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2.0 - Full Kernel Bypass: DPDK
2.1 - The Kernel Fastpath: XDP
2.2 - XDP Internals: Actions and Modes
2.3 - AF_XDP: A Zero-Copy Bridge to Userspace
3 - Building the Scanner
3.0 - Core Design
3.1 - The eBPF Filter Component
3.2 - The Userspace Application
3.2.0 - Setup and Initialization
3.2.1 - The Packet Transmission Loop
3.2.2 - The Packet Reception Loop
4 - Performance Analysis
4.0 - A Note on Benchmarking
4.1 - Head-to-Head: AF_XDP vs. masscan
5 - Extending the AF_XDP Framework
5.0 - High-Speed HTTP/HTTPS Application Fuzzing and L7 DDoS
5.1 - Stateless UDP Fuzzing and DDoS Amplification
5.2 - High-Entropy SYN Flooding
6 - Caveats and Considerations
7 - Conclusion
8 - References
9 - Source Code
--[ 0 - Introduction
The network scanner has always been a fundamental tool in my arsenal. As
network interface speeds have increased, I found my tools were constrained
by the overhead of the operating system's kernel network stack. This has
become a significant bottleneck when doing internet-scale scans.
In this article, I describe the method I used to build a high-performance
port scanner using the Linux kernel's eBPF and AF_XDP subsystems. This
approach creates a kernel fastpath that bypasses the traditional network
stack, allowing my application to interact more directly with the network
driver for line-rate filtering and zero-copy data transfer.
--[ 1 - The Slow Path: Traditional Scanning Methods
---[ 1.0 - Per-Connection Syscall Overhead
My work began by analyzing the conventional port scanning method, which
uses the connect() syscall. For each port, the application creates a socket,
initiates a TCP handshake, and waits for the kernel to report the outcome.
Every socket() and connect() call is a context switch into the kernel,
consuming CPU cycles and introducing significant latency, making it
impractical for my purposes.
---[ 1.1 - Inefficient Packet Filtering with AF_PACKET
I then examined raw sockets (AF_PACKET), which allow a userspace
application to receive raw link-layer frames, bypassing the kernel's
high-level network stack. While this is an improvement for SYN scanning,
it does not provide the performance of a true kernel bypass. Packets are
still delivered via the standard kernel data path, which involves overhead
from context switches and memory copies for every packet received by the
interface. This inherent slowness compared to a direct kernel bypass was
unacceptable for my goals.
--[ 2 - Kernel Bypass and Fastpath Architectures
---[ 2.0 - Full Kernel Bypass: DPDK
To achieve maximum performance, some frameworks like the Data Plane
Development Kit (DPDK) implement a full kernel bypass. They use custom
Poll-Mode Drivers (PMDs) that unbind a network interface from the kernel's
control, giving a userspace application exclusive access. While this is
very fast, it comes with drawbacks: it requires custom drivers, is invasive
to the system, and often requires pinning a CPU core at 100% utilization
for polling.
---[ 2.1 - The Kernel Fastpath: XDP
It is important to clarify that AF_XDP is not a kernel bypass in the same
vein as DPDK. It is a highly efficient kernel fastpath that works in
cooperation with existing kernel drivers. My XDP program is an eBPF program
attached to a low-level hook in the network driver, triggered for every
incoming packet at the earliest possible point.
---[ 2.2 - XDP Internals: Actions and Modes
Once my eBPF program is running at the XDP hook, it can inspect the raw
packet data and return a verdict that determines the packet's fate. The
primary actions are XDP_PASS, XDP_DROP, XDP_TX, and XDP_REDIRECT. The
XDP_REDIRECT action is what allows my program to forward a packet to an
AF_XDP socket in userspace.
You can load XDP programs in three modes, which affects performance:
- Native XDP: The program is loaded directly by a supported network card
driver, providing the highest performance.
- Offloaded XDP: The program is offloaded to and executed directly on the
NIC hardware, requiring specific SmartNICs.
- Generic XDP: The program is hooked later in the kernel's network path,
after an sk_buff has been allocated. This mode serves as a fallback for
testing or for use on unsupported hardware.