112 lines
4.5 KiB
Markdown
112 lines
4.5 KiB
Markdown
# Robovoice DHCP Tunnel Protocol
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## Overview
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Robovoice communicates with a remote STT server by tunneling through the
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DHCP UDP ports (68→67). This exploits a common killswitch exception: VPN
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software (e.g. WireGuard) blocks all traffic except DHCP, which is allowed
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for network connectivity maintenance.
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```
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[Robovoice client] --broadcast UDP :68→:67--> [STT server]
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[Robovoice client] <--unicast UDP :67→:68-- [STT server]
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```
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The client broadcasts NOP heartbeats while PTT is held. The server starts
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recording on the first NOP and stops when it receives OFF or when 150ms
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pass with no NOPs.
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## Transport
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- **Protocol:** UDP (connectionless, unreliable)
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- **Client → Server:** broadcast, source port 68, dest port 67
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- **Server → Client:** unicast, source port 67, dest port 68
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- **Client binds:** to a specific LAN interface IP on port 68 (with
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`SO_REUSEADDR` to coexist with the Windows DHCP service)
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- **No connection state** — purely fire-and-forget datagrams
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## Wire format
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All messages are plain text, newline-terminated (`\n`). Every message starts
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with the 6-byte magic `HKMSTR` to distinguish our traffic from real DHCP.
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### Client → Server
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**NOP (heartbeat while PTT held):**
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```
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HKMSTR <nonce>\n
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```
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Sent every 50ms while PTT is held. The nonce is an incrementing unsigned
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integer that makes each datagram unique. The server discards it — it's
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purely for packet uniqueness, not for any protocol logic.
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**OFF (PTT released):**
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```
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HKMSTR:OFF <nonce>\n
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```
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Sent once when PTT is released. This is the fast-stop signal. If lost, the
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150ms timeout acts as a backstop.
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### Server → Client
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**Partial transcript:**
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```
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HKMSTR:P <text>\n
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```
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Intermediate recognition result. Fire-and-forget. Client logs it but does
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not act on it.
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**Final transcript:**
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```
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HKMSTR:F <text>\n
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```
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Complete utterance. Client feeds this to the TTS engine.
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## Server state machine
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```
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┌──────────────────────────────────────────┐
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│ │
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▼ │
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┌──────────┐ first NOP ┌──────────────┐ │
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│ IDLE │ ──────────► │ RECORDING │ │
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└──────────┘ └──────────────┘ │
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│ │ │
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OFF │ │ 150ms │
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recv'd │ │ silence │
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▼ ▼ │
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┌─────────────┐ │
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│ PROCESSING │ │
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└─────────────┘ │
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│ │
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STT │ │
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done │ │
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▼ │
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send HKMSTR:F ─────────┘
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```
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- **IDLE → RECORDING:** first NOP received, start mic capture
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- **RECORDING → PROCESSING:** OFF received, OR 150ms since last NOP
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- **PROCESSING → IDLE:** STT done, send `HKMSTR:F <text>`
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## Timing
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| Parameter | Value | Purpose |
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|-----------|-------|---------|
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| NOP interval | 50ms | Heartbeat frequency while PTT held |
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| Silence timeout | 150ms | Stop recording if no NOPs (3 missed = lost OFF) |
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| NOP bandwidth | ~20 msg/s × ~20 bytes | ~400 bytes/s — negligible |
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## Why this works
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1. **Outbound broadcast `:68→:67` to `255.255.255.255`** passes the
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WireGuard WFP killswitch (DHCP exception matches this exact pattern)
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2. **Inbound `:67→:68`** has no address restriction in the WFP rule, so
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unicast replies pass through
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3. **Binding to a specific interface IP** (not `0.0.0.0`) wins unicast
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delivery over the Windows DHCP client service
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4. **NOP spam** ensures the ON message gets through even at 5% packet loss
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(3 consecutive NOPs = ~0.01% drop probability)
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5. **150ms timeout** is the backstop for lost OFF — at 50ms intervals, 3
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consecutive NOPs must all be lost to false-stop
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