# Android Cast — On-Demand Remote Access (Reverse Tunnel) Proposals

<!-- doc-meta:start -->
| Field | Value |
|---|---|
| Author | Anton Afanasyeu |
| Revision | R1 |
| Creation date | 2026-06-03 |
| Last modification date | 2026-06-03 |
| Co-authored |  |
| Severity | medium |
| State | pending review |
| Document type | DR |
<!-- doc-meta:end -->

\newpage
\newpage

---


_Date: 2026-06-02 — **merged document**: initial proposal (rev. 1) + infra review addendum (rev. 2)_

---

---

---

---

---


---


---

---


---

## Table of contents

<!-- toc -->
- [Document history](#document-history)
- [Context and constraints](#context-and-constraints)
- [Executive summary](#executive-summary)
  - [Initial proposal (rev. 1)](#initial-proposal-rev-1)
  - [Refined after infra review (rev. 2)](#refined-after-infra-review-rev-2)
- [Stakeholder questions (rev. 2 — added to requirements)](#stakeholder-questions-rev-2-added-to-requirements)
- [Problem statement](#problem-statement)
- [Proposed control plane (heartbeat type: ra)](#proposed-control-plane-heartbeat-type-ra)
  - [Device → BE (poll, random interval 1–7 min when enabled)](#device-be-poll-random-interval-17-min-when-enabled)
  - [BE → device (only if whitelisted + operator requested session)](#be-device-only-if-whitelisted-operator-requested-session)
  - [Disable (user unchecks box or admin revokes)](#disable-user-unchecks-box-or-admin-revokes)
  - [Field reference](#field-reference)
  - [State machine (simplified)](#state-machine-simplified)
- [Network topology — Gentoo FE → Alpine BE (rev. 2)](#network-topology-gentoo-fe-alpine-be-rev-2)
  - [FE ↔ BE tuning (by track)](#fe-be-tuning-by-track)
- [Security model](#security-model)
  - [Key handling options (rev. 1)](#key-handling-options-rev-1)
- [Embedding in the Android app (rev. 2)](#embedding-in-the-android-app-rev-2)
- [Backend integration ease — Alpine BE (rev. 2)](#backend-integration-ease-alpine-be-rev-2)
- [Licensing and open source (rev. 2)](#licensing-and-open-source-rev-2)
- [What “Custom file API” means (rev. 2 — clarified)](#what-custom-file-api-means-rev-2-clarified)
- [Tooling alternatives — full survey (rev. 1)](#tooling-alternatives-full-survey-rev-1)
- [Tooling summary — infra-focused (rev. 2)](#tooling-summary-infra-focused-rev-2)
- [Comparison matrix — initial scoring (rev. 1)](#comparison-matrix-initial-scoring-rev-1)
- [Comparison matrix — FE proxy focus (rev. 2)](#comparison-matrix-fe-proxy-focus-rev-2)
- [WireGuard vs SSH — decision lens (rev. 2)](#wireguard-vs-ssh-decision-lens-rev-2)
- [Recommendations](#recommendations)
  - [Initial (rev. 1)](#initial-rev-1)
  - [Refined (rev. 2 — Gentoo FE → Alpine BE)](#refined-rev-2-gentoo-fe-alpine-be)
- [RBAC roles (BE)](#rbac-roles-be)
- [BE admin UI (crashes look-and-feel)](#be-admin-ui-crashes-look-and-feel)
- [MariaDB schema (sketch — rev. 1)](#mariadb-schema-sketch-rev-1)
- [Android app changes (sketch — rev. 1)](#android-app-changes-sketch-rev-1)
- [Estimates (engineering)](#estimates-engineering)
- [Open questions (merged)](#open-questions-merged)
- [Rev. 3 — WG vs SSH Q&A + bastion deep-dive](#rev-3-wg-vs-ssh-qa-bastion-deep-dive)
  - [Q1 — Exact FE/BE rules: WireGuard (DNAT) vs SSH (no DNAT required)](#q1-exact-febe-rules-wireguard-dnat-vs-ssh-no-dnat-required)
  - [Q2 — Can we use ports other than :80? (heartbeats and everything else)](#q2-can-we-use-ports-other-than-80-heartbeats-and-everything-else)
  - [Q3 — Why is “Proxyable (FE→BE)” low for WG (2) and high for SSH (5)?](#q3-why-is-proxyable-febe-low-for-wg-2-and-high-for-ssh-5)
- [Rev. 3 — Bastion Q&A](#rev-3-bastion-qa)
  - [Q4 — How easy to implement? Command checklist (FE + BE)](#q4-how-easy-to-implement-command-checklist-fe-be)
  - [Q5 — Bastion: file transfers only, or other operations too?](#q5-bastion-file-transfers-only-or-other-operations-too)
- [Appendix 1 — WG test configuration (Linux laptop)](#appendix-1-wg-test-configuration-linux-laptop)
  - [Network topology (v1 WireGuard)](#network-topology-v1-wireguard)
  - [Prerequisites (laptop)](#prerequisites-laptop)
  - [A. HTTP E2E (no tunnel yet)](#a-http-e2e-no-tunnel-yet)
  - [B. UDP / WireGuard from laptop (after open session)](#b-udp-wireguard-from-laptop-after-open-session)
  - [C. Quick checks before blaming WG](#c-quick-checks-before-blaming-wg)
- [Appendix 2 — WG production configuration](#appendix-2-wg-production-configuration)
  - [BE config.php (remote_access)](#be-configphp-remoteaccess)
  - [BE (Alpine) — WireGuard interface](#be-alpine-wireguard-interface)
  - [FE (Gentoo) — UDP DNAT (not nginx)](#fe-gentoo-udp-dnat-not-nginx)
  - [MariaDB / migrations](#mariadb-migrations)
  - [Production verify (on BE or laptop)](#production-verify-on-be-or-laptop)
  - [Android (field device)](#android-field-device)
- [Source linkage](#source-linkage)
<!-- /toc -->

**Documentation index:** [README.md](README.md)

---

## Document history

| Rev | Focus | Summary |
|---|---|---|
| **1** | Initial research memo | On-demand reverse tunnel via heartbeat `type: ra`; compare WG, SSH, tinc, Tailscale, etc.; hybrid **WireGuard outbound + SSH/SFTP bastion** as first pick. |
| **2** | Stakeholder Q&A | Gentoo FE → Alpine BE path, embed WG/SSH in app, licensing, Custom file API clarified; refined pick **reverse SSH/SFTP** (**Proxyable**) over WG (**Lightweight**) for this nginx topology. |
| **3** | WG vs SSH + bastion deep-dive | Exact FE/BE commands (DNAT vs `stream`), port policy, **Proxyable** scoring explained, bastion scope (files vs full debug). |

Both revisions are kept below. Where they differ, **§14 Recommendations** lists both and explains why rev. 2 refined the tunnel choice. **§Rev. 3** adds operator-ready command checklists for the final pick.

