mc-proxy/ARCHITECTURE.md

# ARCHITECTURE.md

mc-proxy is a TLS proxy and router for Metacircular Dynamics services. It
operates in two modes on a per-route basis:

- **L4 (passthrough):** Reads the SNI field from the TLS ClientHello and
  proxies the raw TCP stream to the matched backend without terminating TLS.
  This is the default mode and the original behavior.
- **L7 (terminating):** Terminates the client TLS connection, then reverse
  proxies HTTP/2 (and HTTP/1.1) traffic to the backend. This enables
  HTTP-level features: header inspection, user-agent blocking, request
  routing, and `X-Forwarded-For` injection.

A global firewall evaluates every connection before routing. PROXY protocol
support enables multi-hop deployments where a public edge mc-proxy forwards
through a private network (e.g., Tailscale) to an origin mc-proxy while
preserving the real client IP.

## Table of Contents

1. [System Overview](#system-overview)
2. [Connection Lifecycle](#connection-lifecycle)
3. [PROXY Protocol](#proxy-protocol)
4. [Firewall](#firewall)
5. [Routing](#routing)
6. [L7 Proxying](#l7-proxying)
7. [gRPC Admin API](#grpc-admin-api)
8. [Configuration](#configuration)
9. [Storage](#storage)
10. [Deployment](#deployment)
11. [Security Model](#security-model)
12. [Future Work](#future-work)

---

## System Overview

```
                        ┌──────────────────────────────────────────────┐
                        │                  mc-proxy                    │
    Clients ──────┐     │                                              │
                  │     │  ┌───────┐  ┌──────┐  ┌───┐  ┌─────┐        │
                  ├────▶│  │ PROXY │─▶│ Fire │─▶│SNI│─▶│Route│──┐     │
                  │     │  │ proto │  │ wall │  │   │  │Table│  │     │
                  ├────▶│  └───────┘  └──────┘  └───┘  └─────┘  │     │
                  │     │      │         │ RST           │  │    │     │
    Clients ──────┘     │      │         ▼               │  │    │     │
                        │  (optional)  (blocked)         │  │    │     │
                        │                                │  │    │     │
                        │         ┌──────────────────────┘  │    │     │
                        │         │ L4 route                │    │     │
                        │         ▼                         │    │     │     ┌────────────┐
                        │    ┌──────────┐                   │    │     │────▶│ Backend A  │
                        │    │ Raw TCP  │───────────────────│────│─────│     │ :8443 (TLS)│
                        │    │ Relay    │                   │    │     │     └────────────┘
                        │    └──────────┘                   │    │     │
                        │                                   │    │     │
                        │         ┌─────────────────────────┘    │     │
                        │         │ L7 route                     │     │
                        │         ▼                              │     │     ┌────────────┐
                        │    ┌──────────┐  ┌──────────┐          │     │────▶│ Backend B  │
                        │    │ TLS Term │─▶│ HTTP/2   │──────────│─────│     │ :8080 (h2c)│
                        │    │ (certs)  │  │ Rev Proxy│          │     │     └────────────┘
                        │    └──────────┘  └──────────┘          │     │
                        └──────────────────────────────────────────────┘

    Listener 1 (:443)  ─┐
    Listener 2 (:8443) ─┼─ Each runs the same pipeline; L4/L7 is per-route
    Listener N (:9443) ─┘
```

Key properties:

- **Dual mode, per-route.** Each route is independently configured as L4
  (passthrough) or L7 (terminating). Both modes coexist on the same listener.
- **TLS-only on ingest.** Non-TLS connections are not supported. If the first
  bytes of a connection are not a TLS ClientHello (after any PROXY protocol
  header), the connection is reset.
- **Multiple listeners.** A single mc-proxy instance binds to one or more
  ports. Each listener runs the same pipeline.
- **Global firewall.** Firewall rules apply to all listeners uniformly.
  There are no per-route firewall rules.
- **PROXY protocol.** Listeners can accept PROXY protocol v1/v2 headers to
  learn the real client IP from an upstream proxy. Routes can send PROXY
  protocol v2 headers to downstream backends or proxies.
- **No authentication.** mc-proxy is pre-auth infrastructure. It sits in
  front of services that handle their own authentication via MCIAS.

