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Security in Networks

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This chapter is from the book

This chapter is from the book


Firewalls were officially invented in the early 1990s, but the concept really reflects the reference monitor (described in Chapter 5) from two decades earlier. The first reference to a firewall by that name may be [RAN92]; other early references to firewalls are the Trusted Information Systems firewall toolkit [RAN94] and the book by Cheswick and Bellovin [CHE94, updated as CHE02].

What Is a Firewall?

A firewall is a device that filters all traffic between a protected or "inside" network and a less trustworthy or "outside" network. Usually a firewall runs on a dedicated device; because it is a single point through which traffic is channeled, performance is important, which means nonfirewall functions should not be done on the same machine. Because a firewall is executable code, the attacker could compromise that code and execute from the firewall's device. Thus, the fewer pieces of code on the device, the fewer tools the attacker would have by compromising the firewall. Firewall code usually runs on a proprietary or carefully minimized operating system.

The purpose of a firewall is to keep "bad" things outside a protected environment. To accomplish that, firewalls implement a security policy that is specifically designed to address what bad things might happen. For example, the policy might be to prevent any access from outside (while still allowing traffic to pass from the inside to the outside). Alternatively, the policy might permit accesses only from certain places, from certain users, or for certain activities. Part of the challenge of protecting a network with a firewall is determining which security policy meets the needs of the installation.

People in the firewall community (users, developers, and security experts) disagree about how a firewall should work. In particular, the community is divided about a firewall's default behavior. We can describe the two schools of thought as "that which is not expressly forbidden is permitted" (default permit) and "that which is not expressly permitted is forbidden" (default deny). Users, always interested in new features, prefer the former. Security experts, relying on several decades of experience, strongly counsel the latter. An administrator implementing or configuring a firewall must choose one of the two approaches, although the administrator can often broaden the policy by setting the firewall's parameters.

Design of Firewalls

Remember from Chapter 5 that a reference monitor must be

  • always invoked

  • tamperproof

  • small and simple enough for rigorous analysis

A firewall is a special form of reference monitor. By carefully positioning a firewall within a network, we can ensure that all network accesses that we want to control must pass through it. This restriction meets the "always invoked" condition. A firewall is typically well isolated, making it highly immune to modification. Usually a firewall is implemented on a separate computer, with direct connections only to the outside and inside networks. This isolation is expected to meet the "tamperproof" requirement. And firewall designers strongly recommend keeping the functionality of the firewall simple.

Types of Firewalls

Firewalls have a wide range of capabilities. Types of firewalls include

  • packet filtering gateways or screening routers

  • stateful inspection firewalls

  • application proxies

  • guards

  • personal firewalls

Each type does different things; no one is necessarily "right" and the others "wrong." In this section, we examine each type to see what it is, how it works, and what its strengths and weaknesses are. In general, screening routers tend to implement rather simplistic security policies, whereas guards and proxy gateways have a richer set of choices for security policy. Simplicity in a security policy is not a bad thing; the important question to ask when choosing a type of firewall is what threats an installation needs to counter.

Because a firewall is a type of host, it often is as programmable as a good-quality workstation. While a screening router can be fairly primitive, the tendency is to host even routers on complete computers with operating systems because editors and other programming tools assist in configuring and maintaining the router. However, firewall developers are minimalists: They try to eliminate from the firewall all that is not strictly necessary for the firewall's functionality. There is a good reason for this minimal constraint: to give as little assistance as possible to a successful attacker. Thus, firewalls tend not to have user accounts so that, for example, they have no password file to conceal. Indeed, the most desirable firewall is one that runs contentedly in a back room; except for periodic scanning of its audit logs, there is seldom reason to touch it.

Packet Filtering Gateway

A packet filtering gateway or screening router is the simplest, and in some situations, the most effective type of firewall. A packet filtering gateway controls access to packets based on packet address (source or destination) or specific transport protocol type (such as HTTP web traffic). As described earlier in this chapter, putting ACLs on routers may severely impede their performance. But a separate firewall behind (on the local side) of the router can screen traffic before it gets to the protected network. Figure 7-35 shows a packet filter that blocks access from (or to) addresses in one network; the filter allows HTTP traffic but blocks traffic using the Telnet protocol.

Figure 7-35Figure 7-35 Packet Filter Blocking Addresses and Protocols.

For example, suppose an international company has three LANs at three locations throughout the world, as shown in Figure 7-36. In this example, the router has two sides: inside and outside. We say that the local LAN is on the inside of the router, and the two connections to distant LANs through wide area networks are on the outside. The company might want communication only among the three LANs of the corporate network. It could use a screening router on the LAN at to allow in only communications destined to the host at and to allow out only communications addressed either to address or

Figure 7-36Figure 7-36 Three Connected LANs.

