Firewall (computing)

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In computing, a firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. [1] [2] A firewall typically establishes a barrier between a trusted network and an untrusted network, such as the Internet. [3]



The term firewall originally referred to a wall intended to confine a fire within a line of adjacent buildings. [4] Later uses refer to similar structures, such as the metal sheet separating the engine compartment of a vehicle or aircraft from the passenger compartment. The term was applied in the late 1980s to network technology [5] that emerged when the Internet was fairly new in terms of its global use and connectivity. [6] The predecessors to firewalls for network security were routers used in the late 1980s. Because they already segregated networks, routers could apply filtering to packets crossing them. [7]

Before it was used in real-life computing, the term appeared in the 1983 computer-hacking movie WarGames , and possibly inspired its later use. [8]


Firewalls are categorized as a network-based or a host-based system. Network-based firewalls are positioned between two or more networks, typically between the local area network (LAN) and wide area network (WAN), [9] their basic function is to control the flow of data between connected networks. They are either a software appliance running on general-purpose hardware, a hardware appliance running on special-purpose hardware, or a virtual appliance running on a virtual host controlled by a hypervisor. Firewall appliances may also offer non firewall functionality, such as DHCP [10] [11] or VPN [12] services. Host-based firewalls are deployed directly on the host itself to control network traffic or other computing resources. [13] [14] This can be a daemon or service as a part of the operating system or an agent application for protection.

An illustration of a network-based firewall within a network Firewall.png
An illustration of a network-based firewall within a network

Packet filter

The first reported type of network firewall is called a packet filter, which inspects packets transferred between computers. The firewall maintains an access-control list which dictates what packets will be looked at and what action should be applied, if any, with the default action set to silent discard. Three basic actions regarding the packet consist of a silent discard, discard with Internet Control Message Protocol or TCP reset response to the sender, and forward to the next hop. [15] Packets may be filtered by source and destination IP addresses, protocol, source and destination ports. The bulk of Internet communication in 20th and early 21st century used either Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) in conjunction with well-known ports, enabling firewalls of that era to distinguish between specific types of traffic such as web browsing, remote printing, email transmission, and file transfers. [16] [17]

The first paper published on firewall technology was in 1987 when engineers from Digital Equipment Corporation (DEC) developed filter systems known as packet filter firewalls. At AT&T Bell Labs, Bill Cheswick and Steve Bellovin continued their research in packet filtering and developed a working model for their own company based on their original first-generation architecture. [18] In 1992, Steven McCanne and Van Jacobson released a paper on BSD Packet Filter (BPF) while at Lawrence Berkeley Laboratory. [19] [20]

Connection tracking

Flow of network packets through Netfilter, a Linux kernel module Netfilter-packet-flow.svg
Flow of network packets through Netfilter, a Linux kernel module

From 1989–1990, three colleagues from AT&T Bell Laboratories, Dave Presotto, Janardan Sharma, and Kshitij Nigam, developed the second generation of firewalls, calling them circuit-level gateways. [21]

Second-generation firewalls perform the work of their first-generation predecessors but also maintain knowledge of specific conversations between endpoints by remembering which port number the two IP addresses are using at layer 4 (transport layer) of the OSI model for their conversation, allowing examination of the overall exchange between the nodes. [22]

Application layer

Marcus Ranum, Wei Xu, and Peter Churchyard released an application firewall known as Firewall Toolkit (FWTK) in October 1993. [23] This became the basis for Gauntlet firewall at Trusted Information Systems. [24] [25]

The key benefit of application layer filtering is that it can understand certain applications and protocols such as File Transfer Protocol (FTP), Domain Name System (DNS), or Hypertext Transfer Protocol (HTTP). This allows it to identify unwanted applications or services using a non standard port, or detect if an allowed protocol is being abused. [26] It can also provide unified security management including enforced encrypted DNS and virtual private networking. [27] [28] [29]

As of 2012, the next-generation firewall provides a wider range of inspection at the application layer, extending deep packet inspection functionality to include, but is not limited to:

Endpoint specific

Endpoint-based application firewalls function by determining whether a process should accept any given connection. Application firewalls filter connections by examining the process ID of data packets against a rule set for the local process involved in the data transmission. Application firewalls accomplish their function by hooking into socket calls to filter the connections between the application layer and the lower layers. Application firewalls that hook into socket calls are also referred to as socket filters.[ citation needed ]


Setting up a firewall is a complex and error-prone task. A network may face security issues due to configuration errors. [30]

Firewall policy configuration is based on specific network type (e.g., public or private), and can be set up using firewall rules that either block or allow access to prevent potential attacks from hackers or malware. [31]

See also

Related Research Articles

The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suite are the Transmission Control Protocol (TCP), the User Datagram Protocol (UDP), and the Internet Protocol (IP). Early versions of this networking model were known as the Department of Defense (DoD) model because the research and development were funded by the United States Department of Defense through DARPA.

In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages to other hosts on an Internet Protocol (IP) network. Within an IP network, UDP does not require prior communication to set up communication channels or data paths.

<span class="mw-page-title-main">Network address translation</span> Protocol facilitating connection of one IP address space to another

Network address translation (NAT) is a method of mapping an IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device. The technique was originally used to bypass the need to assign a new address to every host when a network was moved, or when the upstream Internet service provider was replaced, but could not route the network's address space. It has become a popular and essential tool in conserving global address space in the face of IPv4 address exhaustion. One Internet-routable IP address of a NAT gateway can be used for an entire private network.

<span class="mw-page-title-main">Proxy server</span> Computer server that makes and receives requests on behalf of a user

In computer networking, a proxy server is a server application that acts as an intermediary between a client requesting a resource and the server providing that resource.

