how to inspect incoming tcp packets using ebpf

Neter

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23-11-2024

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eBPF (extended Berkeley Packet Filter) is a powerful technology that allows you to run programs in the Linux kernel without needing to load kernel modules. This makes it ideal for tasks like network monitoring and security analysis, including inspecting incoming TCP packets. This article will guide you through the process.

Understanding the Basics

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

• eBPF Programs: These are small, sandboxed programs that run within the kernel. They have access to various kernel data structures, allowing for detailed observation and manipulation of network traffic.

• BPF Maps: These are kernel data structures used for communication between the eBPF program and userspace. They act as shared memory, allowing the program to store and retrieve data.

• perf Tool: We'll use the perf tool to load and interact with our eBPF program. It's a powerful profiling and tracing tool built into the Linux kernel.

• TCP Packet Structure: Familiarity with the structure of a TCP packet (IP header, TCP header, payload) will be helpful for interpreting the data you capture.

Building the eBPF Program

Our eBPF program will attach to the kprobe of the tcp_v4_rcv function (for IPv4) or tcp_v6_rcv (for IPv6). This function is called whenever the kernel receives a new TCP packet. We'll use C for our program.

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

BPF_HASH(packet_info, u32, struct data_t);

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

int kprobe__tcp_v4_rcv(struct pt_regs *ctx) {
    struct sock *sk = (struct sock *)PT_REGS_PARM1(ctx);
    struct inet_sock *inet = inet_csk(sk);
    struct data_t data = {};

    data.saddr = inet->inet_saddr;
    data.daddr = inet->inet_daddr;
    data.sport = inet->inet_sport;
    data.dport = inet->inet_dport;
    data.seq = sk->__sk_common.skc_rcv_nxt; // Or use other relevant TCP sequence number

    packet_info.update(&inet->inet_daddr, &data);
    return 0;
}

// ... (Similarly for tcp_v6_rcv, adapting to IPv6 addresses) ...

This program defines a BPF hash map (packet_info) to store information about each incoming packet. The kprobe__tcp_v4_rcv function extracts source and destination IP addresses, ports, and sequence number from the kernel structures. Then it stores this information in the hash map.

Compiling and Loading the eBPF Program

You'll need the clang compiler and the bcc library installed. Save the code above as tcp_inspector.c. Compile it using:

clang -O2 -emit-llvm -c tcp_inspector.c -o tcp_inspector.bc
llc -march=bpf -filetype=obj tcp_inspector.bc -o tcp_inspector.o

Now load it with perf:

sudo perf probe -p $(pidof your_app) -e tcp_v4_rcv -f tcp_inspector.o

(Replace your_app with the PID of the application receiving TCP packets). For IPv6, you'd similarly use tcp_v6_rcv. You might need to adjust this based on your kernel version and specifics of how you want to track TCP connections. Consider adding more relevant fields as needed (e.g., TCP flags).

Retrieving and Displaying Data

After running the application for a while, you can retrieve the data from the BPF map. This generally requires another userspace program (often written in Python) to interact with perf and extract the data. This would then process and present the collected information in a human-readable format. The example below is a simplified illustration, and a production system would need more robust error handling and potentially different data representation.

Important Considerations

• Error Handling: The code provided is a simplified example and lacks robust error handling. A production-ready program should include checks for potential issues (e.g., map update failures).

• Kernel Version Compatibility: eBPF's capabilities and APIs can evolve across kernel versions. Ensure compatibility with your specific kernel.

• Performance Overhead: eBPF programs, while efficient, still introduce some overhead. Be mindful of the impact on your system's performance, particularly under heavy network load. Consider using sampling techniques if necessary.

• Security: eBPF programs run in the kernel, so it's crucial to ensure the code's security and avoid any potential vulnerabilities.

This comprehensive guide provides a foundation for inspecting incoming TCP packets using eBPF. Remember to expand upon this base with error handling, efficient data retrieval and potentially using more advanced eBPF techniques for advanced network monitoring and analysis. Remember to replace placeholder comments with the appropriate code for IPv6 support.