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Firewall (computing)

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This article is about the network security device. For other uses, see Firewall (disambiguation).
Firewall separating zones of trust

A firewall is an information technology (IT) security device which is configured to permit, deny or proxy data connections set and configured by the organization's security policy. Firewalls can either be hardware and/or software based.

Function

A firewall's basic task is to control traffic between computer networks with different zones of trust. Typical examples are the Internet which is a zone with no trust and an internal network which is (and should be) a zone with high trust. The ultimate goal is to provide controlled interfaces between zones of differing trust levels through the enforcement of a security policy and connectivity model. A zone with an intermediate trust level, situated between the Internet and a trusted internal network, is often referred to as a "perimeter network" or Demilitarized zone (DMZ).

A firewall's function is analogous to firewalls in building construction.

Proper configuration of firewalls demands skill from the firewall administrator. It requires considerable understanding of network protocols and of computer security. Small mistakes can render a firewall worthless as a security tool. Standard Security practices dictate a "default-deny" firewall ruleset.

History

Firewall technology emerged in the late 1980s when the Internet was a fairly new technology in terms of its global use and connectivity. The original idea was formed in response to a number of major internet security breaches, which occurred in the late 1980s. In 1988 an employee at the NASA Ames Research Center in California sent a memo by email to his colleagues that read,

We are currently under attack from an Internet VIRUS! It has hit Berkeley, UC San Diego, Lawrence Livermore, Stanford, and NASA Ames.

The Morris Worm spread itself through multiple vulnerabilities in the machines of the time. Although it was not malicious in intent, the Morris Worm was the first large scale attack on Internet security; the online community was neither expecting an attack nor prepared to deal with one.

First generation - packet filters

The first paper published on firewall technology was in 1988, when Dodong Sean James and Elohra from Digital Equipment Corporation (DEC) developed filter systems known as packet filter firewalls. This fairly basic system was the first generation of what would become a highly evolved and technical internet security feature. At AT&T Bill Cheswick and Steve Bellovin were continuing their research in packet filtering and developed a working model for their own company based upon their original first generation architecture.

Packet filters act by inspecting the "packets" which represent the basic unit of data transfer between computers on the Internet. If a packet matches the packet filter's set of rules, the packet filter will drop (silently discard) the packet, or reject it (discard it, and send "error responses" to the source).

This type of packet filtering pays no attention to whether a packet is part of an existing stream of traffic (it stores no information on connection "state"). Instead, it filters each packet based only on information contained in the packet itself (most commonly using a combination of the packet's source and destination address, its protocol, and, for TCP and UDP traffic, which comprises most internet communication, the port number).

Because TCP and UDP traffic by convention uses well known ports for particular types of traffic, a "stateless" packet filter can distinguish between, and thus control, those types of traffic (such as web browsing, remote printing, email transmission, file transfer), unless the machines on each side of the packet filter are both using the same non-standard ports.

Second generation - "stateful" filters

From 1980-1990 three colleagues from AT&T Bell Laboratories, Dave Presetto, Howard Trickey, and Kshitij Nigam developed the second generation of firewalls, calling them circuit level firewalls.

This technology is generally referred to as a 'stateful firewall' as it maintains records of all connections passing through the firewall, and is able to determine whether a packet is the start of a new connection, or part of an existing connection. Though there's still a set of static rules in such a firewall, the state of a connection can in itself be one of the criteria which trigger specific rules.

This type of firewall can help prevent attacks which exploit existing connections, or certain Denial-of-service attacks, including the SYN flood which sends improper sequences of packets to consume resources on systems behind a firewall..

Third generation - application layer

Publications by Gene Spafford of Purdue University, Bill Cheswick at AT&T Laboratories and Marcus Ranum described a third generation firewall known as application layer firewall, also known as proxy based firewalls. Marcus Ranum's work on the technology spearheaded the creation of the first commercial product. The product was released by DEC who named it the SEAL product. DEC’s first major sale was on June 13, 1991 to a chemical company based on the East Coast of the USA.

The key benefit of application layer filtering is that it can "understand" certain applications and protocols (such as File Transfer Protocol, DNS or web browsing), and can detect whether an unwanted protocol is being sneaked through on a non-standard port, or whether a protocol is being abused in a known harmful way.

This type of filtering can be carried out by proxy servers, but if the filtering is done by a standalone firewall appliance, or in a device for traffic shaping, the technology is likely to be referred to as deep packet inspection.

