What is a Firewall and How does it work?


Whether you just started getting into the subject or are just delving out of curiosity. You’ve probably come across a firewall before and even been blocked by one.

In this guide we will try to explain you what is a firewall and how does it work. Here’s a quick outline to help you navigate through this guide:

1. TCP Network Packets
2. Types of Firewalls
3. Firewall Rules
4. Incoming & Outgoing Traffic
5. Firewall Software & Tools

If you want to find how to unblock yourself from a firewall, please read this article: Unblock my IP from the firewall.

What is a firewall?

A firewall is a system that provides network security. It filters incoming and outgoing network traffic based on a set of user-defined rules.
The firewall’s purpose is to limit or eliminate unwanted network communications while allowing all legitimate communication to flow freely.

In most server infrastructures, firewalls provide an essential layer of security that, combined with other measures, prevent attackers from accessing your servers in malicious ways.

1. TCP Network Packets

Transport Control Protocol or TCP network traffic, moves around a network in packets, which are containers that consist of a packet header.

It contains control information such as:

  • Source and destination addresses
  • Packet sequence information.

This data is also known as a payload. While the control information in each packet helps to ensure that its associated data gets delivered properly, the elements it contains also provides firewalls with a variety of ways to match packets against firewall rules.

It is important to note that successfully receiving incoming TCP packets requires the receiver to send outgoing acknowledgment packets back to the sender. The combination of the control information in the incoming and outgoing packets can be used to determine the connection state such as new, established, related between the sender and receiver.

2. Types of Firewalls

There are three basic types of network firewalls: packet filtering (stateless), stateful, and application layer.

  • Packet filtering, or stateless: firewalls work by inspecting individual packets in isolation. As such, they are unaware of connection state and can only allow or deny packets based on individual packet headers.
  • Stateful firewalls: are able to determine the connection state of packets, which makes them much more flexible than stateless firewalls. They work by collecting related packets until the connection state can be determined, before any firewall rules are applied to the traffic.
  • Application firewalls: go one step further by analyzing the data being transmitted, which allows network traffic to be matched against firewall rules that are specific to individual services or applications. These are also known as proxy-based firewalls.

3. Firewall Rules

Network traffic that go though a firewall is matched against certain rules to determine if it should be allowed or not. An easy way to explain what firewall rules looks like is to image it with some examples.

Suppose you have a server with this list of firewall rules that apply to incoming traffic:

  1. Accept new and established incoming traffic to the public network interface on port 80 and 443 (HTTP and HTTPS web traffic)
  2. Drop incoming traffic from IP addresses of the non-technical employees in your office to port 22 (SSH)
  3. Accept new and established incoming traffic from your office IP range to the private network interface on port 22 (SSH)

PLEASE NOTE: the first word in each of these examples is either “accept”, “reject”, or “drop”. This will define the action that the firewall should do in the event that a piece of network traffic matches a rule. 

  • Accept: means to allow the traffic through, 
  • Reject: means to block the traffic but reply with an “unreachable” error.
  • Drop: means to block the traffic and send no reply. The rest of each rule consists of the condition that each packet is matched against.

Usually, network traffic is matched against a list of firewall rules in a sequence, or chain, from first to last. More specifically, once a rule is matched, the associated action is applied to the network traffic in question. In our example, if an accounting employee attempted to establish an SSH connection to the server they would be rejected based on rule 2, before rule 3 is even checked. A system administrator, however, would be accepted because they would match only rule 3.

Default Policy

It is typical for a chain of firewall rules to not explicitly cover every possible condition. For this reason, firewall chains must always have a default policy specified, which consists only of an action: accept, reject, or drop.

Suppose the default policy for the example chain above was set to drop. If any computer outside of your office attempted to establish an SSH connection to the server, the traffic would be dropped because it does not match the conditions of any rules.

If the default policy were set to accept, anyone, except your own non-technical employees, would be able to establish a connection to any open service on your server. This would be an example of a very poorly configured firewall because it only keeps a subset of your employees out.

4. Incoming and Outgoing Traffic

As network traffic, from the perspective of a server, can be either incoming or outgoing, a firewall maintains a distinct set of rules for either case. Traffic that originates elsewhere, or incoming traffic, is treated differently to outgoing traffic that the server sends.

It is typical for a server to allow most outgoing traffic because the server is usually, to itself, trustworthy. Still, the outgoing rule set can be used to prevent unwanted communication in the case that a server is compromised by an attacker or a malicious executable.

In order to maximize the security benefits of a firewall, you should identify all of the ways you want other systems to interact with your server, create rules that explicitly allow them, then drop all other traffic. Keep in mind that the appropriate outgoing rules must be in place so that a server will allow itself to send outgoing acknowledgements to any appropriate incoming connections. Also, as a server typically needs to initiate its own outgoing traffic for various reasons.

For example, downloading updates or connecting to a database. It is important to include those cases in your outgoing rule set as well.

Writing Outgoing Rules

Suppose our example firewall is set to drop outgoing traffic by default. This means our incoming accept rules would be useless without complementary outgoing rules.

To complement the example incoming firewall rules 1 and 3, from the Firewall Rules section, and allow proper communication on those addresses and ports to occur, we could use these outgoing firewall rules:

  1. Accept established outgoing traffic to the public network interface on port 80 and 443 (HTTP and HTTPS).
  2. Accept established outgoing traffic to the private network interface on port 22 (SSH).

5. Firewall Software and Tools

Now that we’ve gone over how firewalls work, let’s take a look at common software packages that can help us set up a firewall. While there are many other firewall-related packages, these are effective and are the ones you will encounter the most.


Iptables is a standard firewall included in most Linux distributions by default (a modern variant called nftables will soon begin to replace it). It is actually a front end to the kernel-level netfilter hooks that can manipulate the Linux network stack. It works by matching each packet that crosses the networking interface against a set of rules to decide what to do.


UFW, which stands for Uncomplicated Firewall, is an interface to iptables that is geared towards simplifying the process of configuring a firewall.


FirewallD is a complete firewall solution available by default on CentOS 7 servers. Incidentally, FirewallD uses iptables to configure netfilter.


Fail2ban is an intrusion prevention software that can automatically configure your firewall to block brute force login attempts and DDOS attacks.


Now that you understand how firewalls work, you can look confidently into implementing a firewall that will improve your security of your server setup.

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