---

## Context and constraints

- **Need:** occasional remote debug / manual settings on **one field device** (e.g. edit `codecs.json` in app home, inspect logs) without a **persistent** device↔BE tunnel.
- **Stack:** minimalistic LAMP (nginx + php-fpm + MariaDB) on BE; Android app already posts to `/api/heartbeat.php`.
- **Traffic path (rev. 2):** **Mobile app → Gentoo FE** (TLS, firewall, nginx reverse proxy) **→ Alpine BE** (XEN HVM, project services). Heartbeat already follows this; any data tunnel must be tunable on **both** FE and BE.
- **Security:** no standing inbound connection from BE to device; user opt-in; admin whitelist; full audit trail; minimal secret exposure on wire.
- **Ops:** sessions expire on inactivity; 24h DB cleanup; user can disable instantly from app settings.
- **Convenience (rev. 2):** SCP-/SFTP-**like** file exchange is **not mandatory** but highly desirable for `codecs.json` and logs.

---

## Executive summary

### Initial proposal (rev. 1)

- **Recommended:** on-demand reverse tunnel orchestrated by BE heartbeat (`type: ra`) + **WireGuard or SSH over a single ephemeral port** on BE, with **per-session keys** and **PHP admin UI** matching crashes console look-and-feel.
- **Not recommended for v1:** always-on VPN mesh (tinc/Tailscale on every device), public inbound SSH to phones, or BE-initiated connections through carrier NAT.
- **Pattern:** device polls BE at **random 1–7 min** when user enables “Remote access”; BE responds only if device is **whitelisted**; device then **dials out** and attaches tunnel; operator connects to BE **bastion/port**, not to the phone directly.
- **Primary pick (rev. 1):** WireGuard outbound + SSH/SFTP on bastion for operator (combines mobile simplicity with familiar file edit workflow for `codecs.json`).
- **Fallback (rev. 1):** if UDP blocked on target networks, use **chisel/frp over HTTPS** on same `ra` vhost with identical control plane.

### Refined after infra review (rev. 2)

- **Control plane:** unchanged (heartbeat `type: ra`, checkbox, whitelist, jittered poll, device dials out).
- **Tunnel choice (refined):** prefer **reverse SSH (+ SFTP)** over WireGuard for v1 **on this stack**.
- **One word — SSH:** **Proxyable** — TCP (nginx **`stream`**) matches Gentoo FE → Alpine BE like existing HTTP proxy; SFTP gives SCP-like files.
- **One word — WireGuard:** **Lightweight** — excellent embeddable crypto, but **UDP does not ride nginx HTTP reverse-proxy**; FE needs UDP DNAT/firewall rules.
- **Custom file API:** optional third track — HTTPS upload/download via PHP (§10); good Phase-1 or narrow path without sshd.

---

## Stakeholder questions (rev. 2 — added to requirements)

| # | Question | Short answer |
|---|---|---|
| Q1 | Is WireGuard “best”? | Best **crypto/simplicity**, not best **FE→BE proxy fit** for your stack. |
| Q2 | Embed WG in app or separate app? | **Embed** via `wireguard-android` tunnel library + `VpnService`; no standalone WireGuard app. |
| Q3 | Embed SSH or separate app? | **Embed** Dropbear/libssh2/MINA SSHD; no Termux. |
| Q4 | BE integration: WG vs SSH? | **SSH easier** on Alpine (`openssh`, `authorized_keys`, `-R` port). WG needs `wg set` + **UDP** FE→BE. |
| Q5 | Traffic Mobile → Gentoo FE → Alpine BE? | Heartbeat: `proxy_pass :80`. SSH: nginx **`stream`**. WG: **UDP DNAT** (not nginx `location /`). |
| Q6 | Licensing / open source? | See §9; Tailscale server is main license caveat. |
| Q7 | What is “Custom file API”? | HTTPS file transfer via PHP — not SCP, not a VPN; see §10. |
| Q8 | SCP-like file exchange? | Native **SFTP (SSH)**; similar UX via Custom file API; WG needs SSH/SFTP on top for SCP-like UX. |

---

## Problem statement

| Aspect | Detail |
|---|---|
| Trigger | Single device in the field needs interactive access (debug, replace config, pull logs). |
| Anti-goals | No permanent VPN; no open listening port on mobile; no broad fleet exposure. |
| User consent | Checkbox in app settings (“allow remote access at my own risk”). |
| Admin gate | Device ID must be on BE **remote_access_devices** whitelist before any session is offered. |
| Observability | Every enable/disable/connect/disconnect/timeout logged with reason and actor. |
| Path (rev. 2) | Signalling via FE→BE :80; tunnel endpoint on FE forwarded to BE. |

---

## Proposed control plane (heartbeat `type: ra`)

Reuse existing heartbeat endpoint; add branch for remote access.

### Device → BE (poll, random interval 1–7 min when enabled)

```json
{
  "heartbeat": {
    "type": "ra",
    "status": "enable",
    "device_id": "<stable device uuid>",
    "random": "<client session nonce / reconnect token>",
    "app_version": "0.1.0",
    "capabilities": ["ssh", "sftp", "files_app_home"]
  }
}
```

_Also accepted (rev. 2): `"capabilities": ["ssh_reverse", "sftp", "files_app_home"]`._

### BE → device (only if whitelisted + operator requested session)

```json
{
  "heartbeat": {
    "type": "ra",
    "action": "connect",
    "session_id": "uuid",
    "endpoint": "ra.apps.f0xx.org:443",
    "tunnel": "wireguard|ssh|ssh_reverse",
    "credentials": {
      "mode": "ephemeral_pubkey|otp",
      "username": "ra-<session_id>",
      "host_key_fingerprint": "…",
      "remote_forward": "127.0.0.1:<be_port>:127.0.0.1:8022"
    },
    "expires_at": 1717340000
  }
}
```

_WireGuard branch (rev. 1): `"tunnel": "wireguard"` with peer keys in `credentials`. SSH branch (rev. 2): `"tunnel": "ssh_reverse"` as above._

### Disable (user unchecks box or admin revokes)

```json
{ "heartbeat": { "type": "ra", "status": "disable", "device_id": "...", "random": "..." } }
```

### Field reference

| Field | Purpose |
|---|---|
| `random` | Client nonce for matching pending BE session; rotation on reconnect. |
| `session_id` | DB primary key for audit + port/key binding. |
| `status` | `enable` / `disable` / `poll` (optional alias for enable). |
| `action` | BE command: `connect`, `wait`, `deny`, `rotate`. |

