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What Are Firewalls

Firewalls are an integral part of any secure network. As we continue the discussion of the various security features and designs, it is important to take an in-depth look at how firewalls protect a network.

3 Part : 1 - Firewalls

              2 - Types of Firewalls

              3 - Positioning of Firewalls


The term firewall has many definitions in the industry. The definition depends on how and to what extent a firewall is used in a network. Generally, a firewall is a network device that, based on a defined network policy, implements access control for a network.
Apart from doing this basic job, firewalls are often used as network address translating devices, because they often tend to sit on the edge of a network and serve as entry points into the network. 

shows the basic philosophy of a firewall setup.

Some important characteristics distinguish a serious, industrial-strength firewall from other devices that go only halfway toward providing a true security solution are:
  • Logging and notification ability
  • High-volume packet inspection
  • Ease of configuration
  • Device security and redundancy

    Logging and Notification Ability

    A firewall is not much good unless it has a good logging facility. Good logging not only allows network administrators to detect if attacks are being orchestrated against their networks, but it also lets them detect if what is considered normal traffic originating from trusted users is being used for ungainly purposes. Good logging allows network administrators to filter much information based on traffic tagging and get to the stuff that really matters very quickly. Obviously, good logging is different from logging everything that happens.
    "Good logging" also refers to notification ability. Not only do you want the firewall to log the message, but you also want it to notify the administrator when alarm conditions are detected. Notification is often done by software that sorts through the log messages generated by the firewall device. Based on the criticality of the messages, the software generates notifications in the form of pages, e-mails, or other such means to notify a network administrator. The purpose of the notification is to let the administrator make a timely modification to either the configuration or the software image of the firewall itself to decrease the threat and impact of an attack or potential attack.

    High-Volume Packet Inspection

    One test of a firewall is its ability to inspect a large amount of network traffic against a configured set of rules without significantly degrading network performance. How much a firewall should be able to handle varies from network to network, but with today's demanding networks, a firewall should not become a bottleneck for the network it is sitting on. It is important to keep a firewall from becoming a bottleneck in a network because of its placement in the network. Firewalls are generally placed at the periphery of a network and are the only entry point into the network. Consequently, a slowdown at this critical place in the network can slow down the entire network.
    Various factors can affect the speed at which a firewall processes the data passing through it. Most of the limitations are in hardware processor speed and in the optimization of software code that keeps track of the connections being established through the firewall. Another limiting factor is the availability of the various types of interface cards on the firewall. A firewall that can support Gigabit Ethernet in a Gigabit Ethernet environment is obviously more useful than one that can only do Fast Ethernet in a faster network such as Gigabit Ethernet.
    One thing that often helps a firewall process traffic quickly is to offload some of the work to other software. This work includes notifications, URL filter-based access control, processing of firewall logs for filtering important information, and other such functions. These often-resource-intensive functions can take up a lot of the firewall's capacity and can slow it down.

    Ease of Configuration

    Ease of configuration includes the ability to set up the firewall quickly and to easily see configuration errors. Ease of configuration is very important in a firewall. The reason is that many network breaches that occur in spite of a firewall's being in place are not due to a bug in the firewall software or the underlying OS on which the firewall sits. They are due to an error in the firewall's configuration! Some of the "credit" for this goes to the person who configures the firewall. However, an easy-to-configure firewall mitigates many errors that might be produced in setting it up.
    It is important for a firewall to have a configuration utility that allows easy translation of the site security policy into the configuration. It is very useful to have a graphical representation of the network architecture as part of the configuration utility to avoid common configuration errors. Similarly, the terminology used in the configuration utility needs to be in synch with normally accepted security site topological nomenclature, such as DMZ zones, high-security zones, and low-security zones. Use of ambiguous terminology in the configuration utility can cause human error to creep in.
    Centralized administrative tools that allow for the simultaneous management of multiple security devices, including firewalls, are very useful for maintaining uniformly error-free configurations.