---

## Connection Lifecycle

Every inbound connection follows this sequence. Steps 1-5 are common to all
connections. The route's mode determines whether the L4 or L7 path is taken.

```
1. ACCEPT           Listener accepts TCP connection.
2. PROXY PROTOCOL   If listener has proxy_protocol enabled:
                      Parse PROXY v1/v2 header to extract real client IP.
                      If malformed or missing → RST, done.
                    Otherwise: use TCP source IP directly.
3. FIREWALL         Check source IP (real, from step 2) against blocklists:
                      a. Per-IP rate limit check.
                      b. IP/CIDR block check.
                      c. GeoIP country block check.
                    If blocked → RST, done.
4. SNI EXTRACT      Read the TLS ClientHello (without consuming it).
                    Extract the SNI hostname.
                    If no valid ClientHello or no SNI → RST, done.
5. ROUTE LOOKUP     Match SNI hostname against the route table.
                    If no match → RST, done.
                    Route determines mode (L4 or L7).

── L4 path (passthrough) ──────────────────────────────────────────────

6a. BACKEND DIAL    Open TCP connection to the matched backend.
                    If dial fails → RST, done.
7a. PROXY HEADER    If route has send_proxy_protocol enabled:
                      Send PROXY v2 header with real client IP to backend.
8a. FORWARD         Write the buffered ClientHello to the backend.
9a. RELAY           Bidirectional byte copy: client ↔ backend.
10a. CLOSE          Either side closes → half-close propagation → done.

── L7 path (terminating) ──────────────────────────────────────────────

6b. TLS HANDSHAKE   Complete TLS handshake with the client.
                    The buffered ClientHello is replayed into crypto/tls
                    via a prefixConn wrapper. The server certificate is
                    selected from the route's configured cert/key pair
                    using tls.Config.GetCertificate keyed on SNI.
                    If handshake fails → close, done.
7b. HTTP PARSE      Read HTTP/2 (or HTTP/1.1) request from the TLS conn.
8b. L7 POLICY       Apply HTTP-level policies:
                      - User-agent blocking (future)
                      - Header inspection (future)
                    If blocked → HTTP error response, done.
9b. BACKEND DIAL    Open connection to the matched backend.
                    If backend_tls: TLS connection to backend.
                    Otherwise: plaintext (h2c or HTTP/1.1).
                    If dial fails → HTTP 502, done.
10b. PROXY HEADER   If route has send_proxy_protocol enabled:
                      Send PROXY v2 header with real client IP to backend.
11b. REVERSE PROXY  Forward HTTP request to backend, stream response to
                    client. 1:1 connection mapping (no pooling).
12b. CLOSE          Connection closed normally or on error.
```

### SNI Extraction

The proxy peeks at the initial bytes of the connection without consuming
them. It parses just enough of the TLS record layer and ClientHello to
extract the `server_name` extension.

- **L4 routes:** The full ClientHello (including the SNI) is forwarded to
  the backend so the backend's TLS handshake proceeds normally.
- **L7 routes:** The buffered ClientHello is replayed into `crypto/tls.Server`
  via a `prefixConn` wrapper (a `net.Conn` that returns the buffered bytes
  before reading from the underlying connection). The TLS library sees the
  complete ClientHello and performs a normal handshake.

If the ClientHello spans multiple TCP segments, the proxy buffers up to
16 KiB (the maximum TLS record size) before giving up.

### Bidirectional Copy (L4)

After the backend connection is established, the proxy runs two concurrent
copy loops (client->backend and backend->client). When either direction
encounters an EOF or error:

1. The write side of the opposite direction is half-closed.
2. The remaining direction drains to completion.
3. Both connections are closed.

Timeouts apply to the copy phase to prevent idle connections from
accumulating indefinitely (see [Configuration](#configuration)).

---

## PROXY Protocol

mc-proxy supports [PROXY protocol](https://www.haproxy.org/download/2.9/doc/proxy-protocol.txt)
for multi-hop deployments where the real client IP must be preserved across
proxy boundaries.

