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An Internet-connected Linux machine is in a high-risk situation. This book details security steps that a home or small-to-mid-size, non-enterprise business might take to protect itself from potential remote attackers. As with the first edition, this book will provide a description of the need for security measures and solutions built upon the most up-to-date technology available. The content for the Second Edition has been updated to cover the 2.4 kernel, and additional chapters on VPNs, SSH, and Tripwires have been added.
The term firewall has a number of meanings depending on its implementation and purpose. At this opening point in the book, firewall means the Internet-connected machine. This is where your security policies will be implemented. The firewall machine's external network interface card is the connection point, or gateway, to the Internet. The purpose of a firewall is to protect what's on your side of this gateway from what's on the other side.
A simple firewall setup is sometimes called a bastion firewall because it's the main line of defense against attack from the outside. All your security measures are mounted from this one defender of your realm. Consequently, everything possible is done to protect this system. It's your one and only bastion of defense.
Behind this line of defense is your single computer or your group of computers. The purpose of the firewall machine might simply be to serve as the connection point to the Internet for other machines on your LAN.You might be running local, private services behind this firewall, such as a shared printer or shared file systems. Or you might want all your computers to have access to the World Wide Web. One of your machines might host your private financial records.You might want to have Internet access from this machine, but you won't want anyone getting in. At some point, you might want to offer your own services to the Internet. One of the machines might be hosting your own Web site for the Internet.Your setup and goals will determine your security policies.
The firewall's purpose is to enforce the security policies you define.These policies reflect the decisions you've made about which Internet services you want to be accessible to your computers, which services you want to offer the world from your computers, which services you want to offer to specific remote users or sites, and which services and programs you want to run locally for your own private use. Security policies are all about access control and authenticated use of private or protected services, programs, and files on your computers.
Home and small business systems don't face all the security issues of a large, corporate site. The basic ideas and steps are the same. There just aren't so many factors to consider. The emphasis is on protecting your site from unwelcome access from the Internet. (A corporate site would emphasize internal security, which isn't much of an issue for a home-based site.) A packet-filtering firewall is one common approach toand one piece of-network security and controlling access from the outside.
Before going into the details of developing a firewall, this chapter introduces basic underlying concepts and mechanisms a packet-filtering firewall is based on. These concepts include a general frame of reference for what network communication is, how network-based services are identified, what a packet is, and the types of messages and information sent between computers on a network.
The TCP/IP Reference Networking Model
In order to provide a framework for this chapter, and for the rest of the book, I'm going to use a few terms before they're defined in the following sections of this chapter. The definitions are old hat to computer science and networking people, but they might be new to less technically inclined people. If this is all new to you, don't worry. Right now, I'm just trying to give you a conceptual place to hang the upcoming definitions on so that they make more sense.
If you've had formal academic training in networking, you're familiar with the Open Systems Interconnection (OSI) reference model.The OSI reference model was developed in the late 1970s and early 1980s to provide a framework for network interconnection standards. The OSI model is a formal, careful, academic model. Textbooks and academicians use this model as their conceptual framework when talking about networking.
Networking was taking off in the late 1970s and early 1980s, so the world went on during the seven years the OSI reference model was being hammered out. As TCP/IP became the de facto standard for Internet communication between UNIX machines during this time, a second, informal model called the TCP/IP reference model developed. Rather than being an academic ideal, the TCP/IP reference model is based on what manufacturers and developers finally came to agree upon for communication across the Internet. Because the model focuses on TCP/IP from a practical, real-world, developer's point of view, the model is simpler than the OSI model. So where OSI explicitly delineates seven layers, the TCP/IP model clumps them into four layers.
This book uses the TCP/IP reference model. As with most people with a computer science background, I tend to use the OSI vocabulary, but map it into the TCP/IP conceptual model.
Network communication is conceptualized as a layered model, with communication taking place between adjacent layers on an individual computer, and between parallel layers on communicating computers. The program you're running (e.g., your Web browser) is at the top, at the application layer, talking to another program on another computer (e.g., a Web server).
In order for your Web browser client application to send a request for a Web page to the Web server application, it has to make library and system calls that take the information from the Web browser and encapsulate it in a message suitable for transport between the two programs across the network.These messages are either transport-layer TCP segments or UDP datagrams. To construct these messages, the application layer calls the transport layer to provide this service.The transport-layer messages are sent between the Web browser client and the Web server. The transport layer knows how to deliver messages between a program on one computer and a program on the other end of the network. Both the OSI model and TCP/IP model call this layer the transport layer, although the OSI model breaks this layer into several different layers functionally.
In order for these transport-layer messages to be delivered between the two programs, the messages have to be sent between the two computers. To do this, the transport layer calls functions in the operating system that take the TCP or UDP transport message and encapsulate it in an Internet datagram suitable for sending to the other computer.These datagrams are IP packets.The IP Internet packets are sent between the two computers across the Internet. The Internet layer knows how to talk to the computer on the other other end of the network. The TCP/IP reference model calls this layer the Internet layer. The OSI reference model vocabulary is commonly used for this layer, so it's more commonly called the network layer. They are one and the same.
Beneath the network layer is the subnet layer. Again, the packet is encapsulated in an Ethernet header. At the subnet level, the message is now called an Ethernet frame. From the TCP/IP point of view, the subnet layer is a clump of everything that happens to get the IP packet delivered to the next computer.This clump includes all the addressing and delivery details associated with routing the frame between the computers...
|I||Packet-Filtering and Basic Security Measures||1|
|1||Preliminary Concepts Underlying Packet-Filtering Firewalls||3|
|3||ipatables: The Linux Firewall Administration Program||81|
|4||Building and Installing a Standalone Firewall||111|
|II||Advanced Issues, Multiple Firewalls, and Perimeter Networks||181|
|7||NAT - Network Address Translation||265|
|8||Debugging the Firewall Rules||285|
|III||System-Level Security and Monitoring||311|
|9||Verifying That the System Is Running as You Expect||313|
|10||Issues at the UNIX System Administration Level||331|
|11||Secure Shell (SSH)||387|
|13||Intrusion Detection and Incident Reporting Software||433|
|A: Security Resources||451|
|B||Firewall Examples and Support Scripts||461|
Posted July 29, 2002
This book makes the process of designing, understanding, and monitoring a linux-based firewall very easy. Anyone interested in building an inexpensive but very powerful, configurable firewall should STRONGLY consider this book, as it covers practically every aspect of firewall construction and configuration.Was this review helpful? Yes NoThank you for your feedback. Report this reviewThank you, this review has been flagged.
Posted February 21, 2001
I have some experence with firewalls and i can't belve how much i learned about how linux can handle the same jobs at some advance ( and verry expensive) firewalls. My only regret is that VPN was not covered in great depth but threre is a nother book for that some moer light reading will solve thatWas this review helpful? Yes NoThank you for your feedback. Report this reviewThank you, this review has been flagged.
Posted January 29, 2001