Augur: Detecting Filtering via Side Channels
💡 The Core Idea
Augur detects if two hosts (e.g., a Reflector and a Site) are blocked from talking to each other, without having direct control over either of them.
🧠Mechanism: IP ID Side Channel
It leverages the global IP ID counter in IP headers.
- Probe: Measure the Reflector’s current IP ID.
- Perturb: Send a spoofed packet to the Site, pretending to be the Reflector.
- If the Site receives it, it replies to the Reflector.
- The Reflector replies with a RST, incrementing its IP ID.
- Measure: Check the Reflector’s IP ID again.
- Incremented? The hosts communicated (No filtering).
- Not Incremented? The packets were blocked (Filtering detected).