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Firewall (computing)

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This article is about the network security device. For other uses, see Firewall.
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 that is configured to permit or deny network transmissions based upon a set of rules and other criteria. Firewalls are thus a specialized type of router focusing on specific types of network security functions.

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 inspects 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.

Firewalls In Practical Use

Nearly every computer has some sort of firewall capability built into it; Microsoft's Windows operating system, for example, includes a built-in firewall software component designed to protect an individual computer from external threats. But nowadays, even most home networks include a hardware-based firewall because the broadband wired or wireless router most use have firewall capabilities built into them. As originally deployed, firewalls for organizations were designed to protect its intranet from a variety of external threats. These firewalls were merely special purpose routers fitted with hardware and software to perform specific security-oriented tasks. The firewall would be placed at the front end of the network, such that external internet links had to connect through the firewall before being able to access the intranet. As a simple example, a firewall would have an external connection to the internet via an ethernet port, having a globally routable (genuine) IP address. It would also have at least one internal ethernet connection with a local IP address (one routable only within the intranet) that would connect to the entire intranet via other network devices, such as a switch. Connections coming into the firewall would be tested to make sure they did not represent a threat, and then based on a set of programmed rules, would be routed to a destination in the intranet. This early focus of firewalls on intrusion detection and threat identification has given way to many additional security-oriented functions, and modern firewalls (now more commonly referred to as security devices) often have multiple capabilities. At the same time, an organization is likely to have more than one router focused on addressing security-oriented concerns.

Some of the security features firewalls address are: - Intrusion prevention and detection. - VPN connectivity (connecting together physically separate networks or a client to an intranet via secure, encrypted communications). - Content filtering. - Client use monitoring and logging (tracking the use of network resources by individual intranet clients). - Email and web browsing protection (antivirus, antispyware, file blocking, antispam, antiphishing, etc). - Secure routing (directing traffic to specific intranet destinations)

The critical importance of network security has created huge demand for firewall and other security-oriented routers. Companies such as Cisco (ASA5500, PIX, CVPN3000), Juniper (Netscreen), Barracuda, Watchguard (Firebox), Sonicwall, and many others have created a range of products, some integrated and some specialized, for all sizes of organizations, from the home office to large corporations.

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]

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 on 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 (i.e. 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 constitute 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 mainly on the first three layers of the OSI reference model, which means most of the work is done between the network and physical layers, with a little bit of peeking into the transport layer to figure out source and destination port numbers [2]. 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.

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 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. Good examples of application firewalls are MS-ISA (Internet Security and Acceleration) server, McAfee Firewall Enterprise & Palo Alto PS Series firewalls. 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 (MS-ISA) are much slower than hardware based stateful firewalls but dedicated appliances (McAfee & Palo Alto) provide much higher performance levels for Application Inspection.

In 2009/2010 the focus of the most comprehensive firewall security vendors turned to expanding the list of applications such firewalls are aware of now covering hundreds and in some cases thousands of applications which can be identified automatically. Many of these applications can not only be blocked or allowed but manipulated by the more advanced firewall products to allow only certain functionally enabling network security administrations to give users functionality without enabling unnecessary vulnerabilities. As a consequence these advanced version of the "Second Generation" firewalls are being referred to as "Next Generation" and surpass the "Third Generation" firewall. It is expected that due to the nature of malicious communications this trend will have to continue to enable organizations to be truly secure.

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 actually be exploited by certain Denial-of-service attacks which can fill the connection tables with illegitimate connections.

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. authpf on BSD systems loads firewall rules dynamically per user, after authentication via SSH.

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.

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).

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.

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. 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.

See also

References

  1. ^ RFC 1135 The Helminthiasis of the Internet
  2. ^ William R. Cheswick, Steven M. Bellovin, Aviel D. Rubin (2003). "Google Books Link". Firewalls and Internet security: repelling the wily hacker
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