference First Edition

By Bryan Costales and Eric Allman

The sendmail program is a Mail Transport Agent (MTA). It accepts mail from Mail User Agents (MUAs), mail users (humans), and other MTAs. It then delivers that mail to Mail Delivery Agents (MDAs) on the local machine, or transports that mail to another MTA at another machine. The behavior of sendmail is determined by its command line and by commands in its configuration file.

The sendmail program is written and maintained by Eric Allman at sendmail.org. Versions V8.7 and earlier are no longer supported and are no longer considered secure. If you are not currently running V8.8, we recommend you upgrade now. This Desktop Reference covers sendmail version 8.8.5.

This Desktop Reference is a companion to the second edition of the sendmail book by Bryan Costales with Eric Allman, published by O'Reilly & Associates. Section numbers herein reference the section numbers in that book. This is a reference guide only - for detail or tutorial information, refer to the full sendmail book.

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TCP/IP Network Administration Second Edition

By Craig Hunt

The protocol wars are over and TCP/IP won. TCP/IP is now universally recognized as the pre-eminent communications protocol for linking together diverse computer systems. The importance of interoperable data communications and global computer networks is no longer debated. But that was not always the case. When I wrote the first edition of this book, IPX was far and away the leading PC communications protocol. Microsoft did not bundle communications protocols in their operating system. Corporate networks were so dependent on SNA that many corporate network administrators had not even heard of TCP/IP. Even UNIX, the mother of TCP/IP, nursed a large number of pure UUCP networks. Back then I felt compelled to tout the importance of TCP/IP by pointing out that it was used on thousands of networks and hundreds of thousands of computers. How times have changed! Today we count the hosts and users connected to the Internet in the tens of millions. And the Internet is only the tip of the TCP/IP iceberg. The largest market for TCP/IP is in the corporate "intranet." An intranet is a private TCP/IP network used to disseminate information within the enterprise. The competing network technologies have shrunk to niche markets where they fill special needs - while TCP/IP has grown to be the communications software that links the world.

The acceptance of TCP/IP as a worldwide standard and the size of its global user base are not the only things that have changed. In 1991 I lamented the lack of adequate documentation. At the time it was difficult for a network administrator to find the information he or she needed to do the job. Since that time there has been an explosion of books about TCP/IP and the Internet. However, there are still too few books that concentrate on what a system administrator really needs to know about TCP/IP administration and too many books that try to tell you how to surf the Web. In this book I strive to focus on TCP/IP and UNIX, and not to be distracted by the phenomenon of the Internet.

I am very proud of the first edition of TCP/IP Network Administration. In the second edition, I have done everything I can to maintain the essential character of the book while making it better. The Domain Name Service material has been updated to cover the latest version of the BIND 4 software. The email configuration is now based on sendmail version 8, and the operating system examples are from the current versions of Solaris and Linux. The routing protocol coverage has been expanded to include Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP). I have also added new topics such as one-time passwords and configuration servers based on Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP). Despite the additional topics, the book has been kept to a reasonable length.

The bulk of this edition is derived directly from the first edition of the book. To emphasize both that times have changed and that my focus on practical information has not, I have left the introductory paragraphs from the first edition intact.

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DNS and BIND Third Edition

By Cricket Liu & Paul Albitz

You may not know much about the Domain Name System - yet - but whenever you use the Internet, you use DNS. Every time you send electronic mail or surf the World Wide Web, you rely on the Domain Name System.

You see, while you, as a human being, prefer to remember the names of computers, computers like to address each other by number. On an internet, that number is 32 bits long, or between zero and four billion or so.[1] That's easy for a computer to remember, because computers have lots of memory ideal for storing numbers, but it isn't nearly as easy for us humans. Pick ten phone numbers out of the phone book at random, and then try to remember them. Not easy? Now flip to the front of the book and attach random area codes to the phone numbers. That's about how difficult it would be to remember ten arbitrary internet addresses.

[1] And, with IP version 6, it's soon to be a whopping 128 bits long, or between zero and a decimal number with 39 digits.

This is part of the reason we need the Domain Name System. DNS handles mapping between host names, which we humans find convenient, and internet addresses, which computers deal with. In fact, DNS is the standard mechanism on the Internet for advertising and accessing all kinds of information about hosts, not just addresses. And DNS is used by virtually all internetworking software, including electronic mail, remote terminal programs such as telnet, file transfer programs such as ftp, and web browsers such as Netscape Navigator and Microsoft Internet Explorer.

Another important feature of DNS is that it makes host information available all over the Internet. Keeping information about hosts in a formatted file on a single computer only helps users on that computer. DNS provides a means of retrieving information remotely, from anywhere on the network.

More than that, DNS lets you distribute the management of host information among many sites and organizations. You don't need to submit your data to some central site or periodically retrieve copies of the "master" database. You simply make sure your section, called a zone, is up to date on your name servers. Your name servers make your zone's data available to all the other name servers on the network.

Because the database is distributed, the system also needs the ability to locate the data you're looking for by searching a number of possible locations. The Domain Name System gives name servers the intelligence to navigate through the database and find data in any zone.
Of course, DNS does have a few problems. For example, the system allows more than one name server to store data about a zone, for redundancy's sake. But inconsistencies can crop up between copies of the zone data.

But the worst problem with DNS is that despite its widespread use on the Internet, there's really very little documentation about managing and maintaining it. Most administrators on the Internet make do with the documentation their vendors see fit to provide, and with whatever they can glean from following the Internet mailing lists and Usenet newsgroups on the subject.

This lack of documentation means that the understanding of an enormously important internet service - one of the linchpins of today's Internet - is either handed down from administrator to administrator like a closely-guarded family recipe, or relearned repeatedly by isolated programmers and engineers. New administrators of domains suffer through the same mistakes made by countless others.

Our aim with this book is to help remedy this situation. We realize that not all of you have the time or the desire to become DNS experts. Most of you, after all, have plenty to do besides managing a domain or a name server: system administration, network engineering, or software development. It takes an awfully big institution to devote a whole person to DNS. We'll try to give you enough information to allow you to do what you need to do, whether that's running a small domain or managing a multinational monstrosity, tending a single name server or shepherding a hundred of them. Read as much as you need to know now, and come back later if you need to know more.

DNS is a big topic - big enough to require two authors, anyway - but we've tried to present it as sensibly and understandably as possible. The first two chapters give you a good theoretical overview and enough practical information to get by, and later chapters fill in the nitty-gritty details. We provide a roadmap up front, to suggest a path through the book appropriate for your job or interest.

When we talk about actual DNS software, we'll concentrate almost exclusively on BIND, the Berkeley Internet Name Domain software, which is the most popular implementation of the DNS specs (and the one we know best). We've tried to distill our experience in managing and maintaining a domain with BIND into this book - a domain, incidentally, that is one of the largest on the Internet. (We don't mean to brag, but we can use the credibility.) Where possible, we've included the real programs that we use in administration, many of them rewritten into Perl for speed and efficiency.

We hope that this book will help you get acquainted with DNS and BIND if you're just starting out, let you refine your understanding if you're already familiar with them, and provide valuable insight and experience even if you know 'em like the back of your hand.

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