A static packet filtering firewall is the simplest and least expensive type of firewalls, offering minimum security provisions to a low-risk computing environment.
A static packet filter firewall examines both the source and destination addresses of the incoming data packet and applies ACL’s to them. They operates at either the Network or Transport layer. They are known as the First generation of firewall.
Older firewalls that were only packet filters were essentially routing devices that provided access control functionality for host addresses and communication sessions. These devices, also known as stateless inspection firewalls, do not keep track of the state of each flow of traffic that passes though the firewall; this means, for example, that they cannot associate multiple requests within a single session to each other. Packet filtering is at the core of most modern firewalls, but there are few firewalls sold today that only do stateless packet filtering. Unlike more advanced filters, packet filters are not concerned about the content of packets. Their access control functionality is governed by a set of directives referred to as a ruleset. Packet filtering capabilities are built into most operating systems and devices capable of routing; the most common example of a pure packet filtering device is a network router that employs access control lists.
There are many types of Firewall:
Application Level Firewalls – Often called a Proxy Server. It works by transferring a copy of each accepted data packet from one network to another. They are known as the Second generation of firewalls.
An application-proxy gateway is a feature of advanced firewalls that combines lower-layer access control with upper-layer functionality. These firewalls contain a proxy agent that acts as an intermediary between two hosts that wish to communicate with each other, and never allows a direct connection between them. Each successful connection attempt actually results in the creation of two separate connections—one between the client and the proxy server, and another between the proxy server and the true destination. The proxy is meant to be transparent to the two hosts—from their perspectives there is a direct connection. Because external hosts only communicate with the proxy agent, internal IP addresses are not visible to the outside world. The proxy agent interfaces directly with the firewall ruleset to determine whether a given instance of network traffic should be allowed to transit the firewall.
Stateful Inspection Firewall - Packets are captured by the inspection engine operating at the network layer and then analyzed at all layers. They are known as the Third generation of firewalls.
Stateful inspection improves on the functions of packet filters by tracking the state of connections and blocking packets that deviate from the expected state. This is accomplished by incorporating greater awareness of the transport layer. As with packet filtering, stateful inspection intercepts packets at the network layer and inspects them to see if they are permitted by an existing firewall rule, but unlike packet filtering, stateful inspection keeps track of each connection in a state table. While the details of state table entries vary by firewall product, they typically include source IP address, destination IP address, port numbers, and connection state information.
Web Application Firewalls - The HTTP protocol used in web servers has been exploited by attackers in many ways, such as to place malicious software on the computer of someone browsing the web, or to fool a person into revealing private information that they might not have otherwise. Many of these exploits can be detected by specialized application firewalls called web application firewalls that reside in front of the web server.
Web application firewalls are a relatively new technology, as compared to other firewall technologies, and the type of threats that they mitigate are still changing frequently. Because they are put in front of web servers to prevent attacks on the server, they are often considered to be very different than traditional firewalls.
Host-Based Firewalls and Personal Firewalls - Host-based firewalls for servers and personal firewalls for desktop and laptop personal computers (PC) provide an additional layer of security against network-based attacks. These firewalls are software-based, residing on the hosts they are protecting—each monitors and controls the incoming and outgoing network traffic for a single host. They can provide more granular protection than network firewalls to meet the needs of specific hosts.
Host-based firewalls are available as part of server operating systems such as Linux, Windows, Solaris, BSD, and Mac OS X Server, and they can also be installed as third-party add-ons. Configuring a host-based firewall to allow only necessary traffic to the server provides protection against malicious activity from all hosts, including those on the same subnet or on other internal subnets not separated by a network firewall. Limiting outgoing traffic from a server may also be helpful in preventing certain malware that infects a host from spreading to other hosts.11 Host-based firewalls usually perform logging, and can often be configured to perform address-based and application-based access controls
Dynamic Packet Filtering – Makes informed decisions on the ACL’s to apply. They are known as the Fourth generation of firewalls.
Kernel Proxy - Very specialized architecture that provides modular kernel-based, multi-layer evaluation and runs in the NT executive space. They are known as the Fifth generation of firewalls.
The following were incorrect answers:
All of the other types of firewalls listed are more complex than the Packet Filtering Firewall.
Reference(s) used for this question:
HARRIS, Shon, All-In-One CISSP Certification Exam Guide, 6th Edition, Telecommunications and Network Security, Page 630.
and
NIST Guidelines on Firewalls and Firewalls policies, Special Publication 800-4 Revision 1
