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  1. 1. 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.
  2. 2. 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 authorizedcommunications. It is a device or set of devices that is configured to permit or deny network transmissions based upon a set of rules andother criteria.Firewalls can be implemented in either hardware or software, or a combination of both. Firewalls are frequently used to preventunauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leavingthe 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 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.
  3. 3. Contents[hide] • 1 History • 1 . 1 F i r s t g e n e r a t i o n : p a c k e t f i l t
  4. 4. [edit] HistoryThe term firewall/fireblock originally meant a wall to confine a fire or potential fire within a building; cf. firewall (construction). Later usesrefer to similar structures, such as the metal sheet separating the engine compartment of a vehicle or aircraft from the passengercompartment.- The Morris Worm spread itself through multiple vulnerabilities in the machines of the time. Although it was not malicious in intent, theMorris Worm was the first large scale attack on Internet security; the online community was neither expecting an attack nor prepared to dealwith one.[1][edit] First generation: packet filtersThe first paper published on firewall technology was in 1988, when engineers from Digital Equipment Corporation (DEC) developed filtersystems known as packet filter firewalls. This fairly basic system was the first generation of what became a highly evolved and technicalinternet security feature. At AT&T Bell Labs, Bill Cheswick and Steve Bellovin were continuing their research in packet filtering anddeveloped 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 onconnection "state"). Instead, it filters each packet based only on information contained in the packet itself (most commonly using acombination of the packets 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 knownports for particular types of traffic, a "stateless" packet filter can distinguish between, and thus control, those types of traffic (such as webbrowsing, remote printing, email transmission, file transfer), unless the machines on each side of the packet filter are both using the samenon-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 betweenthe 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 thatare configured in the firewall and drops or rejects the packet accordingly. When the packet passes through the firewall, it filters the packet ona protocol/port number basis (GSS). For example, if a rule in the firewall exists to block telnet access, then the firewall will block the IPprotocol for port number 23.[edit] Second generation: application layerMain article: Application layer firewallThe 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
  5. 5. 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 OSImodel, from the application down to the physical Layer. This is similar to a packet filter firewall but here we can also filter information onthe basis of content. Good examples of application firewalls are MS-ISA (Internet Security and Acceleration) server, McAfee FirewallEnterprise & 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 beenabled on the firewall to block that particular word. Software-based firewalls (MS-ISA) are much slower than hardware based statefulfirewalls 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 areaware of now covering hundreds and in some cases thousands of applications which can be identified automatically. Many of theseapplications can not only be blocked or allowed but manipulated by the more advanced firewall products to allow only certain functionallyenabling network security administrations to give users functionality without enabling unnecessary vulnerabilities. As a consequence theseadvanced 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 trulysecure.[edit] Third generation: "stateful" filtersMain article: Stateful firewallFrom 1989-1990 three colleagues from AT&T Bell Laboratories, Dave Presetto, Janardan Sharma, and Kshitij Nigam, developed the thirdgeneration 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 thepacket series. This technology is generally referred to as a stateful packet inspection as it maintains records of all connections passingthrough 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 aninvalid 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 whichtrigger specific rules.This type of firewall can actually be exploited by certain Denial-of-service attacks which can fill the connection tables with illegitimateconnections.[edit] Subsequent developmentsIn 1992, Bob Braden and Annette DeSchon at the University of Southern California (USC) were refining the concept of a firewall. Theproduct known as "Visas" was the first system to have a visual integration interface with colors and icons, which could be easilyimplemented and accessed on a computer operating system such as Microsofts Windows or Apples MacOS. In 1994 an Israeli company
  6. 6. 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 standardizingprotocols 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 useridentities 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 users signature for each connection. authpf on BSD systems loads firewall rules dynamically per user,after authentication via SSH.[edit] TypesThere are several classifications of firewalls depending on where the communication is taking place, where the communication is interceptedand the state that is being traced.[edit] Network layer and packet filtersNetwork layer firewalls, also called packet filters, operate at a relatively low level of the TCP/IP protocol stack, not allowing packets to passthrough 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 connections lifetime (including sessioninitiation, handshaking, data transfer, or completion connection). If a packet does not match an existing connection, it will be evaluatedaccording to the ruleset for new connections. If a packet matches an existing connection based on comparison with the firewalls 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 mayalso be necessary for filtering stateless network protocols that have no concept of a session. However, they cannot make more complexdecisions 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 otherattributes.Commonly used packet filters on various versions of Unix are ipf (various), ipfw (FreeBSD/Mac OS X), pf (OpenBSD, and all other BSDs),