### Use Case: Edge-to-Origin

A common deployment has a public-facing mc-proxy on a VPS forwarding
through Tailscale to a private-network mc-proxy:

```
Client ──→ [edge mc-proxy] ──PROXY v2 + TLS──→ Tailscale ──→ [origin mc-proxy] ──→ backend
              VPS, :443                                         private network
              proxy_protocol = false                             proxy_protocol = true
              send_proxy_protocol = true                         send_proxy_protocol = false
```

Without PROXY protocol, the origin mc-proxy sees the Tailscale peer IP as
the client address. With it, the real client IP is preserved for firewall
evaluation, logging, and `X-Forwarded-For` injection on L7 routes.

### Receiving (Listener Side)

When `proxy_protocol = true` on a listener:

1. After accepting the TCP connection, read the PROXY protocol header
   **before** any TLS data.
2. Accept both v1 (text) and v2 (binary) formats.
3. Extract the real client IP and port from the header.
4. Use the real client IP for all subsequent processing (firewall, logging,
   `X-Forwarded-For`).
5. If the header is malformed, missing, or the connection times out waiting
   for it, reset the connection.

When `proxy_protocol = false` (default), the TCP source IP is used directly.
No PROXY header is expected or tolerated — a connection that starts with a
PROXY header will fail SNI extraction (it's not a TLS ClientHello) and be
reset. This prevents a non-PROXY listener from accidentally trusting
attacker-supplied headers.

### Sending (Route Side)

When `send_proxy_protocol = true` on a route:

1. After connecting to the backend, send a PROXY protocol v2 header before
   any other data.
2. The header contains the real client IP (from the PROXY header on the
   listener, or the TCP source IP if the listener is not PROXY-enabled).
3. For L4 routes: the v2 header is sent, then the buffered ClientHello,
   then raw relay.
4. For L7 routes: the v2 header is sent on the backend connection before
   HTTP traffic begins.

mc-proxy always sends v2 (binary format) when `send_proxy_protocol` is
enabled. v2 is more compact, supports IPv6 natively, and is the modern
standard.

### Header Timeout

A hardcoded 5-second timeout applies to reading the PROXY protocol header.
If the header is not fully received within this window, the connection is
reset. This prevents slowloris-style attacks on PROXY-enabled listeners.

---

## Firewall

The firewall is a global, ordered rule set evaluated on every new
connection before SNI extraction. Rules are evaluated in definition order;
the first matching rule determines the outcome. If no rule matches, the
connection is **allowed** (default allow -- the firewall is an explicit
blocklist, not an allowlist).

The source IP used for firewall evaluation is the **real client IP** --
either from a PROXY protocol header (if the listener is PROXY-enabled) or
the TCP source address.

### Rule Types

| Config Field | Match Field | Example |
|--------------|-------------|---------|
| `blocked_ips` | Source IP address | `"192.0.2.1"` |
| `blocked_cidrs` | Source IP prefix | `"198.51.100.0/24"` |
| `blocked_countries` | Source country (ISO 3166-1 alpha-2) | `"KP"`, `"CN"`, `"IN"`, `"IL"` |

### Blocked Connection Handling

Blocked connections receive a TCP RST. No TLS alert, no HTTP error page, no
indication of why the connection was refused. This is intentional -- blocked
sources should receive minimal information.

### GeoIP Database

mc-proxy uses the [MaxMind GeoLite2](https://dev.maxmind.com/geoip/geolite2-free-geolocation-data)
free database for country-level IP geolocation. The database file is
distributed separately from the binary and referenced by path in the
configuration.

The GeoIP database is loaded into memory at startup and can be reloaded
via `SIGHUP` without restarting the proxy. If the database file is missing
or unreadable at startup and GeoIP rules are configured, the proxy refuses
to start.

---

## Routing

Each listener has its own route table mapping SNI hostnames to backend
addresses. A route entry consists of:

| Field | Type | Default | Description |
|-------|------|---------|-------------|
| `hostname` | string | *(required)* | Exact SNI hostname to match (e.g. `metacrypt.metacircular.net`) |
| `backend` | string | *(required)* | Backend address as `host:port` (e.g. `127.0.0.1:8443`) |
| `mode` | string | `"l4"` | `"l4"` for passthrough, `"l7"` for TLS-terminating reverse proxy |
| `tls_cert` | string | `""` | Path to TLS certificate file (PEM). Required for L7 routes. |
| `tls_key` | string | `""` | Path to TLS private key file (PEM). Required for L7 routes. |
| `backend_tls` | bool | `false` | If true, connect to backend with TLS (re-encrypt). If false, plaintext h2c/HTTP. |
| `send_proxy_protocol` | bool | `false` | If true, send PROXY protocol v2 header to backend. |

Routes are scoped to the listener that accepted the connection. The same
hostname can appear on different listeners with different backends and
different modes.

