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From Wikipedia, the free encyclopediaJump to: navigation, search This article is about the network security device. For other uses, see Firewall. This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (February 2008)

An illustration of where a firewall would be located in a network. An example of a user interface for a firewall on Ubuntu (Gufw)A firewall is a part of a computer system or network that is designed to block unauthorized access while permitting authorized communications. It is a device or set of devices which is configured to permit or deny computer applications based upon a set of rules and other criteria.

Firewalls can be implemented in either hardware or software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.

There are several types of firewall techniques:

1.Packet filter: Packet filtering inspects each packet passing through the network and accepts or rejects it based on user-defined rules. Although difficult to configure, it is fairly effective and mostly transparent to its users. It is susceptible to IP spoofing. 2.Application gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance degradation. 3.Circuit-level gateway: Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking. 4.Proxy server: Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses. Contents [hide] 1 Function 2 History 2.1 First generation: packet filters 2.2 Second generation: application layer 2.3 Third generation: "stateful" filters 2.4 Subsequent developments 3 Types 3.1 Network layer and packet filters 3.2 Application-layer 3.3 Proxies 3.4 Network address translation 4 See also 5 References 6 External links [edit] Function A firewall is a dedicated appliance, or software running on a computer, which inspects network traffic passing through it, and denies or permits passage based on a set of rules/criteria.

It is normally placed between a protected network and an unprotected network and acts like a gate to protect assets to ensure that nothing private goes out and nothing malicious comes in.

A firewall's basic task is to regulate some of the flow of traffic between computer networks of different trust levels. Typical examples are the Internet which is a zone with no trust and an internal network which is a zone of higher trust. 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 within a network is similar to physical firewalls with fire doors in building construction. In the former case, it is used to prevent network intrusion to the private network. In the latter case, it is intended to contain and delay structural fire from spreading to adjacent structures.

[edit] History The term firewall/fireblock originally meant a wall to confine a fire or potential fire within a building; cf. firewall (construction). 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 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.[1] [edit] First generation: packet filters The first paper published on firewall technology was in 1988, when engineers from Digital Equipment Corporation (DEC) developed filter systems known as packet filter firewalls. This fairly basic system was the first generation of what became a highly evolved and technical internet security feature. At AT&T Bell Labs, 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.

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, the port number).

TCP and UDP protocols comprise most communication over the Internet, and 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.

Packet filtering firewalls work on the first three layers of the OSI reference model, which means all the work is done between the network and physical layers. When a packet originates from the sender and filters through a firewall, the device checks for matches to any of the packet filtering rules that are configured in the firewall and drops or rejects the packet accordingly. When the packet passes through the firewall it filters the packet on a protocol/port number basis (GSS). For example if a rule in the firewall exists to block telnet access, then the firewall will block the IP protocol for port number 23.

[edit] Second generation: application layer Main article: Application layer firewall 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 it can detect if an unwanted protocol is sneaking through on a non-standard port or if a protocol is being abused in any harmful way.

An application firewall is much more secure and reliable compared to packet filter firewalls because it works on all seven layers of the OSI reference model, from the application down to the physical Layer. This is similar to a packet filter firewall but here we can also filter information on the basis of content. The best example of an application firewall is ISA (Internet Security and Acceleration) server. An application firewall can filter higher-layer protocols such as FTP, Telnet, DNS, DHCP, HTTP, TCP, UDP and TFTP (GSS). For example, if an organization wants to block all the information related to "foo" then content filtering can be enabled on the firewall to block that particular word. Software-based firewalls are thus much slower than stateful firewalls.

[edit] Third generation: "stateful" filters Main article: Stateful firewall From 1989-1990 three colleagues from AT&T Bell Laboratories, Dave Presetto, Janardan Sharma, and Kshitij Nigam, developed the third generation of firewalls, calling them circuit level firewalls.

Third-generation firewalls, in addition to what first- and second-generation look for, regard placement of each individual packet within the packet series. This technology is generally referred to as a stateful packet inspection 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, a part of an existing connection, or is an invalid packet. Though there is still a set of static rules in such a firewall, the state of a connection can 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.

[edit] Subsequent developments In 1992, Bob Braden and Annette DeSchon at the University of Southern California (USC) were refining the concept of a firewall. The product known as "Visas" was the first system to have a visual integration interface with colors and icons, which could be easily implemented 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.

The existing deep packet inspection functionality of modern firewalls can be shared by Intrusion-prevention systems (IPS).

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

Another axis of development is about integrating identity of users into Firewall rules. Many firewalls provide such features by binding user identities to IP or MAC addresses, which is very approximate and can be easily turned around. The NuFW firewall provides real identity-based firewalling, by requesting the user's signature for each connection.

[edit] Types There are several classifications of firewalls depending on where the communication is taking place, where the communication is intercepted and the state that is being traced.