### State machine (simplified)

```text
[user enables RA] → device polls → BE: wait (not whitelisted / no operator)
                 → admin opens session for device_id → next poll → BE: connect + creds
                 → device opens outbound tunnel → BE marks active → operator uses bastion
[user disables]  → disable heartbeat → tunnel tear-down → BE inactive + audit row
[24h idle]       → BE cron purges stale rows + closes ports
```

---

## Network topology — Gentoo FE → Alpine BE (rev. 2)

```text
[Android app]
    │  HTTPS POST /api/heartbeat.php  (type: ra)
    ▼
[Gentoo FE — apps.f0xx.org]
    │  nginx: proxy_pass http://artc0.intra.raptor.org:80  (existing)
    ▼
[Alpine BE — artc0 / XEN HVM]
    │  php-fpm + MariaDB (control plane)
    │  sshd / ra-bastion (data plane — SSH track)
    │  wg0 + UDP listener (data plane — WG track, if used)

[Operator laptop]
    │  SSH/SFTP to FE or BE bastion port (never to phone IP)
    ▼
  session forwarded to device's reverse tunnel
```

### FE ↔ BE tuning (by track)

| Track | Public entry (FE) | FE config | BE service | Notes |
|---|---|---|---|---|
| **Heartbeat (all)** | `https://apps.f0xx.org/.../heartbeat.php` | `proxy_pass http://artc0:80` | php-fpm | Already works. |
| **Reverse SSH** | `ra.apps.f0xx.org:443` TCP or `:2222` | `stream { proxy_pass artc0:22; }` | `sshd` + `Match User ra-*` | Rev. 2 pick (**Proxyable**). |
| **WireGuard** | `ra.apps.f0xx.org:51820/udp` | **iptables/nft DNAT** → BE | `wg-quick` / dynamic peers | Rev. 1 pick candidate (**Lightweight**). |
| **Custom file API** | `https://apps.f0xx.org/.../api/ra/*` | existing HTTP proxy | php-fpm only | No stream/UDP rules. |

---

## Security model

| Layer | Mechanism |
|---|---|
| Authorization | RBAC roles (§12) + per-device whitelist + user opt-in on device. |
| Authentication | Per-session ephemeral key pair or one-time port + password; no long-lived device secrets in APK. |
| Transport | TLS to BE for heartbeat; tunnel encrypted (WireGuard / SSH). |
| Exposure | BE listens on dedicated **ra** vhost/port; no inbound to mobile; optional IP allowlist for operators. |
| Audit | `remote_access_events` table: who, device, action, reason, ip, timestamps. |
| Cleanup | Inactivity timeout (e.g. 30–60 min); daily purge of records > 24h; credential invalidation on disable. |
| Reconnect | Same `session_id` + rotated `random` within grace window; new keys if expired. |

### Key handling options (rev. 1)

| Mode | Description | Risk |
|---|---|---|
| Ephemeral WG keypair | BE generates keys per session; device installs peer config once | Low; keys short-lived |
| SSH host cert + user cert | Step-CA or internal CA signs 1h user cert | Medium ops; mature tooling |
| Pre-allocated device key | One WG pubkey per device in DB | Higher leak impact; simpler reconnect |
| OTP + reverse SSH | `autossh -R random_port` with password from BE | Simple; weaker than pubkey |

**Key ceremony (rev. 1):** ephemeral WireGuard or SSH **user certificate** per session; optional device **identity key** (Ed25519) registered at whitelist time for authenticating poll requests only (not tunnel).

**Additional (rev. 2):**

- Terminate operator SSH on FE stream or require VPN to Gentoo before bastion.
- Device **outbound-only** to `ra.apps.f0xx.org:443` (TCP).
- Per-session **`ForceCommand internal-sftp`** if shell not required.

---

## Embedding in the Android app (rev. 2)

| Solution | Separate app required? | How to embed | Android notes |
|---|---|---|---|
| **WireGuard** | **No** | [`wireguard-android`](https://git.zx2c4.com/wireguard-android) tunnel AAR / `GoBackend` | `VpnService`; VPN icon while active. |
| **SSH reverse** | **No** | Dropbear + JNI, **libssh2**, **Apache MINA SSHD** | Foreground service; no VPN icon. |
| **frp/chisel client** | **No** | Static binary or Go mobile bind | Extra artifact. |
| **Custom file API** | **No** | OkHttp + JSON | Pure Java/Kotlin. |

You do **not** need Termux or the standalone WireGuard app if libraries are embedded in **Android Cast**.

---

## Backend integration ease — Alpine BE (rev. 2)

| Aspect | Reverse SSH | WireGuard |
|---|---|---|
| Package on Alpine | `openssh` | `wireguard-tools` + kernel mod |
| Session open (PHP) | `authorized_keys` + `-R` port; optional `ForceCommand internal-sftp` | `wg set wg0 peer …` |
| Session close | Remove key; kill session; free port | `wg set … remove` |
| FE cooperation | nginx **`stream`** (TCP) | **UDP DNAT** |
| Operator access | `sftp -P 443 ra-...@ra.apps.f0xx.org` | `wg-quick up` or SSH over WG |
| Jump host `f0xx.org:222` | Natural extension | Separate UDP path |

---

## Licensing and open source (rev. 2)

| Component | License | Ship in app / BE | Notes |
|---|---|---|---|
| **WireGuard** (kernel module) | **GPL-2.0** | BE kernel | Stock Alpine package. |
| **wireguard-go** / Android tunnel | **MIT** / **Apache-2.0** | App embed | Userspace embed. |
| **OpenSSH** | **BSD-style** | BE + client | Permissive. |
| **Dropbear** | **Permissive (MIT-like)** | App embed | Small client. |
| **libssh2** | **BSD-3-Clause** | App embed | |
| **Apache MINA SSHD** | **Apache-2.0** | App (Java) | SSH/SFTP in Java. |
| **chisel** | **MIT** | App/BE | |
| **frp** | **Apache-2.0** | App/BE | |
| **tinc** | **GPL-2.0+** | — | Copyleft; mesh. |
| **Tailscale client** | **BSD-3-Clause** | — | Client OSS. |
| **Headscale / Tailscale server** | **BSD** / **BSL** | — | Self-host Headscale if needed. |
| **Custom file API** | **Project code** | BE PHP | No third-party tunnel license. |

_GPL-2.0 Android app: use permissively licensed clients (MIT/BSD/Apache); review before linking GPL client libs into APK._

---

## What “Custom file API” means (rev. 2 — clarified)

**Not** a tunnel and **not** the SCP protocol. Application-layer remote file exchange on existing PHP:

- Endpoints: `POST /api/ra/upload`, `GET /api/ra/download`, `GET /api/ra/list`.
- Auth: short-lived token from heartbeat `connect` (same session as control plane).
- Scope: **`getFilesDir()`** / `codecs.json` unless Storage permission granted (broader scope needs second consent).
- Operator: crashes-themed **web UI** or `curl` — **SCP-like** push/pull without `scp` binary.
- **Pros:** no FE stream/UDP; minimal attack surface (no shell); reuses LAMP.
- **Cons:** no shell; implement listing/chunking for large logs.