### Match Semantics

- Hostname matching is **exact** and **case-insensitive** (per RFC 6066,
  SNI hostnames are DNS names and compared case-insensitively).
- Wildcard matching is not supported in the initial implementation.
- If duplicate hostnames appear within the same listener, the proxy refuses
  to start.
- If no route matches an incoming SNI hostname, the connection is reset.

### Route Table Source

Route tables are persisted in the SQLite database. On first run, they are
seeded from the TOML configuration. On subsequent runs, the database is
the source of truth. Routes can be added or removed at runtime via the
gRPC admin API.

### Validation

- L7 routes must have `tls_cert` and `tls_key` set and pointing to
  readable files. The proxy validates certificate/key pairs at startup and
  refuses to start if any are invalid.
- L4 routes must not have `tls_cert` or `tls_key` set (they are ignored
  and produce a warning if present).
- `backend_tls` is only meaningful for L7 routes. On L4 routes it is
  ignored (the raw TLS stream passes through regardless).

---

## L7 Proxying

L7 routes terminate TLS at the proxy and reverse proxy HTTP traffic to the
backend. This enables HTTP-level features that are impossible in L4 mode.

### TLS Termination

The proxy uses `crypto/tls.Server` with a `GetCertificate` callback that
selects the certificate based on the SNI hostname. Since SNI has already
been extracted in step 4 of the connection lifecycle, the route (and its
cert/key) is known before the handshake begins.

The `prefixConn` wrapper replays the buffered ClientHello bytes, then reads
from the underlying `net.Conn`. From `crypto/tls`'s perspective, it is
reading a normal TCP connection.

TLS configuration:

- Minimum version: TLS 1.2 (TLS 1.3 preferred).
- Cipher suites: Go's default secure set (no manual override).
- ALPN: `h2` and `http/1.1` advertised. The negotiated protocol determines
  whether the connection uses HTTP/2 or HTTP/1.1.

### HTTP/2 Reverse Proxy

After the TLS handshake, the proxy serves HTTP using Go's `net/http` server
with HTTP/2 enabled. Each request is forwarded to the backend using a
reverse proxy.

**Backend transports:**

| `backend_tls` | Protocol | Description |
|----------------|----------|-------------|
| `false` | h2c | HTTP/2 over plaintext TCP. Used for localhost/trusted-network backends. Supports gRPC and standard HTTP. |
| `true` | h2 (TLS) | HTTP/2 over TLS to backend. Used when backend is remote or requires encryption. |

The reverse proxy handles:

- **HTTP/2 and HTTP/1.1**: Protocol negotiated via ALPN on the client side.
  Backend connections use h2c (or h2 if `backend_tls`). If the backend only
  speaks HTTP/1.1, Go's transport handles the protocol translation.
- **gRPC**: gRPC is HTTP/2 with `content-type: application/grpc`. The
  reverse proxy forwards HTTP/2 frames, including **trailers** (critical
  for gRPC status codes and error details).
- **Streaming**: Server-side, client-side, and bidirectional gRPC streaming
  work through the reverse proxy. Request and response bodies are streamed,
  not buffered.
- **Header injection**: `X-Forwarded-For`, `X-Forwarded-Proto`, and
  `X-Real-IP` are set from the real client IP (PROXY protocol or TCP source).
- **Hop-by-hop headers**: `Connection`, `Transfer-Encoding`, and other
  hop-by-hop headers are stripped per RFC 7230.

### Connection Model

L7 uses 1:1 connection mapping: each client connection maps to one backend
connection. There is no connection pooling. This keeps the implementation
simple and the connection lifecycle predictable. Pooling may be added later
if backend connection setup becomes a bottleneck.

