how to inspect incoming tcp packets using ebpf

Breiz Heurope

3 min read

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11-03-2025

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eBPF (extended Berkeley Packet Filter) is a powerful technology that allows you to write programs that run in the Linux kernel without needing to load kernel modules. This makes it ideal for tasks like network monitoring and analysis, where you need to inspect network traffic in real-time with minimal overhead. This article will guide you through inspecting incoming TCP packets using eBPF.

Understanding the Basics

Before diving into the code, let's understand the fundamental concepts:

• eBPF Programs: These are small programs written in C (or other supported languages) that run inside the kernel. They're sandboxed for security.
• BPF Maps: These are kernel-side data structures used for communication between the eBPF program and userspace. They allow the program to store and retrieve data.
• perf Tool: This is a Linux performance analysis tool that's often used to load and interact with eBPF programs.
• TCP Packet Structure: Familiarizing yourself with the TCP header is essential for understanding what data you can access within the eBPF program. Key fields include source and destination ports, sequence numbers, and flags.

Setting up Your Environment

To follow along, you'll need:

• A Linux system: eBPF is a Linux-specific technology.
• BCC (BPF Compiler Collection): This provides tools and libraries for easier eBPF program development. Install it using your distribution's package manager (e.g., apt-get install linux-headers-$(uname -r) bcc).
• A basic understanding of C programming: While BCC simplifies things, you'll still be working with C code.

A Simple eBPF Program for TCP Packet Inspection

This program will print the source and destination IP addresses and ports of incoming TCP packets:

#include <uapi/linux/bpf.h>
#include <uapi/linux/ptrace.h>
#include <net/sock.h>
#include <bcc/proto.h>

BPF_PERF_OUTPUT(events);

struct data_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
};

int kprobe__tcp_rcv_established(struct pt_regs *ctx, struct sock *sk) {
    struct data_t data = {};
    data.saddr = sk->__sk_common.skc_rcv_saddr;
    data.daddr = sk->__sk_common.skc_daddr;
    data.sport = sk->__sk_common.skc_num;
    data.dport = sk->__sk_common.skc_dport;

    events.perf_submit(ctx, &data, sizeof(data));
    return 0;
};

Explanation:

• #include statements bring in necessary header files.
• BPF_PERF_OUTPUT(events) declares a map for sending data to userspace.
• struct data_t defines the structure for the data we want to capture.
• kprobe__tcp_rcv_established is the eBPF function attached to the tcp_rcv_established kernel function. This function is called when a TCP packet is received in an established connection.
• Inside the function, we extract the relevant fields from the sock structure and store them in data.
• events.perf_submit sends the data to the userspace program.

Compiling and Running the Program

1. Save: Save the code above as tcp_inspect.c.
2. Compile: Use BCC's clang to compile the code: clang -O2 -emit-llvm -c tcp_inspect.c -o tcp_inspect.bc
3. Load and Run: Use the bcc tool to load and run the program: sudo bcc -l tcp_inspect.bc

Interpreting the Output

The output will show the source and destination IP addresses and ports of incoming TCP packets. You'll need to translate the IP addresses from their numerical representation.

More Advanced Techniques

This example is a basic introduction. You can expand upon it to:

• Filter Packets: Add conditions within the eBPF program to only capture packets matching specific criteria (e.g., specific ports or IP addresses).
• Inspect Packet Payloads: Accessing the packet payload requires more complex techniques and careful consideration of security and performance implications.
• Use Different Probes: Explore other kernel functions (kprobes, uprobes, tracepoints) to capture different events related to TCP packets.
• Aggregate Data: Use BPF maps to aggregate data (e.g., count packets per source IP).

Security Considerations

Always exercise caution when running eBPF programs with kernel access. Ensure your code is well-tested and doesn't contain vulnerabilities. The sandboxed nature of eBPF mitigates some risks, but it's not foolproof.

Conclusion

eBPF provides a powerful and efficient way to inspect network traffic. This article demonstrated a basic example of inspecting incoming TCP packets. By exploring the advanced techniques mentioned above, you can create sophisticated network monitoring and analysis tools. Remember to prioritize security and test your programs thoroughly.