**Use when:** Phase-1 before sshd bastion, or permanent narrow path alongside SSH for power users.

_Original rev. 1 one-liner: “Custom HTTPS file API — PHP endpoints; no shell; minimal attack surface; more app code for file ops.”_

---

## Tooling alternatives — full survey (rev. 1)

| Option | Maturity | Mobile fit | BE fit | Pros | Cons / risks |
|---|---|---|---|---|---|
| **WireGuard (outbound from device)** | High | Excellent (userspace WG) | Single UDP listener / per-session peers | Fast, modern crypto, small code | UDP blocked on some carriers; peer lifecycle automation |
| **Reverse SSH (`autossh -R`)** | High | Good (embedded dropbear client) | sshd on bastion | Familiar, SFTP, shell | Key mgmt; battery if misconfigured |
| **SSH via WebSocket (chisel, frp)** | Medium | Good | One HTTPS port | Traverses strict firewalls | Extra binary; audit frp history |
| **OpenVPN / IKEv2 (on-demand)** | High | Heavy | openvpn-access | Full tunnel | Overkill; battery; not file-focused |
| **tinc** | Medium | Poor (daemon, mesh) | mesh coordinator | P2P mesh | Persistent mesh ≠ on-demand; key distribution painful |
| **Tailscale / Headscale** | High | App + VPN | control server | Easy UX | Persistent identity; fleet-wide trust |
| **MeshCentral / RustDesk** | High | Agent app | relay server | Remote desktop | Different UX; screen control not primary need |
| **ADB over TLS tunnel** | Medium | Dev-only | reverse port | Exact Android debug | Dev options; security sensitive |
| **Custom HTTPS file API** | Low (build) | Native | PHP endpoints | No shell; minimal attack surface | No arbitrary shell; more app code |

---

## Tooling summary — infra-focused (rev. 2)

| Option | Embed? | FE→BE fit | SCP-like | License | Verdict |
|---|---|---|---|---|---|
| **Reverse SSH + SFTP** | Yes | **Excellent** (`stream`) | **Native** | BSD | **v1 pick (rev. 2)** |
| **WireGuard outbound** | Yes | Fair (UDP DNAT) | Via SFTP over WG | MIT/GPL | **v1 pick (rev. 1)** / v2 if UDP path ready |
| **Custom file API** | Yes | **Excellent** (HTTP) | Similar UX | Yours | Phase-1 / narrow |
| **frp/chisel** | Yes | Good | Possible | MIT/Apache | Fallback (both revs) |
| **tinc / Tailscale** | VPN app | Poor on-demand | Varies | GPL/BSL | Not recommended |

---

## Comparison matrix — initial scoring (rev. 1)

Scoring: 1 = low, 5 = high. Lower total was interpreted as stronger general fit in rev. 1 table orientation.

| Criterion | WG outbound | Reverse SSH | frp/chisel | tinc | Tailscale | Custom file API |
|---|:---:|:---:|:---:|:---:|:---:|:---:|
| Security | 5 | 4 | 3 | 3 | 4 | 5 |
| Simplicity (ops) | 4 | 4 | 3 | 2 | 3 | 4 |
| On-demand fit | 5 | 5 | 5 | 1 | 2 | 5 |
| File access (app dir) | 4 | 5 | 4 | 3 | 4 | 5 |
| Shell/debug | 4 | 5 | 4 | 3 | 4 | 1 |
| Fits LAMP BE | 4 | 5 | 4 | 3 | 3 | 5 |
| Battery / data | 4 | 3 | 3 | 2 | 2 | 5 |
| **Weighted total** | **31** | **30** | **26** | **17** | **22** | **30** |

_Rev. 1 tie note: WireGuard, reverse SSH, and Custom file API scored highest. Rev. 1 suggested **WireGuard + SFTP/SSH bastion** if shell needed; **Custom file API** if only `codecs.json` + logs._

---

## Comparison matrix — FE proxy focus (rev. 2)

| Criterion | Rev SSH | WG outbound | Custom file API | frp/chisel |
|---|:---:|:---:|:---:|:---:|
| **Proxyable (FE→BE)** | **5** | 2 | **5** | 4 |
| Security | 4 | 5 | 5 | 3 |
| On-demand fit | 5 | 5 | 5 | 5 |
| SCP-like files | **5** | 3* | 4 | 3 |
| BE integration ease | **5** | 3 | **5** | 3 |
| Embed without 3rd-party app | 5 | 5 | **5** | 4 |
| **Total** | **29** | 23 | **29** | 22 |

\*WG files row assumes SSH/SFTP layered on tunnel.

---

## WireGuard vs SSH — decision lens (rev. 2)

| Criterion | WireGuard | Reverse SSH + SFTP |
|---|---|---|
| **One-word headline** | **Lightweight** | **Proxyable** |
| Embed in app | Yes (`VpnService`) | Yes (libssh2/MINA/dropbear) |
| FE nginx HTTP proxy | No (UDP) | Yes (**`stream` TCP**) |
| Gentoo → Alpine path | Firewall UDP DNAT | Same as HTTP ops model |
| SCP-like files | Needs SSH/SFTP on top | **Built-in (SFTP)** |
| Carrier networks | UDP sometimes blocked | TCP 443 usually works |

---

## Recommendations

### Initial (rev. 1)

1. **Phase 0** — Design + threat model (1–2 days): consent copy, port allocation, key ceremony.
2. **Phase 1** — Control plane (1–2 weeks): heartbeat `ra`, whitelist, admin UI, audit, cron; dry-run `wait`/`connect`.
3. **Phase 2** — Tunnel (1–2 weeks): **WireGuard outbound** from app OR **reverse SSH to bastion**; operator connects via existing VPN/SSH to BE network.
4. **Phase 3** — Hardening: rate limits, mTLS on heartbeat, device identity signing, reconnect, pen-test.

**Primary pick (rev. 1):** WireGuard outbound + SSH/SFTP on bastion for operator.

**Fallback (rev. 1):** chisel/frp over HTTPS on same `ra` vhost.

### Refined (rev. 2 — Gentoo FE → Alpine BE)

**Primary v1:** **reverse SSH (+ SFTP)** — path is Mobile → Gentoo FE (nginx) → Alpine BE; **Proxyable** TCP; SFTP for `codecs.json`.

**Secondary:** WireGuard when dedicated **UDP forwarding** on FE is acceptable — **Lightweight** tunnel.

**Optional parallel:** **Custom file API** for whitelist-only file push/pull without internet-facing sshd.

**Phases (merged):**

1. Phase 0 — FE/BE port map + threat model (1–2 d).
2. Phase 1 — heartbeat `ra` + admin UI + optional Custom file API (1–2 w).
3. Phase 2 — reverse SSH/SFTP + Gentoo `stream` → Alpine sshd **or** WG track if chosen (1–2 w).
4. Phase 3 — MFA, reconnect, pen-test.