### Error Handling

| Condition | Client Response |
|-----------|----------------|
| TLS handshake failure | Connection closed (TCP RST) |
| Backend dial failure | HTTP 502 Bad Gateway |
| Backend timeout | HTTP 504 Gateway Timeout |
| Backend returns error | Error response forwarded as-is |

---

## gRPC Admin API

The admin API is optional (disabled if `[grpc]` is omitted from the config).
It listens on a Unix domain socket for security -- access is controlled via
filesystem permissions (0600, owner-only). The API provides runtime management
of routes and firewall rules without restarting the proxy.

### RPCs

| RPC | Description |
|-----|-------------|
| `ListRoutes` | List all routes for a given listener |
| `AddRoute` | Add a route to a listener (write-through to DB) |
| `RemoveRoute` | Remove a route from a listener (write-through to DB) |
| `GetFirewallRules` | List all firewall rules |
| `AddFirewallRule` | Add a firewall rule (write-through to DB) |
| `RemoveFirewallRule` | Remove a firewall rule (write-through to DB) |
| `GetStatus` | Return version, uptime, listener status, connection counts |
| `grpc.health.v1.Health` | Standard gRPC health check (Check, Watch) |

### Input Validation

The admin API validates all inputs before persisting:

- **Route backends** must be valid `host:port` tuples.
- **Route mode** must be `"l4"` or `"l7"`.
- **L7 routes** must include valid `tls_cert` and `tls_key` paths.
- **IP firewall rules** must be valid IP addresses (`netip.ParseAddr`).
- **CIDR firewall rules** must be valid prefixes in canonical form.
- **Country firewall rules** must be exactly 2 uppercase letters (ISO 3166-1 alpha-2).

### Security

The gRPC admin API has no MCIAS integration -- mc-proxy is pre-auth
infrastructure. Access control relies on Unix socket filesystem permissions:

- Socket is created with mode `0600` (read/write for owner only)
- Only processes running as the same user can connect
- No network exposure -- the API is not accessible over TCP

---

## Configuration

TOML configuration file, loaded at startup. The proxy refuses to start if
required fields are missing or invalid.

```toml
# Database. Required.
[database]
path = "/srv/mc-proxy/mc-proxy.db"

# Listeners. Each has its own route table (seeds DB on first run).
[[listeners]]
addr = ":443"
proxy_protocol = false   # accept PROXY protocol headers (default: false)

  # L4 route: passthrough, no TLS termination.
  [[listeners.routes]]
  hostname = "metacrypt.metacircular.net"
  backend  = "127.0.0.1:18443"
  mode     = "l4"

  # L7 route: terminate TLS, reverse proxy HTTP/2.
  [[listeners.routes]]
  hostname = "api.metacircular.net"
  backend  = "127.0.0.1:8080"
  mode     = "l7"
  tls_cert = "/srv/mc-proxy/certs/api.crt"
  tls_key  = "/srv/mc-proxy/certs/api.key"

  # L4 route with PROXY protocol to downstream proxy.
  [[listeners.routes]]
  hostname = "mcias.metacircular.net"
  backend  = "10.0.0.5:443"
  mode     = "l4"
  send_proxy_protocol = true

[[listeners]]
addr = ":8443"

  [[listeners.routes]]
  hostname = "metacrypt.metacircular.net"
  backend  = "127.0.0.1:18443"

# gRPC admin API. Optional -- omit or leave addr empty to disable.
# Listens on a Unix socket; access controlled via filesystem permissions.
[grpc]
addr = "/var/run/mc-proxy.sock"

# Firewall. Global blocklist, evaluated before routing. Default allow.
[firewall]
geoip_db          = "/srv/mc-proxy/GeoLite2-Country.mmdb"
blocked_ips       = ["192.0.2.1"]
blocked_cidrs     = ["198.51.100.0/24"]
blocked_countries = ["KP", "CN", "IN", "IL"]
rate_limit        = 20       # max new connections per IP per rate_window
rate_window       = "10s"

# Proxy behavior.
[proxy]
connect_timeout  = "5s"     # Timeout for dialing backend
idle_timeout     = "300s"   # Close connections idle longer than this
shutdown_timeout = "30s"    # Graceful shutdown drain period

# Logging.
[log]
level = "info"              # debug, info, warn, error
```

### Environment Variable Overrides

Configuration values can be overridden via environment variables using the
prefix `MCPROXY_` with underscore-separated paths:

```
MCPROXY_LOG_LEVEL=debug
MCPROXY_PROXY_IDLE_TIMEOUT=600s
```

Environment variables cannot define listeners, routes, or firewall rules --
these are structural and must be in the TOML file.