Packet filters do not "see inside" a packet; they block or accept packets solely on the basis of the IP addresses and ports. Thus, any details in the packet's data field (for example, allowing certain Telnet commands while blocking other services) is beyond the capability of a packet filter.

Packet filters can perform the very important service of ensuring the validity of inside addresses. Inside hosts typically trust other inside hosts for all the reasons described as characteristics of LANs. But the only way an inside host can distinguish another inside host is by the address shown in the source field of a message. Source addresses in packets can be forged, so an inside application might think it was communicating with another host on the inside instead of an outside forger. A packet filter sits between the inside network and the outside net, so it can know if a packet from the outside is forging an inside address, as shown in Figure 7-37. A screening packet filter might be configured to block all packets from the outside that claimed their source address was an inside address. In this example, the packet filter blocks all packets claiming to come from any address of the form 100.50.25.x (but, of course, it permits in any packets with destination 100.50.25.x).

Figure 7-37Figure 7-37 Filter Screening Outside Addresses.

The primary disadvantage of packet filtering routers is a combination of simplicity and complexity. The router's inspection is simplistic; to perform sophisticated filtering, the filtering rules set needs to be very detailed. A detailed rules set will be complex and therefore prone to error. For example, blocking all port 23 traffic (Telnet) is simple and straightforward. But if some Telnet traffic is to be allowed, each IP address from which it is allowed must be specified in the rules; in this way, the rule set can become very long.

Stateful Inspection Firewall

Filtering firewalls work on packets one at a time, accepting or rejecting each packet and moving on to the next. They have no concept of "state" or "context" from one packet to the next. A stateful inspection firewall maintains state information from one packet to another in the input stream.

One classic approach used by attackers is breaking an attack into multiple packets by forcing some packets to have very short lengths so that a firewall will not be able to detect the signature of an attack split across two or more packets. (Remember that with the TCP protocols, packets can arrive in any order, and the protocol suite is responsible for reassembling the packet stream in proper order before passing it along to the application.) A stateful inspection firewall would track the sequence of packets and conditions from one packet to another to thwart such an attack.

Application Proxy

Packet filters look only at the headers of packets, not at the data inside the packets. Therefore, a packet filter would pass anything to port 25, assuming its screening rules allow inbound connections to that port. But applications are complex and sometimes contain errors. Worse, applications (such as the e-mail delivery agent) often act on behalf of all users, so they require privileges of all users (for example, to store incoming mail messages so that inside users can read them). A flawed application, running with all users' privileges, can cause much damage.

An application proxy gateway, also called a bastion host, is a firewall that simulates the (proper) effects of an application so that the application will receive only requests to act properly. A proxy gateway is a two-headed device: It looks to the inside as if it is the outside (destination) connection, while to the outside it responds just as the insider would.

An application proxy runs pseudoapplications. For instance, when electronic mail is transferred to a location, a sending process at one site and a receiving process at the destination communicate by a protocol that establishes the legitimacy of a mail transfer and then actually transfers the mail message. The protocol between sender and destination is carefully defined. A proxy gateway essentially intrudes in the middle of this protocol exchange, seeming like a destination in communication with the sender that is outside the firewall, and seeming like the sender in communication with the real destination on the inside. The proxy in the middle has the opportunity to screen the mail transfer, ensuring that only acceptable e-mail protocol commands are sent to the destination.

As an example of application proxying, consider the FTP (file transfer) protocol. Specific protocol commands fetch (get) files from a remote location, store (put) files onto a remote host, list files (ls) in a directory on a remote host, and position the process (cd) at a particular point in a directory tree on a remote host. Some administrators might want to permit gets but block puts, and to list only certain files or prohibit changing out of a particular directory (so that an outsider could retrieve only files from a prespecified directory). The proxy would simulate both sides of this protocol exchange. For example, the proxy might accept get commands, reject put commands, and filter the local response to a request to list files.

To understand the real purpose of a proxy gateway, let us consider several examples.

  • A company wants to set up an online price list so that outsiders can see the products and prices offered. It wants to be sure that (a) no outsider can change the prices or product list and (b) outsiders can access only the price list, not any of the more sensitive files stored inside.

  • A school wants to allow its students to retrieve any information from World Wide Web resources on the Internet. To help provide efficient service, it wants to know what sites have been visited, and what files from those sites have been fetched; particularly popular files will be cached locally.

  • A government agency wants to respond to queries through a database management system. However, because of inference attacks against databases, the agency wants to restrict queries that return the mean of a set of fewer than five values.

  • A company with multiple offices wants to encrypt the data portion of all e-mail to addresses at its other offices. (A corresponding proxy at the remote end will remove the encryption.)

  • A company wants to allow dial-in access by its employees, without exposing its company resources to login attacks from remote nonemployees.