---

## RBAC roles (BE)

| Role | Permissions |
|---|---|
| **remote_access_user** | Open/close own sessions for whitelisted devices; view active/inactive list (scoped); SFTP / file API. |
| **admin** | Same as user + add/remove whitelist entries + assign `remote_access_user` role. |
| **root** | Full role management + force-disconnect any session + view all audit logs. |

Integrate with existing `users` / `Rbac`; new permissions: `remote_access_view`, `remote_access_operate`, `remote_access_admin`.

---

## BE admin UI (crashes look-and-feel)

- New nav: **Remote access** under `/app/androidcast_project/crashes/?view=remote_access` (or `/remote/`).
- Reuse `layout.php`, themes, locale picker, session auth.
- Screens (rev. 1):
  1. **Devices** — whitelist, last seen, RA opt-in status (from last heartbeat).
  2. **Sessions** — active / inactive; device, operator, started, last activity, tunnel type, port, status.
  3. **Audit log** — filter by device, user, action, date.
  4. **Open session** — pick device → pending session → waits for next device poll.
- Rev. 2 add-on: **file browser** tab when Custom file API enabled (upload/download `codecs.json`).
- No new frontend framework; same CSS/JS as tickets/graphs.

---

## MariaDB schema (sketch — rev. 1)

```sql
-- remote_access_devices (whitelist)
-- remote_access_sessions (session_id, device_id, operator_user_id, status,
--   tunnel, port, keys_ref, expires_at, last_activity)
-- remote_access_events (audit)
-- remote_access_credentials (optional encrypted blob, TTL)
```

Cron: `scripts/purge_remote_access.php` — inactive > 24h → `closed_timeout`; revoke keys; free ports.

---

## Android app changes (sketch — rev. 1)

- Settings → Developer → **Allow remote access** (off by default, strong warning).
- `RemoteAccessService` (foreground while tunnel active): jittered alarm 1–7 min → POST heartbeat.
- On `action: connect`: start WireGuard/SSH client with supplied creds; expose app files dir only unless Storage permission granted (broader scope needs second consent).
- On disable: stop service, wipe session keys, POST `status: disable`.
- Persist nothing long-term except optional device identity pubkey for poll authentication.

---

## Estimates (engineering)

| Track | Initial delivery | Hardening | Total |
|---|---:|---:|---:|
| Control plane only (heartbeat + DB + admin list) | 5–8 d | 3–5 d | 8–13 d |
| Control plane + Custom file API | 6–9 d | 3–5 d | 9–14 d |
| + WireGuard outbound tunnel (rev. 1 pick) | 8–12 d | 5–8 d | 13–20 d (+ FE UDP) |
| + Reverse SSH/SFTP bastion (rev. 2 pick) | 7–11 d | 4–7 d | 11–18 d |
| + FE nginx `stream` + firewall docs | +1–2 d | +1 d | (included in SSH track) |

_Assumptions: one BE bastion; excludes Play Store policy review._

---

## Open questions (merged)

**Rev. 1:**

- Maximum concurrent RA sessions per BE?
- Store tunnel configs only in RAM vs encrypted MariaDB blob?
- Require operator MFA (TOTP) before `Open session`?
- Play Store disclosure for “remote access” foreground service?

**Rev. 2:**

- Public TCP port: dedicated `2222` vs share `443` with `ssl_preread`?
- SFTP-only vs full shell for `remote_access_user`?
- Store device identity Ed25519 pubkey at whitelist time?
- Rate limit heartbeat `ra` polls?

**Rev. 3:**

- Confirm public RA port(s) with FE firewall owner before Phase 2?
- SFTP-only (`ForceCommand internal-sftp`) vs full shell for v1?

---

## Rev. 3 — WG vs SSH Q&A + bastion deep-dive

_Added 2026-06-02 — concrete FE/BE commands and decision support._

Constants used below (from [INFRA.md](INFRA.md)):

| Symbol | Value |
|---|---|
| **FE** | Gentoo HVM, public `apps.f0xx.org` (TLS **443** terminates here) |
| **BE** | Alpine `artc0.intra.raptor.org` / `10.7.16.128`, app vhost **`listen 80`** |
| **FE→BE (existing HTTP)** | `proxy_pass http://artc0.intra.raptor.org:80` |
| **Operator jump (existing)** | `f0xx.org:222` → Alpine `:22` |

Replace `10.7.16.128` with the live BE IP if it changes.

---

### Q1 — Exact FE/BE rules: WireGuard (DNAT) vs SSH (no DNAT required)

**Short answer:** WireGuard **must** use **UDP DNAT** (or equivalent firewall forward) on Gentoo FE because nginx **`proxy_pass` is HTTP/TCP-only** and cannot carry WG packets. SSH **does not need DNAT** on FE if you use nginx **`stream`** TCP forwarding (same nginx daemon, different block). You *can* use TCP DNAT for SSH as an alternative, but it is redundant when `stream` works.

#### WireGuard track — FE (Gentoo): UDP DNAT + forward allow

Public UDP **51820** on FE → BE **51820**. Example with **nftables** (adjust interface `eth0`, table/chain names to match your FE):

```bash
# FE — one-time / boot-persistent (example)
BE_IP=10.7.16.128
WG_PORT=51820

nft add rule inet nat prerouting iifname "eth0" udp dport $WG_PORT \
  dnat ip to ${BE_IP}:${WG_PORT}
nft add rule inet filter forward ip daddr ${BE_IP} udp dport ${WG_PORT} accept
```

**iptables equivalent:**

```bash
BE_IP=10.7.16.128
WG_PORT=51820

iptables -t nat -A PREROUTING -i eth0 -p udp --dport $WG_PORT \
  -j DNAT --to-destination ${BE_IP}:${WG_PORT}
iptables -A FORWARD -p udp -d ${BE_IP} --dport $WG_PORT -j ACCEPT
```

**Persist:** save rules (`/etc/nftables.conf` or `iptables-save` → Gentoo `iptables` init). **Open UDP 51820** on FE edge firewall (in addition to 443/tcp).