  7. 7. iptables/ipchains (Linux).[edit] Application-layerMain article: Application layer firewallApplication-layer firewalls work on the application level of the TCP/IP stack (i.e., all browser traffic, or all telnet or ftp traffic), and mayintercept all packets traveling to or from an application. They block other packets (usually dropping them without acknowledgment to thesender). 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 andtrojans. The additional inspection criteria can add extra latency to the forwarding of packets to their destination.[edit] ProxiesMain article: Proxy serverA proxy device (running either on dedicated hardware or as software on a general-purpose machine) may act as a firewall by responding toinput 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 notnecessarily cause a security breach exploitable from outside the firewall (as long as the application proxy remains intact and properlyconfigured). Conversely, intruders may hijack a publicly-reachable system and use it as a proxy for their own purposes; the proxy thenmasquerades as that system to other internal machines. While use of internal address spaces enhances security, crackers may still employmethods such as IP spoofing to attempt to pass packets to a target network.[edit] Network address translationMain article: Network address translationFirewalls often have network address translation (NAT) functionality, and the hosts protected behind a firewall commonly have addresses inthe "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 tocompanies or individuals as well as reduce both the amount and therefore cost of obtaining enough public addresses for every computer in anorganization. Hiding the addresses of protected devices has become an increasingly important defense against network reconnaissance
  8. 8. Vesaria, LLC 443.501.4044What are the basic types of firewalls? NEW JERSEY: Conceptually, there are two types of firewalls: 708 Lakeview Drive Lakewood, NJ 087011. MARYLAND: Network layer 722 Dulaney Valley2. Road, Suite 192 Towson, MD 21204 Application layerThey are not as different as you might think, and latest technologies are blurring thedistinction to the point where its no longer clear if either one is ``better or ``worse. Asalways, you need to be careful to pick the type that meets your needs. Firewall FAQ Table ofWhich is which depends on what mechanisms the firewall uses to pass traffic from one Contentssecurity zone to another. The International Standards Organization (ISO) Open SystemsInterconnect (OSI) model for networking defines seven layers, where each layer provides Previousservices that ``higher-level layers depend on. In order from the bottom, these layers are Section: Designphysical, data link, network, transport, session, presentation, application. and ImplementationThe important thing to recognize is that the lower-level the forwarding mechanism, the Issuesless examination the firewall can perform. Generally speaking, lower-level firewalls arefaster, but are easier to fool into doing the wrong thing. Next Section: What are proxy servers and how3.2.1 Network layer firewalls do they work?These generally make their decisions based on the source, destination addresses and ports(see Appendix C for a more detailed discussion of ports) in individual IP packets. A simple Find out morerouter is the ``traditional network layer firewall, since it is not able to make particularly aboutsophisticated decisions about what a packet is actually talking to or where it actually came VESARiAfrom. Modern network layer firewalls have become increasingly sophisticated, and now Firewallmaintain internal information about the state of connections passing through them, the
  9. 9. contents of some of the data streams, and so on. One thing thats an important distinction Testing.about many network layer firewalls is that they route traffic directly though them, so touse one you either need to have a validly assigned IP address block or to use a ``privateinternet address block [3]. Network layer firewalls tend to be very fast and tend to bevery transparent to users. Figure 1: Screened Host FirewallIn Figure 1, a network layer firewall called a ``screened host firewall is represented. In ascreened host firewall, access to and from a single host is controlled by means of a routeroperating at a network layer. The single host is a bastion host; a highly-defended andsecured strong-point that (hopefully) can resist attack.
  10. 10. Figure 2: Screened Subnet FirewallExample Network layer firewall : In figure 2, a network layer firewall called a ``screenedsubnet firewall is represented. In a screened subnet firewall, access to and from a wholenetwork is controlled by means of a router operating at a network layer. It is similar to ascreened host, except that it is, effectively, a network of screened hosts.3.2.2 Application layer firewallsThese generally are hosts running proxy servers, which permit no traffic directly betweennetworks, and which perform elaborate logging and auditing of traffic passing throughthem. Since the proxy applications are software components running on the firewall, it is agood place to do lots of logging and access control. Application layer firewalls can beused as network address translators, since traffic goes in one ``side and out the other, afterhaving passed through an application that effectively masks the origin of the initiatingconnection. Having an application in the way in some cases may impact performance andmay make the firewall less transparent. Early application layer firewalls such as thosebuilt using the TIS firewall toolkit, are not particularly transparent to end users and may
  11. 11. require some training. Modern application layer firewalls are often fully transparent.Application layer firewalls tend to provide more detailed audit reports and tend to enforcemore conservative security models than network layer firewalls. Figure 3: Dual Homed GatewayExample Application layer firewall : In figure 3, an application layer firewall called a``dual homed gateway is represented. A dual homed gateway is a highly secured host thatruns proxy software. It has two network interfaces, one on each network, and blocks alltraffic passing through it.The Future of firewalls lies someplace between network layer firewalls and applicationlayer firewalls. It is likely that network layer firewalls will become increasingly ``awareof the information going through them, and application layer firewalls will becomeincreasingly ``low level and transparent. The end result will be a fast packet-screening
  12. 12. system that logs and audits data as it passes through. Increasingly, firewalls (network andapplication layer) incorporate encryption so that they may protect traffic passing betweenthem over the Internet. Firewalls with end-to-end encryption can be used by organizationswith multiple points of Internet connectivity to use the Internet as a ``private backbonewithout worrying about their data or passwords being sniffed. © 2000 - 2011 Vesaria Network Security Specialists About Vesaria | Legal | Privacy | Contact C