---

## Storage

### SQLite Database

Listeners, routes, and firewall rules are persisted in a SQLite database
(WAL mode, foreign keys enabled, busy timeout 5000ms). The pure-Go driver
`modernc.org/sqlite` is used (no CGo).

**Startup behavior:**

1. Open the database at the configured path. Run migrations.
2. If the database is empty (first run): seed from the TOML config.
3. If the database has data: load from it. TOML listener/route/firewall
   fields are ignored.

The TOML config continues to own operational settings: proxy timeouts,
log level, gRPC config, GeoIP database path.

**Write-through pattern:** The gRPC admin API writes to the database first,
then updates in-memory state. If the database write fails, the in-memory
state is not modified.

### Schema

```sql
CREATE TABLE listeners (
    id             INTEGER PRIMARY KEY,
    addr           TEXT    NOT NULL UNIQUE,
    proxy_protocol INTEGER NOT NULL DEFAULT 0
);

CREATE TABLE routes (
    id                  INTEGER PRIMARY KEY,
    listener_id         INTEGER NOT NULL REFERENCES listeners(id) ON DELETE CASCADE,
    hostname            TEXT    NOT NULL,
    backend             TEXT    NOT NULL,
    mode                TEXT    NOT NULL DEFAULT 'l4' CHECK(mode IN ('l4', 'l7')),
    tls_cert            TEXT    NOT NULL DEFAULT '',
    tls_key             TEXT    NOT NULL DEFAULT '',
    backend_tls         INTEGER NOT NULL DEFAULT 0,
    send_proxy_protocol INTEGER NOT NULL DEFAULT 0,
    UNIQUE(listener_id, hostname)
);

CREATE TABLE firewall_rules (
    id    INTEGER PRIMARY KEY,
    type  TEXT NOT NULL CHECK(type IN ('ip', 'cidr', 'country')),
    value TEXT NOT NULL,
    UNIQUE(type, value)
);
```

### Data Directory

```
/srv/mc-proxy/
├── mc-proxy.toml               Configuration
├── mc-proxy.db                 SQLite database
├── mc-proxy.sock               Unix socket for gRPC admin API
├── certs/                      TLS certificates for L7 routes
│   ├── api.crt
│   └── api.key
├── GeoLite2-Country.mmdb       GeoIP database (if using country blocks)
└── backups/                    Database snapshots
```

L4 routes do not use any TLS material -- the raw TLS stream passes through.
L7 routes require certificate/key pairs in the `certs/` directory (or any
path specified in the route config). The gRPC admin API uses a Unix socket.

---

## Deployment

### Binary

Single static binary, built with `CGO_ENABLED=0`. No runtime dependencies
beyond the configuration file, optional GeoIP database, and TLS
certificates for any L7 routes.

### Container

Multi-stage Docker build:

1. **Builder**: `golang:<version>-alpine`, static compilation.
2. **Runtime**: `alpine`, non-root user (`mc-proxy`), port exposure
   determined by configuration.

### systemd

| File | Purpose |
|------|---------|
| `mc-proxy.service` | Main proxy service |
| `mc-proxy-backup.service` | Oneshot database backup (VACUUM INTO) |
| `mc-proxy-backup.timer` | Daily backup timer (02:00 UTC, 5-minute jitter) |

The proxy binds to privileged ports (443) and should use `AmbientCapabilities=CAP_NET_BIND_SERVICE`
in the systemd unit rather than running as root.

Standard security hardening directives apply per engineering standards
(`NoNewPrivileges=true`, `ProtectSystem=strict`, etc.).

### Multi-Hop Deployment

A typical production topology uses two mc-proxy instances:

```
Internet ──→ [edge mc-proxy] ──Tailscale──→ [origin mc-proxy] ──→ backends
               VPS, :443                      private network
```

**Edge mc-proxy** (public VPS):
- Listeners with `proxy_protocol = false` (clients connect directly).
- All routes are L4 passthrough with `send_proxy_protocol = true`.
- Firewall rules for GeoIP and IP blocking applied here.