**Verify from outside:**

```bash
# should show BE wg listening (once BE step done)
nc -u -v apps.f0xx.org 51820
```

#### WireGuard track — BE (Alpine): interface + listen

```bash
# BE
apk add wireguard-tools
modprobe wireguard 2>/dev/null || echo "wireguard module required"

cat >/etc/wireguard/wg0.conf <<'EOF'
[Interface]
Address = 10.66.66.1/24
ListenPort = 51820
PrivateKey = <BE_WG_PRIVATE_KEY>
# PostUp/PostDown: optional iptables for operator subnet
EOF

wg-quick up wg0
rc-update add wg-quick.wg0 default   # if using OpenRC
```

**Per RA session** (PHP or admin script adds ephemeral peer — example):

```bash
wg set wg0 peer <DEVICE_EPHEMERAL_PUBKEY> allowed-ips 10.66.66.2/32
# on session end:
wg set wg0 peer <DEVICE_EPHEMERAL_PUBKEY> remove
```

Device connects **outbound UDP** to `apps.f0xx.org:51820` (DNAT lands on BE). Operator joins same WG net (second peer) or uses SSH **over** the tunnel (see Q5).

---

#### SSH track — FE (Gentoo): nginx **`stream`** (TCP proxy, not DNAT)

Preferred: public TCP **443** on `ra.apps.f0xx.org` (or dedicated **2222**) → BE **22**. No kernel DNAT if nginx terminates the TCP hop.

Add to FE nginx (often `/etc/nginx/nginx.conf`, **outside** `http {}`):

```nginx
stream {
    upstream be_sshd {
        server 10.7.16.128:22;
    }
    server {
        listen 443;                    # or 2222 if 443 shared with TLS SNI split
        proxy_pass be_sshd;
        proxy_connect_timeout 30s;
    }
}
```

If **443** is already used for HTTPS on the same IP, either:

- use **`listen 2222`** for RA SSH, or
- use **`ssl_preread`** on 443 to split HTTPS vs SSH by ClientHello (more complex).

**Reload FE nginx:**

```bash
nginx -t && nginx -s reload
```

**Verify:**

```bash
ssh -p 443 -o StrictHostKeyChecking=accept-new ra-test@ra.apps.f0xx.org
# or: nc -v ra.apps.f0xx.org 2222
```

#### SSH track — optional FE DNAT (parallel to WG, for comparison only)

If you refused nginx `stream`, SSH could mirror WG with **TCP DNAT**:

```bash
# FE — iptables example (TCP 2222 → BE:22)
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 2222 \
  -j DNAT --to-destination 10.7.16.128:22
iptables -A FORWARD -p tcp -d 10.7.16.128 --dport 22 -j ACCEPT
```

This works but **duplicates** what `stream` already does and bypasses nginx logging/timeouts. **Recommendation:** use **`stream`**, not DNAT, for SSH.

#### SSH track — BE (Alpine): sshd + reverse-forward slot

```bash
# BE
apk add openssh
rc-update add sshd default
rc-service sshd start

# Dedicated match block — /etc/ssh/sshd_config.d/ra.conf
cat >/etc/ssh/sshd_config.d/ra.conf <<'EOF'
Match User ra-*
    AllowTcpForwarding yes
    GatewayPorts no
    X11Forwarding no
    PermitTTY yes
    # v1 file-only: ForceCommand internal-sftp
EOF
rc-service sshd restart
```

**Per session — device outbound reverse forward** (embedded client runs equivalent):

```bash
# On device (conceptual — app embeds this):
ssh -N -R 127.0.0.1:18022:127.0.0.1:8022 ra-<session_id>@ra.apps.f0xx.org -p 443
```

**BE — operator reaches device file area via forwarded port:**

```bash
# On BE (after device connected):
sftp -P 18022 ra-<session_id>@127.0.0.1
# or scp -P 18022 ...
```

---

#### Side-by-side: what runs where

| Step | WireGuard | Reverse SSH |
|---|---|---|
| **FE edge open** | UDP **51820** | TCP **443** or **2222** |
| **FE mechanism** | **nft/iptables DNAT** | nginx **`stream`** (or optional TCP DNAT) |
| **BE daemon** | `wg0` / `wg set` | `sshd` + ephemeral `Match User ra-*` |
| **Device direction** | Outbound UDP to FE public IP | Outbound TCP to FE → proxied to BE sshd |
| **Operator path** | WG peer IP or SSH over WG | SFTP/SSH to BE forwarded port (see Q5) |

---

### Q2 — Can we use ports other than `:80`? (heartbeats and everything else)

**Two different “ports” — do not conflate them.**

| Leg | Port today | Can change? | Notes |
|---|---|---|---|
| **Phone → FE** | **443** (HTTPS) | Yes, but **443 is correct** for TLS + carrier-friendly egress | App already targets `https://apps.f0xx.org/...`; users never hit `:80` directly. |
| **FE → BE (heartbeat, crashes, graphs)** | **80** (HTTP) | **Yes** | Requires new FE `proxy_pass` upstream **and** BE `listen` block. Not forbidden — just **unused today**. |
| **RA data plane (WG)** | **51820/udp** (proposed) | Must be **non-80** | UDP tunnel; unrelated to HTTP vhost. |
| **RA data plane (SSH)** | **443/tcp** or **2222/tcp** | Must be **non-80** | Raw TCP/`stream`; not HTTP `location /`. |

**Why heartbeats stay on the existing `:80` FE→BE path (recommended for v1):**

1. **Already deployed** — [nginx.fe-apps.f0xx.org.snippet](../examples/crash_reporter/backend/nginx.fe-apps.f0xx.org.snippet) and BE `listen 80` fragment are the repo source of truth; zero new FE/BE holes for control plane.
2. **Same security zone** as crash upload and graphs — one ops playbook.
3. **No benefit** to moving heartbeat to BE `:443` internally — the phone still speaks HTTPS to FE; internal hop is your choice.
4. **Port 8089** on BE is **Janus/other** — do **not** reuse for androidcast ([INFRA.md §2](INFRA.md)).

**If you intentionally want heartbeat on BE `:443` internally:**

```bash
# FE — change upstream in server block (example)
# proxy_pass https://artc0.intra.raptor.org:443;
# BE — ensure listen 443 ssl server includes /api/heartbeat.php location
```

That is a **separate migration**; not required for RA.

**Summary:** Public **443** for all HTTPS (including heartbeat). Internal FE→BE **80** for LAMP today — **can** change, **should not** for v1 unless you have another reason. RA tunnels **must** use **dedicated non-80** ports (UDP 51820 or TCP 443/2222 stream).

---

### Q3 — Why is “Proxyable (FE→BE)” low for WG (2) and high for SSH (5)?

The rev. 2 score measures one thing: **“Can we forward this service from Gentoo FE to Alpine BE the same way we already forward crashes/hub HTTP?”**

| | **Reverse SSH** | **WireGuard** |
|---|---|---|
| **Protocol** | TCP | UDP |
| **Existing FE tool** | nginx **`stream`** → `10.7.16.128:22` | **Not** nginx `location` / `proxy_pass` |
| **FE change type** | Add `stream {}` block; reload nginx | **Firewall DNAT** + forward rules; separate from nginx HTTP config in repo |
| **Ops familiarity** | Same team that edits `nginx.conf` | Often different (iptables/nft, UDP, kernel module) |
| **Debugging** | `nginx -t`, error log, `curl`/ `nc` TCP | `nft list ruleset`, UDP blocked on some carriers, `wg show` on BE |
| **Matches INFRA docs** | Extends nginx pattern | New parallel path not in current snippets |

**Score meaning:**

- **5 (SSH):** “Drop in a `stream` upstream to BE — same deployment rhythm as today’s reverse proxy.”
- **2 (WG):** “Works, but **not** proxyable via HTTP nginx; needs **DNAT** (or a UDP proxy like `udp2raw`, which is extra complexity). Partial credit — not zero — because FE→BE forwarding is still one hop.”