    Device Security and Redundancy

    The security of the firewall device itself is a critical component of the overall security that a firewall can provide to a network. A firewall that is insecure itself can easily allow intruders to break in and modify the configuration to allow further access into the network. There are two main areas where a firewall needs to have strength in order to avoid issues surrounding its own security:
  • The security of the underlying operating system- If the firewall software runs on a separate operating system, the vulnerabilities of that operating system have the potential to become the vulnerabilities of the firewall itself. It is important to install the firewall software on an operating system known to be robust against network security threats and to keep patching the system regularly to fill any gaps that become known.
  • Secure access to the firewall for administrative purposes- It is important for a firewall to have secure mechanisms available for allowing administrative access to it. Such methods can include encryption coupled with proper authentication mechanisms. Weakness in the implementation of such access mechanisms can allow the firewall to become an easy target for intrusions of various kinds.

    An issue related to device security is the firewall's ability to have a redundant presence with another firewall in the network. Such redundancy allows the backup device to take up the operations of a faulty primary device. In the case of an attack on the primary device that leaves it nonoperational, redundancy also allows for continued operation of the network.

    Types of Firewalls

    In order to gain a thorough understanding of firewall technology, it is important to understand the various types of firewalls. These various types of firewalls provide more or less the same functions that were outlined earlier. However, their methods of doing so provide differentiation in terms of performance and level of security offered.
    The firewalls discussed in this section are divided into five categories based on the mechanism that each uses to provide firewall functionality:
  • Circuit-level firewalls
  • Proxy server firewalls
  • Nonstateful packet filters
  • Stateful packet filters
  • Personal firewalls
These various types of firewalls gather different types of information from the data flowing through them to keep track of legitimate and illegitimate traffic and to protect against unauthorized access. The type of information they use often also determines the level of security they provide.

Circuit-Level Firewalls

These firewalls act as relays for TCP connections. They intercept TCP connections being made to a host behind them and complete the handshake on behalf of that host. Only after the connection is established is the traffic allowed to flow to the client. Also, the firewall makes sure that as soon as the connection is established, only data packets belonging to the connection are allowed to go through.
Circuit-level firewalls do not validate the payload or any other information in the packet, so they are fairly fast. These firewalls essentially are interested only in making sure that the TCP handshake is properly completed before a connection is allowed. Consequently, these firewalls do not allow access restrictions to be placed on protocols other than TCP and do not allow the use of payload information in the higher-layer protocols to restrict access.

Proxy Server Firewalls

Proxy server firewalls work by examining packets at the application layer. Essentially a proxy server intercepts the requests being made by the applications sitting behind it and performs the requested functions on behalf of the requesting application. It then forwards the results to the application. In this way it can provide a fairly high level of security to the applications, which do not have to interact directly with outside applications and servers.
Proxy servers are advantageous in the sense that they are aware of application-level protocols and they can restrict or allow access based on these protocols. They also can look into the data portions of the packets and use that information to restrict access. However, this very capability of processing the packets at a higher layer of the stack can contribute to the slowness of proxy servers. Also, because the inbound traffic has to be processed by the proxy server as well as the end-user application, further degradation in speed can occur. Proxy servers often are not transparent to end users who have to make modifications to their applications in order to use the proxy server. For each new application that must go through a proxy firewall, modifications need to be made to the firewall's protocol stack to handle that type of application.

Nonstateful Packet Filters

Nonstateful packet filters are fairly simple devices that sit on the periphery of a network and, based on a set of rules, allow some packets through while blocking others. The decisions are made based on the addressing information contained in network layer protocols such as IP and, in some cases, information contained in transport layer protocols such as TCP or UDP headers as well.
Nonstateful packet filters are fairly simple devices, but to function properly they require a thorough understanding of the usage of services required by a network to be protected. Although these filters can be fast because they do not proxy any traffic but only inspect it as it passes through, they do not have any knowledge of the application-level protocols or the data elements in the packet. Consequently, their usefulness is limited. These filters also do not retain any knowledge of the sessions established through them. Instead, they just keep tabs on what is immediately passing through.. The use of simple and extended access lists (without the established keyword) on routers are examples of such firewalls.