Subsequent developments

In 1992, Bob Braden and Annette DeSchon at the University of Southern California (USC) were developing their own fourth generation packet filter firewall system. The product known as "Visas" was the first system to have a visual integration interface with colours and icons, which could be easily implemented to and accessed on a computer operating system such as Microsoft's Windows or Apple's MacOS. In 1994 an Israeli company called Check Point Software Technologies built this into readily available software known as FireWall-1.

A second generation of proxy firewalls was based on Kernel Proxy technology. This design is constantly evolving but its basic features and codes are currently in widespread use in both commercial and domestic computer systems. Cisco, one of the largest internet security companies in the world released their PIX product to the public in 1997.

Some modern firewalls leverage their existing deep packet inspection engine by sharing this functionality with an Intrusion-prevention system (IPS).

Types

There are several classifications of firewalls depending on:

  • Whether the communication is being done between a single node and the network, or between two or more networks.
  • Whether the communication is intercepted at the network layer, or at the application layer.
  • Whether the communication state is being tracked at the firewall or not.

With regard to the scope of filtered communications there exist:

  • Personal firewalls, a software application which normally filters traffic entering or leaving a single computer. This filtering may be based on the traffic itself or on the identity of the process which is attempting to listen for or send data.
  • Network firewalls, normally running on a dedicated network device or computer positioned on the boundary of two or more networks or DMZs (demilitarized zones). Such a firewall filters all traffic entering or leaving the connected networks.

The latter definition corresponds to the conventional, traditional meaning of "firewall" in networking.

In reference to the layers where the traffic can be intercepted, three main categories of firewalls exist:

These network-layer and application-layer types of firewall may overlap, even though the personal firewall does not serve a network; indeed, single systems have implemented both together.

There's also the notion of application firewalls which are sometimes used during wide area network (WAN) networking on the world-wide web and govern the system software. An extended description would place them lower than application layer firewalls, indeed at the Operating System layer, and could alternately be called operating system firewalls.

Lastly, depending on whether the firewalls keeps track of the state of network connections or treats each packet in isolation, two additional categories of firewalls exist:

Network layer

Main article: Network layer firewall

Network layer firewalls operate at a (relatively) low level of the TCP/IP protocol stack as IP-packet filters, not allowing packets to pass through the firewall unless they match the rules. The firewall administrator may define the rules; or default built-in rules may apply (as in some inflexible firewall systems).

A more permissive setup could allow any packet to pass the filter as long as it does not match one or more "negative-rules", or "deny rules". Today network firewalls are built into most computer operating systems and network appliances.

Modern firewalls can filter traffic based on many packet attributes like source IP address, source port, destination IP address or port, destination service like WWW or FTP. They can filter based on protocols, TTL values, netblock of originator, domain name of the source, and many other attributes.

Application-layer

Main article: Application layer firewall

Application-layer firewalls work on the application level of the TCP/IP stack (i.e., all browser traffic, or all telnet or ftp traffic), and may intercept all packets traveling to or from an application. They block other packets (usually dropping them without acknowledgement to the sender). In principle, application firewalls can prevent all unwanted outside traffic from reaching protected machines.

By inspecting all packets for improper content, firewalls can restrict or prevent outright the spread of networked computer worms and trojans. In practice, however, this becomes so complex and so difficult to attempt (given the variety of applications and the diversity of content each may allow in its packet traffic) that comprehensive firewall design does not generally attempt this approach.

The XML firewall exemplifies a more recent kind of application-layer firewall.

Proxies

Main article: Proxy server

A proxy device (running either on dedicated hardware or as software on a general-purpose machine) may act as a firewall by responding to input packets (connection requests, for example) in the manner of an application, whilst blocking other packets.

Proxies make tampering with an internal system from the external network more difficult and misuse of one internal system would not necessarily cause a security breach exploitable from outside the firewall (as long as the application proxy remains intact and properly configured). Conversely, intruders may hijack a publicly-reachable system and use it as a proxy for their own purposes; the proxy then masquerades as that system to other internal machines. While use of internal address spaces enhances security, crackers may still employ methods such as IP spoofing to attempt to pass packets to a target network..

Network address translation

Main article: Network address translation

Firewalls often have network address translation (NAT) functionality, and the hosts protected behind a firewall commonly have addresses in the "private address range", as defined in RFC 1918. Firewalls often have such functionality to hide the true address of protected hosts.

Management

The Middlebox Communication (midcom) Working Group of the Internet Engineering Task Force (IETF) is working on standardizing protocols for managing firewalls and other middleboxes.

See also

References

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