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Network Security First-Step: Firewalls

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Feb 8, 2012.

Chapter Description

This chapter dissects a firewall’s duties to understand what makes a firewall operate and how it does its job.

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Firewall Operational Overview

Every long journey begins with the first step. Before delving too deeply into other areas of security appliance behavior, it is essential to understand how a firewall performs its magic.

Most firewalls (most, not all) rely on Stateful Packet Inspection (SPI) to keep track of all outbound packets and the responses these packets might generate. Keeping track of the hosts on the protected network that are generating outbound packets keeps rogue or unsolicited WAN packets from entering an external interface.

In other words, a firewall that uses SPI, as discussed in Chapter 5, “Overview of Security Technologies,” watches all traffic that originates from an inside host, tracks the conversation from that host to the desired destination, and ensures that the inbound response to that request makes it back to the host that started the whole thing in the first place.

The critical dual purposes of packet inspection and filtering (blocking) of packets is one of the most fundamental responsibilities of a firewall. The following list includes the most common rules and features of firewalls:

  • Filter incoming network traffic based on source or destination: Blocking unwanted incoming traffic is the most common feature of a firewall and is the main reason for a firewall—stopping unwanted traffic from entering your network. This unwanted traffic is usually from attackers, thus the need to keep it out.
  • Filter outgoing network traffic based on source or destination: Many firewalls can also screen network traffic from your internal network to the Internet. For example, you might want to prevent employees from accessing inappropriate websites. You might also place a firewall between your network and a business partner with rules to keep each of you safe.
  • Filter network traffic based on content: More advanced firewalls can screen network traffic for unacceptable content. For example, a firewall integrated with a virus scanner can prevent files that contain viruses from entering your network. Other firewalls integrate with email services to screen out unacceptable email.
  • Detect and filter malware: The rise and proliferation of botnets and malware have driven firewall manufacturers to implement features designed to detect infected hosts through packet inspections. This is a good example of how security is ever changing and the security of the network must continue to advance as well because what was secure yesterday might not be tomorrow.
  • Make internal resources available: Although the primary purpose of a firewall is to prevent unwanted network traffic from passing through it, you can also configure many firewalls to enable selective access to internal resources, such as a public web server, while still preventing other access from the Internet to your internal network. In many cases, you can accomplish this by using a DMZ, which is where the public web server would be located. (DMZs are discussed later in the section “Essentials First: Life in the DMZ.”)
  • Allow connections to internal network: A common method for employees to connect to a network is using virtual private networks (VPN). VPNs enable secure connections from the Internet to a corporate network. For example, telecommuters and traveling employees can use a VPN to connect to the corporate network. VPNs can also connect branch offices to each other over the Internet, saving on WAN costs.
  • Report on network traffic and firewall activities: When screening network traffic to and from the Internet, you need to know what your firewall is doing, who tried to break in to your network, and who tried to access inappropriate material on the Internet. Most firewalls include a reporting mechanism of some kind. A good firewall can also log activity to a syslog or other type of archival storage receptacle. Perusing firewall logs after an attack occurs is one of a number of forensic tools you have at your disposal.

Firewalls in Action

These might be new concepts for you, and hopefully you are not thoroughly confused at this point. Look at Figure 7-2 for a bit more clarity of this process. Please refer to the list, which explains the steps a bit more in depth.

Figure 7-2

Figure 7-2 Firewall in Operation

Before looking at the list of steps, you need to know that many firewalls have only two physical interfaces, and 99 percent of them are based on Ethernet. These interfaces are called inside (protected) and outside (unprotected) and are deployed in relation to your network; some have DMZ interfaces as well. Thus, in practice, the outside interface connects to the Internet and the inside interface connects to your internal network:

Figure 7-2 shows a high-level view of the following:

  1. Host A is an Apple Macbook Pro that opens a web browser and wants to view a web page from the www.avoidwork.com web server. This action causes Host A to send the request to view this web page out through the firewall across the Internet and to the web server.
  2. The firewall sees the request originated with Host A and is destined for www.avoidwork.com.

    1. The firewall records (tracks) the outbound request and expects that the reply will come only from the www.avoidwork.com web server.
    2. A session marker is placed in the firewall’s session state table that tracks the communication process from start to finish.
    3. Connection metrics, such as time opened and so forth, are also placed with the marker in the session state table record maintained by the firewall for this conversation.
  3. The Avoidwork.com web server replies to the web page request from Host A, which is then transmitted back through the Internet and to the firewall.
  4. The firewall checks its session state table to see whether the metrics being maintained for this session match the outbound connection. If all the stored connection details match exactly, the firewall enables the inbound traffic.

The information contained in the firewall’s state table records and tracks information such as who needed www information from the avoidwork.com server, when they asked for it, how they asked for it, and so forth. This provides an added level of protection over and above the “can I enter or not” rules because if a certain traffic type is allowed in but the host did not ask for it (attack), it’s denied. Because a firewall maintains connection state information about inbound and outbound connections, the possibility of a hacker “spoofing” or “forging” a packet with the intention of penetrating your network becomes more difficult. When attackers try to send packets to get through a firewall, incorrect or missing connection state information means that the session is terminated and most likely logged for later review.

Implementing a Firewall

The choice of firewalls is almost mind-boggling these days; they come in every shape, size, and capacity. When I am designing a firewall solution for a customer, the first thing I want to know is what will the firewall’s responsibilities be?

