Chapter 1 is a brief introduction to networking concepts. It covers the OSI and TCP/IP networking models with explanations of their layers, TCP and UDP as Layer 4 protocols, and then rounds off the chapter with a discussion on IP addresses, Subnetting, and Supernetting.

Chapter 2 discusses possible security threats and vulnerabilities found at each of the OSI layers. The goal here is to understand where and how these threats can affect us and to stay protected from attackers. It then rounds off the discussion by sketching out the basic steps required to protect the services that run on our system.

Chapter 3 introduces two tools needed to build Linux firewalls and QoS. We first learn the workings of netfilter, which is a packet-filtering framework, and implement what we have learned to build a basic firewall for a Linux workstation. We then see how to perform advanced routing and traffic shaping using the IP and TC tools provided by the iproute2 package. The chapter ends with another example scenario where we implement the concepts learned in the chapter.

Chapter 4 discusses NAT, the types of NAT, how they work, and how they can be implemented with Linux by giving practical examples. It also describe packet mangling, when to use it, and why to use it.

Chapter 5 covers Layer 7 filtering in detail. We see how to install the L7-filter package, apply the necessary Linux kernel and iptables patches, and test our installation. We then learn the different applications of L7-filter and see how to put them to practical use. We also see how to install and use IPP2P, which is an alternative to the L7-filter package, but only for P2P traffic, and finally we set up a test between the two packages.

Chapter 6 raises two very popular scenarios, for which we design, implement, and test firewalls and a small QoS configuration. In the first scenario, we configure Linux as a SOHO router. Being a relatively smaller network with few devices, we learn how to adapt to what we have learned in the earlier chapters to suit this environment and build a secure network. We implement transparent proxies using squid and iptables so that children/minors cannot access malicious or pornographic web content. Our firewall setup implements NAT to redirect traffic from certain ports to other hosts using Linux. This configuration is tested by checking the NAT table and seeing how the kernel analyzes our rules.

As part of QoS, we split the bandwidth between the devices in a SOHO environment using HTB. Assuming a 1Mbps connection, we design a policy to split it between the 4 devices creating 4 HTB child classes for the 4 devices. In the end, we test our QoS configuration using the tc class show command.

In the second scenario, we configure Linux as router for a typical small to medium company.

Chapter 7 covers the design of a firewall system for a hypermarket having its headquarters in one location, one store in the same city, and several stores in other cities. The hypermarket has an application that uses MSSQL databases in each location, which are replicated at the headquarters. All locations have IP Analog Telephone Adapters with subscriptions at the main provider (the HQ provider). In this example we use, just like in the real H.323 as the VoIP protocol. We set up all remote locations to have an encrypted VPN connection using ip tunnel to connect to the headquarters. Users are shown how to create a QOS script with HTB that controls bandwidth usage based on priorities.

The next firewall taken up is that for a small ISP setup that has one internet connection, an access network, a server farm, and the internal departments. The setup of firewall scripts for each of them and methods to handle the tricky wireless server are covered. The QoS is handled by the intranet server, the wireless server, and the Core router.

Chapter 8 covers the design of a three-layered network deployed at a large provider of Internet and IP telephony services, the three layers being Core, Distribution, and Access. It explains network configuration first on the core and distribution levels and then moves on to building firewalls. The huge size of the network also means that there is a need to tackle newer security threats. We have four Cores running BGP under Zebra and each one is peculiar in its own way. There are three data services that this ISP can provide to its customers: Internet access, national network access, and metropolitan network access. This chapter will show you how to handle QoS so as to limit this traffic as needed.

In this book, you will find a number of styles of text that distinguish between different kinds of information. Here are some examples of these styles, and an explanation of their meaning.

There are three styles for code. Code words in text are shown as follows: "To limit upload, we will mark packets in the PREROUTING chain of the mangle table".

A block of code will be set as follows:

#Drop SSH packets except from admins
$IPT -A INPUT -s ! 1.2.3.16/28 -p tcp --dport 22 -j DROP

When we wish to draw your attention to a particular part of a code block, the relevant lines or items will be made bold:

tc filter add dev eth0 protocol ip parent 1:0 prio 5 u32 match ip src 1.2.3.34 flowid 1:100

New terms and important words are introduced in a bold-type font. Words that you see on the screen, in menus or dialog boxes for example, appear in our text like this: "In the IP: Netfilter Configuration section you will find the options needed for NAT".

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