this post was submitted on 19 Mar 2026
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I built a minimalist SPA tool using eBPF/XDP to keep ports invisible. First project in Go! (github.com)
submitted 4 days ago by lucy@lemy.nl to c/linux@lemmy.ml
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Hi guys, I wanted to share a project I’ve been working on called xSpa. It's an implementation of Single Packet Authorization that works at the XDP level.

I built this because I wanted something faster and more DDoS-resilient than traditional port-knocking or SPA tools that rely on userspace processing or iptables. Here, the "drop-all" logic happens right at the driver level.

Key bits:

 L1 verification (SipHash) in kernel space.

 L2 (ChaCha20-Poly1305) in Go userspace.

 It uses the eBPF ring buffer for communication.

This is my first Go project and my first shot at Open Source. I’m still a bit of a noob when it comes to kernel-level programming, so I’d love to get some feedback on the architecture and security. If anyone has time to check the code, I’d love to hear your thoughts on how to make it better.

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[–] non_burglar@lemmy.world 0 points 3 days ago (1 children)

Sorry, I'm failing to understand what this does, mostly because your description is mixing tcpip, socket, and xdp terminology.

It's an implementation of Single Packet Authorization that works at the XDP level.

Sure, but to do what, exactly? Is this meant to run on Linux routers as a prefilter? Or on hosts themselves? In the second case, why not just use 802.11x?

I don't get the use case, or what this has to do with DDOS.

[–] lucy@lemy.nl 2 points 3 days ago (1 children)

Thanks for the feedback! You’re right, my description was a bit messy. Let me clarify the goal and the use case

The goal to make a VPS on the public internet completely invisible to unauthorized scans while allowing access to services (like SSH) only after a valid SPA packet is received

Traditional SPA tools (like fwknop) often use libpcap to sniff packets or iptables to block them. In both cases, unauthorized packets still hit the kernel network stack. If an attacker floods the port with junk, the kernel still spends CPU cycles processing them before they get dropped

By using XDP, I move the "drop-all" logic to the earliest possible stage. This is where the DDoS resilience comes from we drop unauthorized traffic before sk_buff allocation, making it significantly cheaper in terms of CPU resources

It’s designed to run on the host itself, acting as a "stealth" firewall

A major advantage here is that xSpa doesn't touch the netfilter/iptables stack. Unlike fwknop, which has to dynamically manage firewall rules (often leading to conflicts with Docker, UFW, or complex rulesets), xSpa operates at a lower level. It’s completely independent of whatever you have going on in your iptables configuration

And you can’t use 802.1X to protect an SSH port on a public cloud VPS

[–] non_burglar@lemmy.world 1 points 3 days ago

How does it route an xdp packet across an IP network of unknown hops? I thought xdp required the same broadcast domain for hosts to be able to use it.