Stateful Packet Filters

Stateful packet filters are more intelligent than simple packet filters in that they can block pretty much all incoming traffic and still can allow return traffic for the traffic generated by machines sitting behind them. They do so by keeping a record of the transport layer connections that are established through them by the hosts behind them.
Stateful packet filters are the mechanism for implementing firewalls in most modern networks. Stateful packet filters can keep track of a variety of information regarding the packets that are traversing them, including the following:
  • Source and destination TCP and UDP port numbers
  • TCP sequence numbering
  • TCP flags
  • TCP session state based on the RFCed TCP state machine
  • UDP traffic tracking based on timers
Stateful firewalls often have built-in advanced IP layer handling features such as fragment reassembly and clearing or rejecting of IP options.
Many modern stateful packet filters are aware of application layer protocols such as FTP and HTTP and can perform access-control functions based on these protocols' specific needs.

Personal Firewalls

Personal firewalls are firewalls installed on personal computers. They are designed to protect against network attacks. These firewalls are generally aware of the applications running on the machine and allow only connections established by these applications to operate on the machine.
A personal firewall is a useful addition to any PC because it increases the level of security already offered by a network firewall. However, because many of the attacks on today's networks originate from inside the protected network, a PC firewall is an even more useful tool, because network firewalls cannot protect against these attacks. Personal firewalls come in a variety of flavors. Most are implemented to be aware of the applications running on the PC. However, they are designed to not require any changes from the user applications running on the PC, as is required in the case of proxy servers.


Positioning of Firewalls

Positioning a firewall is as important as using the right type of firewall and configuring it correctly. Positioning a firewall determines which traffic will be screened and whether there are any back doors into the protected network. Some of the basic guidelines for positioning a firewall are as follows:
  • Topological location of the firewall- It is often a good idea to place a firewall on the periphery of a private network, as close to the final exit and initial entry point into the network as possible. The network includes any remote-access devices and VPN concentrators sitting on the its periphery. This allows the greatest number of devices on the private network to be protected by the firewall and also helps keep the boundary of the private and public network very clear. A network in which there is ambiguity as to what is public and what is private is a network waiting to be attacked.
    Certain situations might also warrant placing a firewall within a private network in addition to placing a firewall at the entry point. An example of such a situation is when a critical segment of the network, such as the segment housing the financial or HR servers, needs to be protected from the rest of the users on the private network.
    Also, in most cases firewalls should not be placed in parallel to other network devices such as routers. This can cause the firewall to be bypassed. You should also avoid any other additions to the network topology that can result in the firewall's getting bypassed.
  • Accessibility and security zones- If there are servers that need to be accessed from the public network, such as Web servers, it is often a good idea to put them in a demilitarized zone (DMZ) built on the firewall rather than keep them inside the private network. The reason for this is that if these servers are on the internal network and the firewall has been asked to allow some level of access to these servers from the public network, this access opens a door for attackers. They can use this access to gain control of the servers or to stage attacks on the private network using the access holes created in the firewall. A DMZ allows publicly accessible servers to be placed in an area that is physically separate from the private network, forcing the attackers who have somehow gained control over these servers to go through the firewall again to gain access to the private network.
  • Asymmetric routing- Most modern firewalls work on the concept of keeping state information for the connections made through them from the private network to the public network. This information is used to allow only the packets belonging to the legitimate connections back into the private network. Consequently, it is important that the exit and entry points of all traffic to and from the private network be through the same firewall. If this is not the case, a firewall may drop packets belonging to legitimate connections started from the internal network for which it has no state information. This scenario is known as asymmetric routing.
  • Layering firewalls- In networks where a high degree of security is desired, often two or more firewalls can be deployed in series. If the first firewall fails, the second one can continue to function. This technique is often used as a safeguard against network attacks that exploit bugs in a firewall's software. If one firewall's software is vulnerable to an attack, hopefully the software of the second firewall sitting behind it will not be. Firewalls from different vendors are often used in these setups to ensure that one incorrect or compromised implementation can be backed up by the other vendor's implementation.
Positioning a firewall can be a complicated issue in a large network with multiple subsegments and entry points. Often a network that has not used a firewall in the past needs to be restructured to allow a firewall to be placed properly to protect it. This is necessary to create a single point of entry and exit and to remove the issue of asymmetric routing.


Firewalls are a critical component of any secure network. Firewalls in one form or another provide restricted access to a network based on a defined security policy. In order to make the best use of a firewall's capabilities, however, it is critical to position it in the network where it can provide the most security coverage possible.

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