Introduction to eBPF and Cilium

Imagine being able to run programs directly at the core of your operating system—right within the kernel. There's no need to imagine, though, because Extended Berkeley Packet Filters (eBPF) make this possible.

eBPF allows you to execute functions safely and securely within the Linux kernel. While it has a variety of use cases, one of its most exciting applications is the enhanced observability it provides through tools such as Cilium.

In this article, we'll unpack eBPF, explain how it works, and why it's such an important technology. By the end, you'll also see why Cilium leverages eBPF for its superior performance in cloud-native environments.

What is eBPF?

eBPF is an open-source technology that allows small programs to be run directly within the Linux kernel in response to system events. It adds additional functionalities to the Linux Kernel without tampering with its source code or loading kernel modules.

eBPF provides a way to safely extend the kernel's capabilities by letting you attach small, secure programs to different hooks or points in the kernel where events occur (like network packets arriving, file system operations, or tracing system performance). These eBPF programs can inspect or manipulate data at these points.

The "extended" part of eBPF refers to its evolution from the original BPF. BPF was created in the early 1990s to analyze and filter network traffic. Back then, it gave engineers a way to monitor and control data packets flowing through a network.

Fast forward to today, and eBPF has expanded far beyond its networking roots. While BPF focused primarily on packet filtering, eBPF can now handle almost any kernel-level event.

The magic of eBPF lies in its ability to inject small, custom programs into the kernel using system calls that are triggered by specific events. This enables real-time processing of data — whether it's monitoring network traffic, enhancing security, or improving system performance — without modifying the kernel itself.

How does eBPF work?

As mentioned earlier, eBPF works by injecting custom programs into the kernel that can inspect or act on events without affecting system stability. These programs run in a secure, sandboxed environment and are triggered by specific events in the operating system.

An operating system is broken down into three layers. These are the physical, kernel, and user layers.

 Figure 1: Operating system architecture

The physical layer includes hardware components such as the network interface, storage, etc. Basically, this is where the physical operations happen.

Above that sits the kernel layer—the core of the operating system. The kernel manages all interactions between the hardware and the software and ensures that system resources are properly allocated.

The user layer is where applications are installed and executed. Applications interact with the kernel to access system resources but don't need to worry about how to communicate with different types of hardware. Instead, they rely on the kernel to handle these interactions.

eBPF programs are loaded into the kernel by user-space applications, which means developers can write these programs in high-level languages (such as C or even Python in some cases).

After writing the program, it is compiled into bytecode and sent to the kernel using system calls. Once inside the kernel, the eBPF program attaches itself to the event hooks where it needs to run, such as networking events, process tracing, or system calls.

When an application in the user layer makes a system call (a request to the kernel for a service, such as accessing a network card or reading from disk), this triggers an event within the kernel. At this point, eBPF programs can be executed in response to that event.

When the event occurs, the program is triggered, and it processes data in real-time without having to switch back and forth between user space and kernel space. This makes eBPF extremely efficient.

Why is eBPF important?

eBPF provides the extensibility, safety, and performance improvements that traditional kernel modules could never achieve. Below are some of the key benefits of this technology:

Isolated risk management

Traditionally, extending kernel functionality meant writing kernel modules — custom pieces of code that had to be deeply integrated into the kernel itself.

This approach came with significant risk because if a kernel module had a bug or malfunctioned, it could cause system-wide instability. Moreover, maintaining compatibility with kernel updates often became a nightmare for developers.

eBPF offers a safer solution. Instead of modifying the kernel directly, eBPF programs run isolated from the core kernel, which means they can be introduced and removed without risking kernel crashes.

This isolation makes it easier to extend the kernel while minimizing the chance of causing broader system issues.

Limited Kernel bootups

One of the most frustrating aspects of working with kernel modules is the need to restart the kernel when deploying or removing modules. This can be highly disruptive, especially in production environments where uptime is critical.

With eBPF, there's no need to restart the kernel when deploying new functionality. You can inject eBPF programs dynamically and in real-time, allowing changes to be made without downtime. This flexibility is a huge advantage for systems that require high availability.

High-end performance

eBPF also brings a significant performance boost. Because eBPF operates within the kernel and can hook into almost any part of the system, it can make decisions and take action at the earliest possible moment.

eBPF programs run directly where they are needed, avoiding the overhead of transferring data between the user layer and the kernel.

This efficiency means that you can monitor, filter, or modify system operations with minimal latency. This is particularly important for tasks like real-time traffic observation.

Cilium

eBPF has gravitated towards three primary use cases which are network filtering, security policies, and observability. These core functions have made eBPF a foundation for several advanced projects, such as bcc, Calico, Falco, and the focus of this article, Cilium.

What is Cilium?

Cilium is a platform built for cloud environments that helps manage networking and security, especially for Kubernetes. Instead of using older tools like iptables, Cilium relies on eBPF technology to handle network traffic more efficiently.

Cilium can monitor and manage traffic in real-time. This allows you to control how services connect and secure them better. Since it's built on eBPF, it's fast, scalable, and gives you deeper visibility into what's happening inside your network.

eBPF and Cilium

With eBPF working inside the Linux kernel, Cilium can observe traffic as it happens, providing:

• Fast, reliable network connections with little delay.
• Real-time security enforcement that adapts as your network changes.
• Detailed traffic monitoring to see exactly what's happening in your system.

What sets Cilium apart is how it uses eBPF to do all of this inside the kernel, making it much faster and more efficient than traditional methods.

Conclusion

eBPF allows Linux to do more by running small programs inside the kernel. This helps improve performance, security, and visibility without disrupting the system.

Cilium takes advantage of this technology to offer a more powerful way to handle cloud-native networking and security.

If you need a solution that can handle complex networks and give you detailed insights, try Cilium. It's built on eBPF, so it's fast and reliable.

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