The type of firewall you install depends on your exact requirements for protection and management, and the size of your network, or what is to be protected by the firewall. Firewalls usually fall into one of the following categories:

  • Personal firewall: A personal firewall is usually a piece of software installed on a single PC to protect only that PC. These types of firewalls are usually deployed on home PCs with broadband connections or remote employees. Of course, any time someone wants to deploy a firewall, it is a good idea. You can find some of the more well-known personal firewalls at these websites:



    Operating system manufacturers such as Apple and Microsoft have responded to this need by integrating personal firewalls within them. Apple’s OS X comes with an IP firewall and Windows has a similar firewall, it is just not as secure as the one in OS X. Most antivirus companies have expanded their products to include all sorts of protection through the use of their product suites.

  • All-in-one firewall/routers: These kinds of firewalls are widely used by broadband (cable or DSL) subscribers who have the benefit of a single device that offers the following features and functionality: router, Ethernet switch, wireless access point, and a firewall. If this type of firewall appeals to you, ensure that you take care to determine the firewall’s capabilities, and be skeptical of the security you can gain from these devices, regardless of who makes them. WARNING: Do not be tricked into assuming that a home router has a good firewall built into it; do your research first. I especially advise people to check on how the manufacturer supports what it makes; for example, if it does not take phone calls, you might want to continue shopping.
  • Small-to-medium office firewalls: These firewalls, such as the Cisco ASA 5505 and 5510 or the older PIX 501 and 506, are designed to provide security and protection for small office home office (SOHO) types of requirements. In most cases, they have expansion slots allowing for additional network connections or advanced feature cards to be installed.
  • Enterprise firewalls: These firewalls, such as the Cisco ASA 5520 and up, are designed for larger organizations with thousands of users. These larger models are needed when there are demands for larger numbers of connections, capacity, and features. As a result, they have additional features and capacity, such as more memory and extra interfaces along with slots for advanced feature cards to be added. An example in some cases would be an IPS module.

Normally, a firewall is installed where your internal network connects to the Internet. Although larger organizations also place firewalls between different parts of their internal network that require different levels of security, most firewalls are placed to screen traffic passing between an internal network and the Internet. For example, if a large organization enables business partners to connect directly to its network, you typically find a firewall controlling what is allowed into its network from the partners. This placement of an internal firewall is definitely considered best practice.

Determine the Inbound Access Policy

As network traffic passes through a firewall, the traffic is subject to the rules defined within the firewall. Because 99 percent of all networks use private IP addresses on the inside of their networks, you can expect almost every firewall to be using Network Address Translation (NAT)—as discussed in Chapter 5.

If all your LAN traffic were destined for the Internet, the inbound access policy would be straightforward in its design. The firewall permits only inbound traffic in response to requests from hosts on the internal LAN. The firewall tracks all outbound requests in its state table, as previously discussed.

However, there will come a time when specific requests from the outside must be allowed and controlled through the firewall. Notice that we did not say that this was a good idea or that you should do it, we are just acknowledging that it’s a business function that a security professional must support.

Allowing direct access from the Internet (outside) through your firewall is perilous but common practice. The key to security in these types of implementations is to strictly define the traffic types you will allow and the port number. For example, permitting IP to any location inside your network is inappropriate. For example, you should permit only inbound traffic from the Internet HTTP (port 80) traffic to your web server (IP address: Allowing only HTTP (port 80) traffic to the web server from the Internet is much smarter than allowing every kind of TCP/IP protocol and port.

A strongly recommended best practice is to add layers of security in the form of a personal firewall, intrusion detection system (IDS), and antivirus software. Also, before you implement these devices as layers, make sure your security policies outline the best practices and what steps are needed to maintain security. A layered security model should be used to protect your network; the more layers, the harder it is for an attacker to penetrate your network. The use of layers is sort of like the joke told between hunters. When you see a hungry and angry bear in the woods start to charge you, as you begin to run remember you do not have to be faster than the bear, just faster than the other hunter! Layering network security definitely helps make your network less appealing than your competitors. Another layer would be to integrate an IPS in a firewall, making a layered defense.

Determine Outbound Access Policy

All firewalls screen traffic coming into a firewall from the Internet, but a well-implemented and designed firewall also screens outgoing user traffic. Spoiled employees are not going to like this, but the truth of the matter is that companies pay for Internet connections in support of their business, NOT to let employees surf, watch video, stream music, or look at pictures they are not supposed to.

You might also want to use your firewall to control what IP addresses are allowed to exit; specifically, you should allow only IP addresses that are found on your internal network out, thus preventing spoofing of IP addresses.

Perhaps there are also certain places on the Internet where you do not want users to go. Alternatively, you might want to specify the locations they are allowed to go because every other destination will be denied by default. Recall the earlier discussion of proxy servers and how they can be used to control and monitor traffic that leaves your network. They are a good example of a device that defines an outbound access policy. Remember, employees and contractors are bound to rules, whether they be policies or service-level agreements (SLA), and good behavior is not optional—it’s mandatory—and so are accurate logging and event correlation.

In addition, recall the earlier discussion about placing a firewall between your network and connections to business partners. This type of firewall usage and placement is also where you would apply and control traffic bound from your network to theirs. The next section looks at the next aspect of firewall and network security: the Demilitarized Zone (DMZ).

5. Essentials First: Life in the DMZ | Next Section Previous Section

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