**Origin mc-proxy** (private network):
- Listeners with `proxy_protocol = true` (receives real client IP from edge).
- Mix of L4 and L7 routes depending on the backend service.
- L7 routes terminate TLS and reverse proxy HTTP/2 to local backends.
- Firewall rules applied using the real client IP from the PROXY header.

### Graceful Shutdown

On `SIGINT` or `SIGTERM`:

1. Stop accepting new connections on all listeners.
2. Wait for in-flight connections to complete (up to `shutdown_timeout`).
3. Force-close remaining connections.
4. Exit.

On `SIGHUP`:

1. Reload the GeoIP database from disk.
2. Continue serving with the updated database.

Routes and firewall rules can be modified at runtime via the gRPC admin API
(write-through to SQLite). Listener changes (adding/removing ports) require
a full restart. TOML configuration changes (timeouts, log level, GeoIP path)
also require a restart.

---

## Security Model

mc-proxy is infrastructure that sits in front of authenticated services.
It has no authentication or authorization of its own.

### Threat Mitigations

| Threat | Mitigation |
|--------|------------|
| SNI spoofing | L4: Backend performs its own TLS handshake -- spoofed SNI fails certificate validation at the backend. L7: The proxy validates the certificate against the SNI during its own handshake, so a mismatched SNI results in a handshake failure before reaching the backend. |
| TLS private key compromise (L7) | Keys are stored on the filesystem with restrictive permissions. The proxy process runs as a dedicated non-root user. Keys are loaded into memory at startup; the file can be made read-only after startup. Consider integration with metacrypt for key storage (future). |
| Client IP spoofing via PROXY protocol | PROXY protocol headers are only trusted on listeners explicitly configured with `proxy_protocol = true`. Non-PROXY listeners reject connections that start with a PROXY header (they fail SNI extraction). Edge listeners facing the public internet should never enable `proxy_protocol`. |
| Resource exhaustion (connection flood) | Idle timeout closes stale connections. Per-IP rate limiting. Per-listener connection limits (future). |
| GeoIP evasion via IPv6 | GeoLite2 database includes IPv6 mappings. Both IPv4 and IPv6 source addresses are checked. |
| GeoIP evasion via VPN/proxy | Accepted risk. GeoIP blocking is a compliance measure, not a security boundary. Determined adversaries will bypass it. |
| Slowloris / slow ClientHello | Hardcoded 10-second timeout on the SNI extraction phase. 5-second timeout on PROXY protocol header parsing. |
| Backend unavailability | Connect timeout prevents indefinite hangs. L4: connection reset. L7: HTTP 502 response. |
| Information leakage (L4) | Blocked connections receive only a TCP RST. No version strings, no error messages, no TLS alerts. |
| Information leakage (L7) | Error responses are minimal (502/504 with no detail). The proxy does not expose its version or software name in HTTP headers. |
| HTTP request smuggling (L7) | Go's `net/http` server handles HTTP parsing. The reverse proxy uses `httputil.ReverseProxy` which strips hop-by-hop headers. HTTP/2 framing prevents classic smuggling vectors. |

### Security Invariants

1. **L4 routes never terminate TLS.** The proxy cannot read application-layer
   traffic on L4 routes. It never modifies the byte stream.
2. **L7 routes terminate TLS at the proxy.** The proxy can read HTTP headers
   and request metadata on L7 routes. It cannot read encrypted backend
   traffic when `backend_tls = true`.
3. Firewall rules are always evaluated before any routing decision.
4. PROXY protocol is only parsed on explicitly enabled listeners.
5. The proxy never logs request or response bodies -- only metadata (source
   IP, SNI hostname, backend, timestamps, bytes transferred, HTTP status
   codes on L7 routes).

---

## Future Work

Items are listed roughly in priority order:

| Item | Description |
|------|-------------|
| **ACME integration** | Automatic certificate provisioning via Let's Encrypt for L7 routes, removing the need for manual cert management. |
| **MCP integration** | Wire the gRPC admin API into the Metacircular Control Plane for centralized management. |
| **Connection pooling** | Pool backend connections for L7 routes to reduce connection setup overhead under high request volume. |
| **Metacrypt key storage** | Store L7 TLS private keys in metacrypt rather than on the filesystem. |