doytsujin / xdp-tutorial

XDP tutorial

XDP Hands-On Tutorial

This repository contains a tutorial that aims to introduce you to the basic steps needed to effectively write programs for the eXpress Data Path (XDP) system in the Linux kernel, which offers high-performance programmable packet processing integrated with the kernel.

The tutorial is composed of a number of lessons, each of which has its own repository. Start with the lessons starting with “basicXX”, and read the README.org file in each repository for instructions for that lesson.

Keep reading below for an introduction to XDP and an overview of what you will learn in this tutorial, or jump straight to the first lesson.

Table of Contents

• Introduction
• First step: Setup dependencies
• How the lessons are organised
  • Basic setup lessons
  • Packet processing lessons
  • Advanced lessons

Introduction

XDP is a part of the upstream Linux kernel, and enables users to install packet processing programs into the kernel, that will be executed for each arriving packet, before the kernel does any other processing on the data. The programs are written in restricted C, and compiled into the eBPF byte code format that is executed and JIT-compiled in the kernel, after being verified for safety. This approach offers great flexibility and high performance, and integrates well with the rest of the system. For a general introduction to XDP, read the academic paper (pdf), or the Cilium BPF reference guide.

This tutorial aims to be a practical introduction to the different steps needed to successfully write useful programs using the XDP system. We assume you have a basic understanding of Linux networking and how to configure it with the iproute2 suite of tools, but assume no prior experience with eBPF or XDP.

The tutorial is a work in progress, and was initially created for use as a live tutorial at the Netdev Conference in Prague in March 2019. Since the kernel BPF subsystem continues to develop at a rapid pace, this tutorial has not kept up with all developments. However, everything presented here will work with recent kernels, and this tutorial functions as a self-contained introduction that anyone can go through to learn the XDP basics. Input and contributions to advance towards this goal are very welcome; just open issues or pull requests in the Github repository.

First step: Setup dependencies

Before you can start completing step in this tutorial, you will need to install a few dependencies on your system. These are described in setup_dependencies.org.

We also provide a helper script that will set up a test environment with virtual interfaces for you to test your code on. This is introduced in the basic lessons, and also has it’s own README file.

How the lessons are organised

The tutorial is organised into a number of lessons; each lesson has its own subdirectory, and the lessons are grouped by category:

• Basic setup (directories starting with basicXX)
• Packet processing (directories starting with packetXX)
• Advanced topics (directories starting with advancedXX)

We recommend you start with the “basic” lessons, and follow the lessons in each category in numerical order. Read the README.org file in each lesson directory for instructions on how to complete the lesson.

Basic setup lessons

We recommend you start with these lessons, as they will teach you how to compile and inspect the eBPF programs that will implement your packet processing code, how to load them into the kernel, and how to inspect the state afterwards. As part of the basic lessons you will also be writing an eBPF program loader that you will need in subsequent lessons.

Packet processing lessons

Once you have the basics figured out and know how to load programs into the kernel, you are ready to start processing some packets. The lessons in the packet progressing category will teach you about the different steps needed to process data packets, including parsing, rewriting, instructing the kernel about what to do with the packet after processing, and how to use helpers to access existing kernel functionality.

Advanced lessons

After having completed the lessons in the basic and packet processing categories, you should be all set to write your first real XDP program that will do useful processing of the packets coming into the system. However, there are some slightly more advanced topics that will probably be useful once you start expanding your program to do more things.

The topics covered in the advanced lessons include how to make eBPF programs in other parts of the kernel interact with your XDP program, passing metadata between programs, best practices for interacting with userspace and kernel features, and how to run multiple XDP programs on a single interface.