[edit] Network layer and packet filters Network layer firewalls, also called packet filters, operate at a relatively low level of the TCP/IP protocol stack, not allowing packets to pass through the firewall unless they match the established rule set. The firewall administrator may define the rules; or default rules may apply. The term "packet filter" originated in the context of BSD operating systems.

Network layer firewalls generally fall into two sub-categories, stateful and stateless. Stateful firewalls maintain context about active sessions, and use that "state information" to speed packet processing. Any existing network connection can be described by several properties, including source and destination IP address, UDP or TCP ports, and the current stage of the connection's lifetime (including session initiation, handshaking, data transfer, or completion connection). If a packet does not match an existing connection, it will be evaluated according to the ruleset for new connections. If a packet matches an existing connection based on comparison with the firewall's state table, it will be allowed to pass without further processing.

Stateless firewalls require less memory, and can be faster for simple filters that require less time to filter than to look up a session. They may also be necessary for filtering stateless network protocols that have no concept of a session. However, they cannot make more complex decisions based on what stage communications between hosts have reached.

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, of the source, and many other attributes.

Commonly used packet filters on various versions of Unix are ipf (various), ipfw (FreeBSD/Mac OS X), pf (OpenBSD, and all other BSDs), iptables/ipchains (Linux).

[edit] 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 acknowledgment to the sender). In principle, application firewalls can prevent all unwanted outside traffic from reaching protected machines.

On inspecting all packets for improper content, firewalls can restrict or prevent outright the spread of networked computer worms and trojans. The additional inspection criteria can add extra latency to the forwarding of packets to their destination.

[edit] 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.

[edit] 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. Originally, the NAT function was developed to address the limited number of IPv4 routable addresses that could be used or assigned to companies or individuals as well as reduce both the amount and therefore cost of obtaining enough public addresses for every computer in an organization. Hiding the addresses of protected devices has become an increasingly important defense against network reconnaissance.

[edit] See also Access control list Bastion host Circuit-level gateway Comparison of firewalls Computer security Egress filtering End-to-end connectivity Firewall pinhole Firewalls and Internet Security (book) Golden Shield Project (aka Great Firewall of China) List of Linux router or firewall distributions Mangled packet network reconnaissance Packet Personal firewall Sandbox (computer security) Screened-subnet firewall Stateful firewall Stateful packet inspection Unified threat management Virtual firewall [edit] References 1.^ RFC 1135 The Helminthiasis of the Internet [edit] External links Wikimedia Commons has media related to: Firewall

Internet Firewalls: Frequently Asked Questions, compiled by Matt Curtin, Marcus Ranum and Paul Robertson. Evolution of the Firewall Industry - Discusses different architectures and their differences, how packets are processed, and provides a timeline of the evolution. A History and Survey of Network Firewalls - provides an overview of firewalls at the various ISO levels, with references to the original papers where first firewall work was reported. Software Firewalls: Made of Straw? Part 1 and Software Firewalls: Made of Straw? Part 2 - a technical view on software firewall design and potential weaknesses [show]v • d • eFirewall software Comparison of firewalls · List of router or firewall distributions Windows Commercial Check Point Integrity · Jetico Firewall · Kaspersky Internet Security · McAfee Personal Firewall Plus · Microsoft Forefront Threat Management Gateway · Norton 360 · Online Armor Personal Firewall · Outpost Firewall Pro · Sunbelt Personal Firewall · Windows Firewall · Windows Live OneCare · WinGate · WinRoute · ZoneAlarm Freeware Comodo Internet Security · Core Force · Online Armor Personal Firewall · PC Tools Firewall Plus · ProtoWall · ZoneAlarm Open-source PeerBlock

Mac OS X NetBarrier X4

Unix-like Arptables · ClarkConnect · ClearOS · EBox · Endian Firewall · FireHOL · Firestarter · IPCop · IPFilter · Ipfirewall · Iplist · Iptables · L7-filter · M0n0wall · MoBlock · Netfilter · NuFW · PF · pfSense · Sentry Firewall · Shorewall · SmoothWall · Zeroshell

Cross-platform PeerGuardian

Appliances Cisco Secure Integrated Software · Novell BorderManager · Vyatta · ZoneAlarm Z100G · Zorp firewall

Application firewall · Context-based access control · Personal firewall [show]v • d • eMalware Infectous malware Computer virus · Macro virus · List of computer viruses · Computer worm · List of computer worms · Timeline of notable computer viruses and worms Concealment Trojan horse · Rootkit · Backdoor Malware for profit Privacy-invasive software · Spyware · Botnet · Keystroke logging · Web threats · Fraudulent dialer · Malbot By operating system Linux malware · Palm OS Viruses Protection Antivirus software · Defensive computing · Firewall · Intrusion detection system · Data loss prevention software Law enforcement Computer surveillance · Operation: Bot Roast

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