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Firewalls protect against the spread of fire or other danger.
In the traditional meaning in construction, a firewall consists of a windowless, fireproof wall (or a wall of substantially heavier construction than other walls in the building) built to prevent fire from spreading beyond one section of a building. Such firewalls form the built-up equivalent of firebreaks in a landscape. Their function in containing the undesirable resembles the use of bulkheads in shipbuilding and aircraft construction.
By extension, the computing world uses the term firewall for a piece of hardware or software which functions in a networked environment to prevent some communications forbidden by the network policy. It has the basic task of preventing intrusion from a connected network device into other networked devices..
Network security analysts distinguish between:
The latter definition corresponds to the conventional meaning of "firewall" in networking, and the remainder of this article addresses this type of firewall. Two main categories of such firewalls exist:
These two types of firewall may overlap; indeed, single systems have implemented both together.
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".
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 such things as viruses. However, in practice, 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.
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.
Firewalls often have network address translation 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.
Proper configuration of firewalls demands skill. It requires considerable understanding of network protocols and of computer security. Small mistakes can render a firewall worthless as a security tool.
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