A firewall sits between a network and the Internet, controlling data flow in and out of the network to stop potential security threats.
What is a firewall?
A firewall is a security system that monitors and controls network traffic based on security rules. Firewalls usually sit between a trusted network and an untrusted network; frequently, the untrusted network is the Internet. For example, office networks often use a firewall to protect their network from online threats.
Firewalls decide whether to allow incoming and outgoing traffic to pass through. They can be built into the hardware, software, or a combination. The term “firewall” is borrowed from a construction practice of building walls between or through the middle of buildings designed to contain a fire. Similarly, network firewalls work to contain online threats.
Why use a firewall?
The primary use case for a firewall is security. Firewalls can intercept incoming malicious traffic before it reaches the network and prevent sensitive information from leaving the network.
Firewalls can also be used for content filtering. For example, a school can configure a firewall to prevent users on their network from accessing adult material. Similarly, in some nations, the government runs a firewall that can prevent people inside that nation-state from accessing certain parts of the Internet.
This article will focus on firewalls configured for security, of which there are several kinds.
What are the different types of firewalls?
Proxy-based firewalls:
These are proxies that sit between clients and servers. Clients connect to the firewall, and the firewall inspects the outgoing packets, after which it will create a connection to the intended recipient (the web server). Similarly, when the web server attempts to send a response to the client, the firewall will intercept that request, inspect the packets, and then deliver that response in a separate connection between the firewall and the client. A proxy-based firewall effectively prevents a direct connection between the client and server.
A proxy-based firewall is kind of like a bouncer at a bar. This bouncer stops guests before they enter the bar to ensure they are not underage, armed, or in any other way a threat to the bar and its patrons. The bouncer also stops patrons on their way out to ensure they can get home safely and do not plan to drink and drive.
The downside of having a bouncer at the bar is that when many people try to enter or leave the bar simultaneously, there will be a long line, and several people will experience delays. Similarly, a significant drawback of a proxy-based firewall is that it can cause latency, particularly during heavy traffic.
*A proxy is a computer that acts as a gateway between a local network and a more extensive network, such as the Internet.
Stateful firewalls:
A “stateful” application saves data from previous events and interactions in computer science. A stateful firewall saves information regarding open connections and uses this information to analyze incoming and outgoing traffic rather than inspecting each packet. Because they do not review every packet, stateful firewalls are faster than proxy-based firewalls.
Stateful firewalls rely on a lot of contexts when making decisions. For example, suppose the firewall records outgoing packets on one connection requesting a certain kind of response. In that case, it will only allow incoming packets on that connection if they provide the asked answer.
Stateful firewalls can also protect ports by keeping them all closed unless incoming packets request access to a specific port. This can mitigate an attack known as port scanning.
A known vulnerability associated with stateful firewalls is that they can be manipulated by tricking clients into requesting specific information. Once the client requests that response, the attacker can send malicious packets matching that criterion through the firewall; for example, insecure websites can use JavaScript code to create these kinds of forged requests from a web browser.
A network port is where information is sent; it’s not a physical place but a communications endpoint. Learn more about ports >>
Next-generation firewalls (NGFW):
NGFWs are firewalls with the capabilities of traditional firewalls but also employ a host of added features to address threats on other OSI model layers. Some NGFW-specific features include:
- Deep packet inspection (DPI) – NGFWs perform much more in-depth review of packets than traditional firewalls. This deep inspection can examine packet payloads and which application the packet accesses. This allows the firewall to enforce more granular filtering rules.
- Application awareness – Enabling this feature makes the firewall aware of which applications are running and which ports those applications use. This can protect against certain types of malware that aim to terminate a running process and then take over its port.
- Identity awareness lets a firewall enforce rules based on identity, such as which computer is being used, which user is logged in, etc.
- Sandboxing – Firewalls can isolate pieces of code associated with incoming packets and execute them in a “sandbox” environment to ensure they are not behaving maliciously. The results of this sandbox test can then be used as criteria when deciding whether or not to let the packets enter the network.
Web application firewalls (WAF):
While traditional firewalls help protect private networks from malicious web applications, WAFs help protect web applications from malicious users. A WAF helps protect web applications by filtering and monitoring HTTP traffic between a web application and the Internet. It covers web applications from attacks like cross-site forgery, cross-site scripting (XSS), file inclusion, and SQL injection.
A shield is placed between the web application and the Internet by deploying a WAF before a web application. While a proxy-based firewall protects a client machine’s identity using an intermediary, a WAF is a reverse proxy, protecting the server from exposure by having clients pass through the WAF before reaching the server.
A WAF operates through a set of rules, often called policies. These policies aim to protect against vulnerabilities in the application by filtering out malicious traffic. The value of a WAF comes in part from the speed and ease with which policy modification can be implemented, allowing for faster response to varying attack vectors; during a DDoS attack, rate limiting can be quickly implemented by modifying WAF policies.
Firewall-as-a-service (FWaaS):
Firewall-as-a-service (FWaaS) is a newer model for delivering firewall capabilities via the cloud. This service may also be called a “cloud firewall.” FWaaS forms a virtual barrier around cloud platforms, infrastructure, and applications, just as traditional firewalls form a barrier around an organization’s internal network. FWaaS is often better suited for protecting cloud and multi-cloud assets than conventional firewalls.
What is a ‘network firewall’?
A “network firewall” is any firewall that defends a network. By definition, almost all security firewalls are network firewalls, although firewalls can also protect individual machines.
While firewalls are an essential component of network security, this area has many other aspects to it as well, including access control, user authentication, and DDoS mitigation. Learn more about network security.
Are firewalls software-based or hardware-based?
Originally, firewalls were hardware appliances (see the history of firewalls section below). While some hardware firewalls are still in use, many modern firewalls are software-based, meaning they can run on several different types of hardware. FWaaS, meanwhile, is hosted in the cloud.
What is the history of firewalls?
Firewalls date back to the late 1980s. The first firewalls allowed or blocked individual data packets. They decided which packet to enable and which to stop by inspecting their network layer and transport layer headers to see their source and destination IP address and port (like viewing the “to” and “from” sections of an email). This prevented illegitimate traffic from getting through and stopped many malware attacks.
The next generation of firewalls added stateful capabilities. And newer generations (such as NGFWs) added the ability to inspect traffic at the application layer.
Just as firewall capabilities have evolved, so has the way firewalls are deployed. Originally, firewalls were physical hardware appliances that were plugged into a company’s networking infrastructure. But as business processes moved to the cloud, funneling all network traffic through a physical box became inefficient. Firewalls today can also run in software or virtually in the cloud.