Each of these requirements can be met with a proxy. In the first case, the proxy would monitor the file transfer protocol data to ensure that only the price list file was accessed, and that file could only be read, not modified. The school's requirement could be met by a logging procedure as part of the web browser. The agency's need could be satisfied by a special-purpose proxy that interacted with the database management system, performing queries but also obtaining the number of values from which the response was computed and adding a random minor error term to results from small sample sizes. The requirement for limited login could be handled by a specially written proxy that required strong user authentication (such as a challenge–response system), which many operating systems do not require. These functions are shown in Figure 7-38.

Figure 7-38Figure 7-38 Actions of Firewall Proxies.

The proxies on the firewall can be tailored to specific requirements, such as logging details about accesses. They can even present a common user interface to what may be dissimilar internal functions. Suppose the internal network has a mixture of operating system types, none of which support strong authentication through a challenge– response token. The proxy can demand strong authentication (name, password, and challenge–response), validate the challenge–response itself, and then pass on only simple name and password authentication details in the form required by a specific internal host's operating system.

The distinction between a proxy and a screening router is that the proxy interprets the protocol stream to an application, to control actions through the firewall on the basis of things visible within the protocol, not just on external header data.


A guard is a sophisticated firewall. Like a proxy firewall, it receives protocol data units, interprets them, and passes through the same or different protocol data units that achieve either the same result or a modified result. The guard decides what services to perform on the user's behalf in accordance with its available knowledge, such as whatever it can reliably know of the (outside) user's identity, previous interactions, and so forth. The degree of control a guard can provide is limited only by what is computable. But guards and proxy firewalls are similar enough that the distinction between them is sometimes fuzzy. That is, we can add functionality to a proxy firewall until it starts to look a lot like a guard.

Guard activities can be quite sophisticated, as illustrated in the following examples:

  • A university wants to allow its students to use e-mail up to a limit of so many messages or so many characters of e-mail in the last so many days. Although this result could be achieved by modifying e-mail handlers, it is more easily done by monitoring the common point through which all e-mail flows, the mail transfer protocol.

  • A school wants its students to be able to access the World Wide Web but, because of the slow speed of its connection to the web, it will allow only so many characters per downloaded image (that is, allowing text mode and simple graphics, but disallowing complex graphics, animation, music, or the like).

  • A library wants to make available certain documents but, to support fair use of copyrighted matter, it will allow a user to retrieve only the first so many characters of a document. After that amount, the library will require the user to pay a fee that will be forwarded to the author.

  • A company wants to allow its employees to fetch files via ftp. However, to prevent introduction of viruses, it will first pass all incoming files through a virus scanner. Even though many of these files will be nonexecutable text or graphics, the company administrator thinks that the expense of scanning them (which should pass) will be negligible.

Each of these scenarios can be implemented as a modified proxy. Because the proxy decision is based on some quality of the communication data, we call the proxy a guard. Since the security policy implemented by the guard is somewhat more complex than the action of a proxy, the guard's code is also more complex and therefore more exposed to error. Simpler firewalls have fewer possible ways to fail or be subverted.

Personal Firewalls

Firewalls typically protect a (sub)network of multiple hosts. University students and employees in offices are behind a real firewall. Increasingly, home users, individual workers, and small businesses use cable modems or DSL connections with unlimited, always-on access. These people need a firewall, but a separate firewall computer to protect a single workstation can seem too complex and expensive. These people need a firewall's capabilities at a lower price.

A personal firewall is an application program that runs on a workstation to block unwanted traffic, usually from the network. A personal firewall can complement the work of a conventional firewall by screening the kind of data a single host will accept, or it can compensate for the lack of a regular firewall, as in a private DSL or cable modem connection.

Just as a network firewall screens incoming and outgoing traffic for that network, a personal firewall screens traffic on a single workstation. A workstation could be vulnerable to malicious code or malicious active agents (ActiveX or Java applets), leakage of personal data stored on the workstation, and vulnerability scans to identify potential weaknesses. Commercial implementations of personal firewalls include Norton Personal Firewall from Symantec, McAfee Personal Firewall, and Zone Alarm from Zone Labs.

The personal firewall is configured to enforce some policy. For example, the user may decide that certain sites, such as computers on the company network, are highly trustworthy, but most other sites are not. The user defines a policy permitting download of code, unrestricted data sharing, and management access from the corporate segment, but not from other sites. Personal firewalls can also generate logs of accesses, which can be useful to examine in case something harmful does slip through the firewall.