**Important nuance:** Low **Proxyable** does **not** mean WG is insecure or bad on the phone — it means **your FE→BE operational model** favours TCP/nginx. That is why rev. 2 refined the v1 pick to SSH despite WG being **Lightweight** on-device.

**Custom file API also scores 5** because it reuses existing **`proxy_pass http://artc0:80`** with new PHP locations — no `stream`, no DNAT.

---

## Rev. 3 — Bastion Q&A

### Q4 — How easy to implement? Command checklist (FE + BE)

Rough effort (after control plane PHP/DB exists):

| Track | FE ops | BE ops | App embed | Typical first-time |
|---|---|---|---|---|
| **Reverse SSH + SFTP** | 1 nginx `stream` block + reload | `openssh`, `sshd_config.d/ra.conf`, session keys | libssh2/MINA client, `-R` forward | **~0.5–1 day** infra |
| **WireGuard outbound** | UDP DNAT + firewall + persist | `wireguard-tools`, `wg0`, dynamic peers | `wireguard-android` + `VpnService` | **~1–2 days** infra |
| **Custom file API only** | none (HTTP already proxied) | PHP endpoints only | OkHttp upload/download | **~0 FE days** |

#### SSH bastion — minimal ordered checklist

**FE (Gentoo):**

```bash
# 1. Edit nginx — add stream {} → 10.7.16.128:22 (see Q1)
nginx -t && nginx -s reload

# 2. Firewall — allow inbound TCP on chosen RA port (443 or 2222)
# (example — adjust for your firewall frontend)
iptables -A INPUT -p tcp --dport 2222 -j ACCEPT
```

**BE (Alpine):**

```bash
apk add openssh
mkdir -p /etc/ssh/sshd_config.d
# ra.conf Match User ra-* (see Q1)
rc-service sshd restart

# Per session (automate in PHP):
# echo "<PUBKEY>" >> /home/ra-<session_id>/.ssh/authorized_keys
# adduser -D -s /bin/sh ra-<session_id>
```

**App (Android):** foreground service → heartbeat poll → on `connect`, run embedded SSH client with `-R 127.0.0.1:<be_port>:127.0.0.1:8022` → local SFTP server or `internal-sftp` on device side (design choice).

**Operator:**

```bash
# Via existing jump, then BE localhost forward:
ssh -J f0xx.org:222 sftp -P 18022 ra-<session_id>@127.0.0.1
```

#### WireGuard — minimal ordered checklist

**FE (Gentoo):**

```bash
BE_IP=10.7.16.128
WG_PORT=51820
# nft DNAT rules (Q1) + persist + open UDP WG_PORT on edge
```

**BE (Alpine):**

```bash
apk add wireguard-tools
# wg0.conf + wg-quick up wg0
# per session: wg set wg0 peer ... allowed-ips ...
```

**App (Android):** `VpnService` + userspace WG → outbound to `apps.f0xx.org:51820`.

**Operator (files still need a channel):**

```bash
# Option A — add operator as second WG peer (PHP generates config)
# Option B — SSH/SFTP over WG inner IP (still need sshd on device or BE hop)
wg-quick up operator-ra-<session_id>
sftp user@10.66.66.2
```

WG **alone** does not give SFTP; you almost always add **SSH on top** or fall back to **Custom file API** — extra moving parts vs SSH-only track.

---

### Q5 — Bastion: file transfers only, or other operations too?

| Capability | Custom file API | SSH/SFTP bastion | WireGuard alone | WG + SSH on tunnel |
|---|---|---|---|---|
| Edit `codecs.json` | Yes (scoped paths) | Yes (SFTP) | No | Yes |
| Pull log files | Yes | Yes | No | Yes |
| Interactive shell (`logcat`, `run-as`, debug cmds) | **No** | **Yes** (if not `ForceCommand internal-sftp`) | No | Yes |
| Port forwarding (ADB-like / local tools) | No | **Yes** (`-L`/`-R`) | Yes (L3) | Yes |
| Packet capture / low-level network | No | Limited | Yes (L3 VPN) | Yes |

**What “bastion” means in this project:**

1. **Narrow bastion (file-only v1):** `ForceCommand internal-sftp`, chroot or path jail to app home — enough for **codecs.json + logs** with smallest attack surface. Matches your stated primary need.
2. **Full debug bastion:** interactive shell + reverse forwards — for **logcat**, running one-off commands, inspecting `/data/data/...` (where policy allows), or forwarding a port to a laptop tool. Same SSH track, one config flag difference.
3. **WireGuard “bastion”:** really a **VPN endpoint**, not a file bastion — gives IP reachability; you still mount **SFTP/SSH or file API** for codecs.json unless you build a custom agent.

**Practical recommendation for your decision:**

| Your priority | Best fit |
|---|---|
| **Only** replace `codecs.json` + download logs | **Custom file API** or **SFTP-only SSH** — bastion = file transfer only |
| Occasional **shell debug** on one field device | **Reverse SSH bastion** with shell allowed (not SFTP-only) |
| Lowest FE friction + familiar ops | **Reverse SSH** (`stream` to BE) — scores **Proxyable 5** |
| Minimum bytes on wire / best battery on tunnel | **WireGuard** — but budget **+SSH or file API** for files |

**Rev. 3 decision hint:** If bastion = **files only**, Custom file API + existing `:80` heartbeat is the shortest path (no FE `stream`, no UDP DNAT). If bastion = **files + shell debug**, **reverse SSH** wins on infra fit. Pure **WG** only wins if you explicitly want L3 VPN and accept FE UDP DNAT + a second protocol for files.

---

## Appendix 1 — WG test configuration (Linux laptop)

_References: [REMOTE_ACCESS_IMPL.md](REMOTE_ACCESS_IMPL.md), [REMOTE_ACCESS_VALIDATION.md](REMOTE_ACCESS_VALIDATION.md)._

**DNS (prod):** `ra.apps.f0xx.org` is a **CNAME** of `apps.f0xx.org` / `f0xx.org` — same public edge IP for HTTPS heartbeats and UDP WireGuard.

