On a computer, every application constantly exchanges data. Yet, few tools allow you to know precisely what is being transmitted. Portmaster fills this gap with an open-source application firewall that asks for your permission before each connection.
Here is an overview of this innovative firewall, useful for any system administrator or user concerned about their privacy.
Portmaster: an innovative and accessible open-source application firewall
Portmaster is not a firewall like the others. Rather than simply blocking or allowing a port, it examines each network connection in real time and associates it with the specific application that initiated it. Imagine being able to tell your system: “This program has the right to communicate with this server, but not that one.” It’s this granularity that makes it so valuable.
This approach replaces traditional port filtering with flexible control based on three criteria: the executable, the traffic destination, and the decision to block or allow. This process is comparable to a stationmaster directing each train according to its destination and the passenger it carries.
The interface provides a clear overview, including the binary name, the contacted domain, the target country, the network operator (ASN), and the verdict applied to the connection. This clarity facilitates immediate decision-making.
How does Portmaster integrate with Windows and Linux systems?
On Windows, Portmaster installs a kernel driver that sits between the IP network layer and the data link layer. This method ensures that it sees all outgoing traffic without unduly slowing down the system. It’s a bit like placing a control station just before trains depart.
On Linux, there are no proprietary kernel drivers. The software relies on iptables with nfqueue to capture packets, correlated with eBPF to identify the source process. This combination leverages native Linux kernel features and avoids overloading user space.
In both cases, priority is given to performance and precision of supervision, with an economy of resources reminiscent of the art of good briquetting on a campfire: exactly what is needed, neither too much nor too little.
A complete functional set to enhance privacy and security
Portmaster doesn’t stop at filtering! It also includes a secure DNS resolver It encrypts your DNS queries and applies upstream filtering, blocking unwanted domains before they are even accessed. This feature is the first line of defense against tracking and malware.
The software includes regularly updated blocklists for ads, telemetry, and malware. Integration is local, eliminating the need for external or complex network solutions like Pi-hole, with the added benefit of protection that follows the machine everywhere.
Finally, Portmaster offers an anonymization network called Safety Privacy Network, a paid service very similar to a multi-relay, multi-address VPN. It uses successive-layered encryption that routes each connection through a relay close to the destination.
Getting started and fine-tuning application by application
Upon launch, a wizard guides you through activating filtering and secure DNS. Then, the main page displays all live connections. You can immediately see which applications are communicating, with which servers, and whether access is allowed or blocked.
Each application has its own detailed profile. You can completely block traffic, restrict internet access while preserving the local network, or target blocking to P2P or direct connections without DNS. These powerful options allow for precise control over each program’s activity—a luxury that has become readily available.
Note that the function to block incoming connections is enabled by default, which reduces the risk of an application serving as a sneaky entry point into your machine.
Concrete uses that speak for themselves
During installation, some users discover that familiar applications no longer work. This is often because they need to contact remote servers to function, such as DeepL, which performs translation via the cloud. The firewall blocks these connections, offering a level of control that may seem drastic but is essential for privacy.
The software also reveals that programs renowned for their security, such as Malwarebytes, initiate more connections than expected. This highlights the amount of telemetry and automated communication that is commonplace, even under Linux, where one would assume privacy is protected.
In this regard, he points out that telemetry is not unique to Windows: some recent Linux distributions or applications also collect data without your knowledge.
Uncompromising security management under Linux and Windows
Portmaster does not replace an antivirus program, but acts as a complement. It functions as a communication filter, not as a code execution control. Under Linux, it works ideally with an antivirus like ClamAV, thus embodying a key principle of computer security: defense in depth.
This technical choice demonstrates a well-considered understanding of the current needs of users and system administrators. It’s not a mere gadget, but rather a pragmatic tool with proven effectiveness.
On Windows, it complements the native firewall, providing visibility and granular control that is otherwise lacking. All of this is contained within a fluid, user-friendly, and well-documented interface.