Firewall (networking)

In computing, a firewall is a piece of hardware and/or software which functions in a networked environment to prevent some communications forbidden by the security policy, analogous to the function of firewalls in building construction.

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 and wit 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.

A firewall is also called a Border Protection Device (BPD) especially in NATO contexts. In BSD context, it is often referred to as packet filter.

Contents

1 Types of firewalls

2 Network layer firewalls

3 Application-layer firewalls

4 Proxies

5 Network address translation

6 Implementations

7 See also

8 External links

Types of firewalls

There are three basic criteria used to distinguish between different types of firewalls:

whether the communication is being done between a single node and the network, or between two or more networks
whether the communication is intercepted at the network layer, or at the application layer
whether the communication state is being tracked at the firewall or not

With regard to the scope of filtered communication there exist:

personal firewalls, a software application which normally filters traffic entering or leaving a single computer
network firewalls, normally running on a dedicated network device or computer positioned on the boundary of two or more networks or DMZs (demilitarized zones). Such a firewall filters all traffic entering or leaving the connected networks.

The latter definition corresponds to the conventional, traditional meaning of "firewall" in networking.

In reference to the layers where the traffic can be intercepted, two main categories of firewalls exist:

These two types of firewall may overlap; indeed, single systems have implemented both together.

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, source port, destination IP 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 viruses. 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

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

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.

Implementations

External links

Matt Curtin and Marcus J. Ranum Internet Firewalls: Frequently Asked Questions
Firewalls 101 - Explains the inner workings of a firewall and some common firewall features in an easy to understand manner.
Firewall White papers - White papers on Web Services XML/SOAP and CORBA IIOP firewall security.
Home PC Firewall Guide - includes info on free firewalls
ShieldsUp - a web service which can evaluate some aspects of firewall effectiveness
What is a Firewall - a guide to computing firewalls, with links to current downloads.
A FreeBSD security how-to (ipfw)