Combining a virus scanner with a personal firewall is both effective and efficient. Typically, users forget to run virus scanners daily, but they do remember to run them occasionally, such as sometime during the week. However, leaving the virus scanner execution to the user's memory means that the scanner detects a problem only after the fact—such as when a virus has been downloaded in an e-mail attachment. With the combination of a virus scanner and a personal firewall, the firewall directs all incoming e-mail to the virus scanner, which examines every attachment the moment it reaches the target host and before it is opened.

A personal firewall runs on the very computer it is trying to protect. Thus, a clever attacker is likely to attempt an undetected attack that would disable or reconfigure the firewall for the future. Still, especially for cable modem, DSL, and other "always on" connections, the static workstation is a visible and vulnerable target for an ever-present attack community. A personal firewall can provide reasonable protection to clients that are not behind a network firewall.

Comparison of Firewall Types

We can summarize the differences among the several types of firewalls we have studied in depth. The comparisons are shown in Table 7-8.

Table 7-8 Comparison of Firewall Types.

Packet Filtering

Stateful Inspection

Application Proxy


Personal Firewall


More complex

Even more complex

Most complex

Similar to packet filtering firewall

Sees only addresses and service protocol type

Can see either addresses or data

Sees full data portion of packet

Sees full text of communication

Can see full data portion of packet

Auditing difficult

Auditing possible

Can audit activity

Can audit activity

Can—and usually does—audit activity

Screens based on connection rules

Screens based on information across packets—in either header or data field

Screens based on behavior of proxies

Screens based on interpretation of message content

Typically, screens based on information in a single packet, using header or data

Complex addressing rules can make configuration tricky

Usually preconfigured to detect certain attack signatures

Simple proxies can substitute for complex addressing rules

Complex guard functionality can limit assurance

Usually starts in "deny all inbound" mode, to which user adds trusted addresses as they appear

Example Firewall Configurations

Let us look at several examples to understand how to use firewalls. We present situations designed to show how a firewall complements a sensible security policy and architecture.

The simplest use of a firewall is shown in Figure 7-39. This environment has a screening router positioned between the internal LAN and the outside network connection. In many cases, this installation is adequate when we need only screen the address of a router.

Figure 7-39Figure 7-39 Firewall with Screening Router.

However, to use a proxy machine, this organization is not ideal. Similarly, configuring a router for a complex set of approved or rejected addresses is difficult. If the firewall router is successfully attacked, then all traffic on the LAN to which the firewall is connected is visible. To reduce this exposure, a proxy firewall is often installed on its own LAN, as shown in Figure 7-40. In this way the only traffic visible on that LAN is the traffic going into and out of the firewall.

Figure 7-40Figure 7-40 Firewall on Separate LAN.

For even more protection, we can add a screening router to this configuration, as shown in Figure 7-41. Here, the screening router ensures address correctness to the proxy firewall (so that the proxy firewall cannot be fooled by an outside attacker forging an address from an inside host); the proxy firewall filters traffic according to its proxy rules. Also, if the screening router is subverted, only the traffic to the proxy firewall is visible—not any of the sensitive information on the internal protected LAN.

Figure 7-41Figure 7-41 Firewall with Proxy and Screening Router.

Although these examples are simplifications, they show the kinds of configurations firewalls protect. Next, we review the kinds of attacks against which firewalls can and cannot protect.

What Firewalls Can—and Cannot—Block

As we have seen, firewalls are not complete solutions to all computer security problems. A firewall protects only the perimeter of its environment against attacks from outsiders who want to execute code or access data on the machines in the protected environment. Keep in mind these points about firewalls.

  • Firewalls can protect an environment only if the firewalls control the entire perimeter. That is, firewalls are effective only if no unmediated connections breach the perimeter. If even one inside host connects to an outside address, by a modem for example, the entire inside net is vulnerable through the modem and its host.

  • Firewalls do not protect data outside the perimeter; data that have properly passed (outbound) through the firewall are just as exposed as if there were no firewall.

  • Firewalls are the most visible part of an installation to the outside, so they are the most attractive target for attack. For this reason, several different layers of protection, called defense in depth, are better than relying on the strength of just a single firewall.

  • Firewalls must be correctly configured, that configuration must be updated as the internal and external environment changes, and firewall activity reports must be reviewed periodically for evidence of attempted or successful intrusion.

  • Firewalls are targets for penetrators. While a firewall is designed to withstand attack, it is not impenetrable. Designers intentionally keep a firewall small and simple so that even if a penetrator breaks it, the firewall does not have further tools, such as compilers, linkers, loaders, and the like, to continue an attack.

  • Firewalls exercise only minor control over the content admitted to the inside, meaning that inaccurate data or malicious code must be controlled by other means inside the perimeter.

Firewalls are important tools in protecting an environment connected to a network. However, the environment must be viewed as a whole, all possible exposures must be considered, and the firewall must fit into a larger, comprehensive security strategy. Firewalls alone cannot secure an environment.

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