In computer security, a DMZ or demilitarized zone is a physical or logical subnetwork that contains and exposes an organization's external-facing services to an untrusted, usually larger, network such as the Internet. The purpose of a DMZ is to add an additional layer of security to an organization's local area network (LAN): an external network node can access only what is exposed in the DMZ, while the rest of the organization's network is protected behind a firewall. The DMZ functions as a small, isolated network positioned between the Internet and the private network.

This page provides an index of articles thought to be Internet or Web related topics.

IPX/SPX stands for Internetwork Packet Exchange/Sequenced Packet Exchange. IPX and SPX are networking protocols used initially on networks using the Novell NetWare operating systems. They also became widely used on networks deploying Microsoft Windows LANS, as they replaced NetWare LANS, but are no longer widely used. IPX/SPX was also widely used prior to and up to Windows XP, which supported the protocols, while later Windows versions do not, and TCP/IP took over for networking.

Deep packet inspection (DPI) is a type of data processing that inspects in detail the data being sent over a computer network, and may take actions such as alerting, blocking, re-routing, or logging it accordingly. Deep packet inspection is often used for baselining application behavior, analyzing network usage, troubleshooting network performance, ensuring that data is in the correct format, checking for malicious code, eavesdropping, and internet censorship, among other purposes. There are multiple headers for IP packets; network equipment only needs to use the first of these for normal operation, but use of the second header is normally considered to be shallow packet inspection despite this definition.

<span class="mw-page-title-main">Port forwarding</span> Application of network address translation

In computer networking, port forwarding or port mapping is an application of network address translation (NAT) that redirects a communication request from one address and port number combination to another while the packets are traversing a network gateway, such as a router or firewall. This technique is most commonly used to make services on a host residing on a protected or masqueraded (internal) network available to hosts on the opposite side of the gateway, by remapping the destination IP address and port number of the communication to an internal host.

<span class="mw-page-title-main">Internet security</span> Branch of computer security

Internet security is a branch of computer security. It encompasses the Internet, browser security, web site security, and network security as it applies to other applications or operating systems as a whole. Its objective is to establish rules and measures to use against attacks over the Internet. The Internet is an inherently insecure channel for information exchange, with high risk of intrusion or fraud, such as phishing, online viruses, trojans, ransomware and worms.

<span class="mw-page-title-main">Application firewall</span> Layer 7/application layer network security system

An application firewall is a form of firewall that controls input/output or system calls of an application or service. It operates by monitoring and blocking communications based on a configured policy, generally with predefined rule sets to choose from. The application firewall can control communications up to the application layer of the OSI model, which is the highest operating layer, and where it gets its name. The two primary categories of application firewalls are network-based and host-based.

In computer networks, a tunneling protocol is a communication protocol which allows for the movement of data from one network to another. It involves allowing private network communications to be sent across a public network through a process called encapsulation.

Context-based access control (CBAC) is a feature of firewall software, which intelligently filters TCP and UDP packets based on application layer protocol session information. It can be used for intranets, extranets and internets.

<span class="mw-page-title-main">Steven M. Bellovin</span>

Steven M. Bellovin is a researcher on computer networking and security. He has been a professor in the Computer Science department at Columbia University since 2005. Previously, Bellovin was a Fellow at AT&T Labs Research in Florham Park, New Jersey.

A network socket is a software structure within a network node of a computer network that serves as an endpoint for sending and receiving data across the network. The structure and properties of a socket are defined by an application programming interface (API) for the networking architecture. Sockets are created only during the lifetime of a process of an application running in the node.

Windows Filtering Platform (WFP) is a set of system services in Windows Vista and later that allows Windows software to process and filter network traffic. Microsoft intended WFP for use by firewalls, antimalware software, and parental controls apps. Additionally, WFP is used to implement NAT and to store IPSec policy configuration.

In computing, Microsoft's Windows Vista and Windows Server 2008 introduced in 2007/2008 a new networking stack named Next Generation TCP/IP stack, to improve on the previous stack in several ways. The stack includes native implementation of IPv6, as well as a complete overhaul of IPv4. The new TCP/IP stack uses a new method to store configuration settings that enables more dynamic control and does not require a computer restart after a change in settings. The new stack, implemented as a dual-stack model, depends on a strong host-model and features an infrastructure to enable more modular components that one can dynamically insert and remove.

A distributed firewall is a security application on a host machine of a network that protects the servers and user machines of its enterprise's networks against unwanted intrusion. A firewall is a system or group of systems that implements a set of security rules to enforce access control between two networks to protect the "inside" network from the "outside" network. They filter all traffic regardless of its origin—the Internet or the internal network. Usually deployed behind the traditional firewall, they provide a second layer of defense. The advantages of the distributed firewall allow security rules (policies) to be defined and pushed out on an enterprise-wide basis, which is necessary for larger enterprises.

The Stream Control Transmission Protocol (SCTP) is a computer networking communications protocol in the transport layer of the Internet protocol suite. Originally intended for Signaling System 7 (SS7) message transport in telecommunication, the protocol provides the message-oriented feature of the User Datagram Protocol (UDP), while ensuring reliable, in-sequence transport of messages with congestion control like the Transmission Control Protocol (TCP). Unlike UDP and TCP, the protocol supports multihoming and redundant paths to increase resilience and reliability.

Port Control Protocol (PCP) is a computer networking protocol that allows hosts on IPv4 or IPv6 networks to control how the incoming IPv4 or IPv6 packets are translated and forwarded by an upstream router that performs network address translation (NAT) or packet filtering. By allowing hosts to create explicit port forwarding rules, handling of the network traffic can be easily configured to make hosts placed behind NATs or firewalls reachable from the rest of the Internet, which is a requirement for many applications.


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