BPF and XDP Reference Guide


This documentation section is targeted at developers and users who want to understand BPF and XDP in great technical depth. While reading this reference guide may help broaden your understanding of Cilium, it is not a requirement to use Cilium. Please refer to the Getting Started guide and eBPF Datapath for a higher level introduction.

BPF is a highly flexible and efficient virtual machine-like construct in the Linux kernel allowing to execute bytecode at various hook points in a safe manner. It is used in a number of Linux kernel subsystems, most prominently networking, tracing and security (e.g. sandboxing).

Although BPF exists since 1992, this document covers the extended Berkeley Packet Filter (eBPF) version which has first appeared in Kernel 3.18 and renders the original version which is being referred to as “classic” BPF (cBPF) these days mostly obsolete. cBPF is known to many as being the packet filter language used by tcpdump. Nowadays, the Linux kernel runs eBPF only and loaded cBPF bytecode is transparently translated into an eBPF representation in the kernel before program execution. This documentation will generally refer to the term BPF unless explicit differences between eBPF and cBPF are being pointed out.

Even though the name Berkeley Packet Filter hints at a packet filtering specific purpose, the instruction set is generic and flexible enough these days that there are many use cases for BPF apart from networking. See Further Reading for a list of projects which use BPF.

Cilium uses BPF heavily in its data path, see eBPF Datapath for further information. The goal of this chapter is to provide a BPF reference guide in order to gain understanding of BPF, its networking specific use including loading BPF programs with tc (traffic control) and XDP (eXpress Data Path), and to aid with developing Cilium’s BPF templates.