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A firewall has the basic task of controlling traffic between different zones of trust. Typical zones of trust include the Internet (a zone with no trust) and an Internal Network (a zone with high trust). The ultimate goal is to provide controlled connectivity between zones of differing trust levels through the enforcement of a security policy and connectivity model based on the Least Privilege principle. Proper configuration of firewalls demands skill from the Administrator . It requires considerable understanding of network protocols and of computer security. Small mistakes can render a firewall worthless as a security tool. TYPES OF FIREWALLS There are three basic types of firewalls depending on:
With regard to the scope of filtered communications there exist:
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 Firewall s 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 track packet states, two additional categories of firewalls exist: NETWORK LAYER FIREWALLS : ''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 system 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 FIREWALLS : ''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 even prevent the spread of the likes of Virus es. 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 Masquerade s 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 Firewalls often have Network Address Translation (NAT) functionality, and the hosts protected behind a firewall commonly use so-called "private address space", as defined in RFC 1918. Administrators often set up such scenarios in an effort (of debatable effectiveness) to disguise the internal address or network. MANAGEMENT The Middlebox Communication (midcom) Working Group of the Internet Engineering Task Force is working on standardizing protocols for managing firewalls and other Middlebox es. See, e.g., Middlebox Communications (MIDCOM) Protocol Semantics . IMPLEMENTATIONS
USE CASE SCENARIO The simplest form could be like this:
ONLINE FIREWALL CHECK These sites offer free online portscan services to check your firewall security. Please note that online port probes are not 100% bulletproof, as they always check the ''public'' IP address, which may be a proxy server. Online portscans are easy to use and offer basic insights, but to ensure network security, use tools like Nmap .
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