Ultimate Guide on SPI Firewall

SPI Firewall

A firewall exists to protect you from different attacks and viruses that seek to invade and steal your data. Just as there are different levels of protection you can get from different VPN service providers, firewalls also vary in the degree of protection they offer.

Table of Content

1 . What is SPI?

2. What Is an SPI Firewall?

3. How SPI Firewalls Work?

SPI firewall is an example among a lot of firewalls you can choose from but what is an SPI firewall and how does it work?

What is SPI?


SPI is short for Stateful Packet Inspection (also known as dynamic packet filtering) and it’s a technology that monitors active internet connections, and checks that incoming data packets correspond to the internet connection. By this, it decides on which connections to deny or grant access through the firewall.

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Devices send data in bits called packets for easy processing from the receiving end. So a large data unit would be split into smaller units before being sent. Hackers can however tamper with these packets to cause harm to the receiver and so an SPI firewall is important to check that the packets are not harmful and from a legitimate source, and that they correspond to an already-existing connection. Packets that don’t correspond to the already existing connection are discarded thereby reducing the risk of being hacked.

What Is an SPI Firewall?


An SPI firewall protects the receiving server from attacks by examining every incoming packet of data to ensure it corresponds to already existing connections.

Unlike the Stateful Packet Inspection, stateless firewalls base their examination of incoming traffic on static values like the source and destination address without any consideration for the packet’s connection traffic. The same criteria are used for different packets without considering the packet’s connection traffic. And so this type of firewall cannot be customized to deny or allow connections. They don’t also authenticate packets of data, and can’t detect the legitimacy of the data packets. In comparison to the SPI firewall, stateless firewalls are not as safe but are faster.

How SPI Firewalls Work?


SPI firewalls remember the features of a connection and validate incoming packets with this information. It examines packets, establishes rules, and then stores the information gotten so that every other incoming request would be inspected based on set criteria. You can say an SPI firewall sees a packet beyond just its contents.

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Due to the already collected information on a connection and established criteria, the SPI firewall doesn’t have to inspect every packet thoroughly, making it work fast especially when in comparison to Deep Packet Inspection (DPI). DPI deconstructs the data packets to check for their integrity and look for any malicious code within. It’s used for different purposes including security, internet censorship, network management, or data mining. With DPI, you get top security but slow speed.


Hackers are always looking for means to infiltrate and steal personal and sensitive data for their malicious acts. They can infiltrate a data packet and get into the receiving server to cause havoc and that’s why firewalls are necessary. There are different types of firewalls for the inspection of data packets and the denial or acceptance of connections and the SPI firewall is one of them. But what is an SPI firewall? By this article, you have undoubtedly gotten not just the answer to that question, but also how this firewall works.