### Network topology (v1 WireGuard)

```text
┌─────────────────┐     HTTPS :443       ┌──────────────────┐   HTTP :80    ┌─────────────────┐
│ Android device  │ ───────────────────► │ Gentoo FE        │ ────────────► │ Alpine BE       │
│ or Linux laptop │   apps.f0xx.org      │ apps.f0xx.org    │  artc0:80     │ PHP heartbeat   │
│ (device sim)    │   /crashes/api/      │ TLS + nginx      │               │ remote_access   │
└────────┬────────┘   heartbeat.php      └────────┬─────────┘               └────────┬────────┘
         │                                         │                                    │
         │ UDP :51820                              │ UDP DNAT :51820                    │ wg0 :51820
         │ ra.apps.f0xx.org (CNAME)                ▼                                    ▼
         └──────────────────────────────► ┌──────────────────┐               ┌─────────────────┐
                                          │ Ephemeral modem  │ ── LAN ─────► │ 10.7.16.128     │
                                          │ (RPi NAT/fw)     │               │ ListenPort 51820│
                                          └──────────────────┘               └─────────────────┘
         Internet / global network ───────► modem ───────► FE ───────► BE
```

**Control plane** = HTTP only (no new nginx `location` for RA). **Data plane** = UDP to `ra.apps.f0xx.org:51820` → FE DNAT → BE `wg0`.

### Prerequisites (laptop)

```bash
# Gentoo / Debian / Fedora — wireguard-tools + jq + curl
sudo emerge -av net-vpn/wireguard-tools   # or: apt install wireguard-tools jq curl
```

### A. HTTP E2E (no tunnel yet)

```bash
cd examples/crash_reporter/backend
export CRASHES_BASE=https://apps.f0xx.org/app/androidcast_project/crashes
export ADMIN_USER=admin ADMIN_PASS='…'

./scripts/ra_e2e_cli.sh
# expect: whitelist → open_session → connect (may wait ~45s for min_poll_interval_s)
```

### B. UDP / WireGuard from laptop (after open session)

**Option 1 — device sim with tunnel bring-up:**

```bash
export CRASHES_BASE=https://apps.f0xx.org/app/androidcast_project/crashes
export RA_DEVICE_ID=ra-laptop-$(hostname -s)

# Operator: whitelist device + open session in ?view=remote_access (or re-run E2E)
./scripts/ra_device_sim.sh poll --until connect --apply-wg
```

**Option 2 — manual:**

```bash
source scripts/ra_lib.sh
RESP="$(ra_ra_post enable "$RA_DEVICE_ID" "$(cat /tmp/androidcast-ra-${RA_DEVICE_ID}.random)" cli-sim)"
ra_wg_quick_from_connect "$RESP" | sudo tee /tmp/ra-laptop.conf
sudo wg-quick up /tmp/ra-laptop.conf
sudo wg show
ping -c3 10.66.66.1          # BE wg0 — OK if routed
sudo wg-quick down /tmp/ra-laptop.conf
```

### C. Quick checks before blaming WG

| Check | Command | Expect |
|-------|---------|--------|
| CNAME | `dig +short ra.apps.f0xx.org` | same A/AAAA as `apps.f0xx.org` |
| UDP port | `nc -vzu ra.apps.f0xx.org 51820` | open / reachable (not TCP) |
| HTTP | `curl -sS -o /dev/null -w '%{http_code}\n' https://apps.f0xx.org/app/androidcast_project/crashes/` | 200 or 302 |
| BE wg | on BE: `wg show wg0` | interface up, peers after connect |

If HTTP E2E passes but `wg-quick up` fails → FE DNAT or firewall on modem/FE, not PHP.

---

## Appendix 2 — WG production configuration

_References: [REMOTE_ACCESS_IMPL.md](REMOTE_ACCESS_IMPL.md) § Production deploy, [REMOTE_ACCESS_VALIDATION.md](REMOTE_ACCESS_VALIDATION.md)._

### BE `config.php` (`remote_access`)

```php
'remote_access' => [
    'wg_endpoint' => 'ra.apps.f0xx.org:51820',
    'wg_server_public_key' => '<output of wg show wg0 public-key on BE>',
    'wg_interface' => 'wg0',
    'provision_peers' => true,
    'require_wg_tools' => true,
    'session_ttl_s' => 3600,
    'min_poll_interval_s' => 45,
],
```

### BE (Alpine) — WireGuard interface

```bash
apk add wireguard-tools wireguard-tools-wg
# /etc/wireguard/wg0.conf — example skeleton:
# [Interface]
# Address = 10.66.66.1/24
# ListenPort = 51820
# PrivateKey = <server private>
sudo wg-quick up wg0
sudo wg show wg0 public-key   # → paste into config.php
```

PHP-FPM user (`nginx` on Alpine) must run `wg set` when `provision_peers=true`:

```bash
sudo -u nginx wg show wg0
```

Cron (hourly purge): `php81 scripts/purge_remote_access.php --hours=24`

### FE (Gentoo) — UDP DNAT (not nginx)

HTTP stays as today: `https://apps.f0xx.org` → `http://artc0.intra.raptor.org:80`.

WireGuard uses **UDP DNAT** on the FE edge (and/or modem) — **not** an nginx `location`:

```bash
BE_IP=10.7.16.128    # Alpine BE on LAN
WG_PORT=51820

# nftables example (adjust iface names):
# table ip nat {
#   chain prerouting {
#     type nat hook prerouting priority dstnat;
#     udp dport $WG_PORT dnat to $BE_IP:$WG_PORT
#   }
# }
```

Ensure **UDP $WG_PORT** is allowed on FE (and modem) toward the BE. `ra.apps.f0xx.org` CNAME ensures devices and laptops target the same edge as `apps.f0xx.org`.

### MariaDB / migrations

Migration **007** (`remote_access_*` tables). See `sql/migrations/007_remote_access.sql`.

### Production verify (on BE or laptop)

```bash
cd examples/crash_reporter/backend
BASE=https://apps.f0xx.org/app/androidcast_project/crashes ./scripts/verify_remote_access_prod.sh
RA_E2E=1 BASE="$BASE" ./scripts/verify_remote_access_prod.sh
```

**Verified 2026-06-04:** HTTP `ra_e2e_cli.sh` on prod returned `connect` with live `peer_public_key` and endpoint `ra.apps.f0xx.org:51820`. UDP handshake is validated separately (Appendix 1 § B).

### Android (field device)

Developer settings → Remote access → **WireGuard**; polls `…/api/heartbeat.php` (`type: ra`). See `REMOTE_ACCESS_IMPL.md` § App.

---

## Source linkage

- Editable source: `docs/20260602_REVERSE_SSH_proposals_summary.md`
- PDF: `docs/20260602_REVERSE_SSH_proposals_summary.pdf`
- Regenerate: `bash scripts/build-reverse-ssh-proposals-pdf.sh`
- All `docs/*.md` PDFs: `bash scripts/build-all-docs-pdf.sh`
