In computing, a firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. A firewall typically establishes a barrier between a trusted internal network and untrusted external network, such as the Internet.
Firewalls are often categorized as either network firewalls or host-based firewalls. Network firewalls filter traffic between two or more networks and run on network hardware. Host-based firewalls run on host computers and control network traffic in and out of those machines.
The term firewall originally referred to a wall intended to confine a fire within a line of adjacent buildings. 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 term was applied in the late 1980s to network technology that emerged when the Internet was fairly new in terms of its global use and connectivity. The predecessors to firewalls for network security were the routers used in the late 1980s, because they separated networks from one another, thus halting the spread of problems from one network to another.
Before it was used in real-life computing, the term appeared in the 1983 computer-hacking movie WarGames, and possibly inspired its later use.
The first reported type of network firewall is called a packet filter. Packet filters act by inspecting packets transferred between computers. When a packet does not match the packet filter's set of filtering rules, the packet filter either drops (silently discards) the packet, or rejects the packet (discards it and generates an Internet Control Message Protocol notification for the sender) else it is allowed to pass. Packets may be filtered by source and destination network addresses, protocol, source and destination port numbers. The bulk of Internet communication in 20th and early 21st century used either Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) in conjunction with well-known ports, enabling firewalls of that era to distinguish between, and thus control, specific types of traffic (such as web browsing, remote printing, email transmission, file transfer), unless the machines on each side of the packet filter used the same non-standard ports.
The first paper published on firewall technology was in 1987, when engineers from Digital Equipment Corporation (DEC) developed filter systems known as packet filter firewalls. At AT&T Bell Labs, Bill Cheswick and Steve Bellovin continued their research in packet filtering and developed a working model for their own company based on their original first generation architecture.
Second-generation firewalls perform the work of their first-generation predecessors but also maintain knowledge of specific conversations between endpoints by remembering which port number the two IP addresses are using at layer 4 (transport layer) of the OSI model for their conversation, allowing examination of the overall exchange between the nodes.
This type of firewall is potentially vulnerable to denial-of-service attacks that bombard the firewall with fake connections in an attempt to overwhelm the firewall by filling its connection state memory.
Marcus Ranum, Wei Xu, and Peter Churchyard released an application firewall known as Firewall Toolkit (FWTK) in October 1993. This became the basis for Gauntlet firewall at Trusted Information Systems.
The key benefit of application layer filtering is that it can understand certain applications and protocols (such as File Transfer Protocol (FTP), Domain Name System (DNS), or Hypertext Transfer Protocol (HTTP)). This is useful as it is able to detect if an unwanted application or service is attempting to bypass the firewall using a disallowed protocol on an allowed port, or detect if a protocol is being abused in any harmful way.
As of 2012, the so-called next-generation firewall (NGFW) is a wider or deeper inspection at the application layer. For example, the existing deep packet inspection functionality of modern firewalls can be extended to include:
Firewalls are generally categorized as network-based or host-based. Network-based firewalls are positioned on the gateway computers of LANs, WANs and intranets. They are either software appliances running on general-purpose hardware, or hardware-based firewall computer appliances. Firewall appliances may also offer other functionality to the internal network they protect, such as acting as a DHCP or VPN server for that network. Host-based firewalls are positioned on the host itself and control network traffic in and out of those machines. The host-based firewall may be a daemon or service as a part of the operating system or an agent application such as endpoint security or protection. Each has advantages and disadvantages. However, each has a role in layered security.
Firewalls also vary in type depending on where communication originates, where it is intercepted, and the state of communication being traced.
Network layer firewalls, also called packet filters,[a] operate at a relatively low level of the TCP/IP stack, blocking packets unless they match the established rule set. The firewall administrator may define the rules; or default rules may apply.
Network layer firewalls generally fall into two sub-categories, stateful and stateless.
Commonly used packet filters on various versions of Unix are ipfirewall (FreeBSD, Mac OS X (< 10.7)), NPF (NetBSD), PF (Mac OS X (> 10.4), OpenBSD, and some other BSDs), iptables/ipchains (Linux) and IPFilter. Prior to iptables, ipchains was the predominant software package for creating Linux firewalls.
Application-layer firewalls work on the application layer of the Internet protocol suite (e.g., browser, telnet or FTP traffic), and may intercept all packets traveling to or from an application. Application firewalls function by determining whether a process should accept any given connection. Application firewalls filter connections by examining the process ID of data packets against a rule set for the local process involved in the data transmission. Application firewalls accomplish their function by hooking into socket calls to filter the connections between the application layer and the lower layers. Application firewalls that hook into socket calls are also referred to as socket filters.
The extent of the filtering that occurs is defined by the provided rule set. Given the variety of software that exists, application firewalls only have more complex rule sets for the standard services, such as sharing services. Application firewalls work much like a packet filter but application filters apply filtering rules (allow/block) on a per-process basis instead of filtering connections on a per-port basis. Generally, prompts are used to define rules for processes that have not yet received a connection. It is rare to find application firewalls not combined or used in conjunction with a packet filter.
Per-process rule sets used by application firewalls have limited efficacy in filtering every possible association that may occur with other processes. Also, these per-process rule sets cannot defend against modification of the process via exploitation, such as memory corruption exploits. Because of these limitations, application firewalls are beginning to be supplanted by a new generation of application firewalls that rely on mandatory access control (MAC), also referred to as sandboxing, to protect vulnerable services.
A proxy server (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, while blocking other packets. A proxy server is a gateway from one network to another for a specific network application, in the sense that it functions as a proxy on behalf of the network user. Proxies make tampering with an internal system from the external network more difficult, so that misuse of one internal system would not necessarily cause a security breach exploitable from outside the firewall.[b]
Firewalls often have network address translation (NAT) functionality, and the hosts protected behind a firewall commonly have addresses in a "private address range", such as those defined in RFC 1918 for IPv4.