reverse-proxy/docs/architecture/config.md

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Configuration

What It Is

The configuration system defines how the proxy is configured, how configuration is loaded, and how dynamic configuration can be reloaded without restarting the process.

Why It Exists

The proxy needs to be configurable without hard-coding domains, upstream addresses, or TLS settings. The configuration system separates immutable startup parameters (bind addresses, TLS mode) from runtime-adjustable parameters (site definitions, rate limits) using the ArcSwap pattern proven in the alknet project.

Architecture

config.toml
    │
    ▼
┌──────────────────────┐
│  serde::Deserialize   │
│  (TOML → Config)     │
└──────────┬───────────┘
           │
           ▼
┌──────────────────────┐
│  StaticConfig         │
│  (immutable)         │
│                      │
│  health_check_port   │
│  admin_socket_path   │
│  log_level           │
│  log_format          │
│                      │
│  listeners[]         │
│  ┌────────────────┐  │
│  │ Listener 1      │  │
│  │ bind_addr       │  │
│  │ http_port       │  │
│  │ https_port      │  │
│  │ tls.mode        │  │
│  │ tls.acme_domains│  │
│  │ tls.acme_cache_dir│ │
│  │ tls.acme_directory│ │
│  │ tls.cert_path   │  │
│  │ tls.key_path    │  │
│  └────────────────┘  │
│  ┌────────────────┐  │
│  │ Listener N      │  │
│  │ ...             │  │
│  └────────────────┘  │
└──────────────────────┘

┌──────────────────────┐
│  DynamicConfig        │
│  (hot-reloadable)     │
│                       │
│  sites[]              │
│  rate_limit           │
│  body_limit           │
│                       │
│  ← ArcSwap →          │
│  ConfigReloadHandle    │
│  .reload(new_config)  │
└───────────────────────┘

Static vs Dynamic Configuration

This split follows the pattern established in alknet (ADR-030) and adapted for our simpler use case. See ADR-019 for the rationale behind the [[listeners]] configuration format.

StaticConfig

Immutable after startup. Changes require a process restart.

Field	Type	Description
listeners	Vec<ListenerConfig>	Independent TLS endpoints, each with its own bind address and TLS config (see ADR-019)
health_check_port	u16	Port for local health check endpoint (default: 9900; set to 0 to disable; see ADR-013)
admin_socket_path	String	Unix domain socket path for admin API (default: /run/reverse-proxy/admin.sock; empty string to disable; see ADR-014)
log_level	"trace", "debug", "info", "warn", "error"	Logging verbosity
log_format	"text" or "json"	Log output format

ListenerConfig (per-listener static config):

Field	Type	Description
bind_addr	String	IP address to bind to (must be explicit, no 0.0.0.0; see ADR-016)
http_port	u16	Port for HTTP→HTTPS redirect (default: 80; set to 0 to disable)
https_port	u16	Port for TLS listener (default: 443)
tls.mode	"acme" or "manual"	Certificate provisioning mode
tls.acme_domains	Vec<String>	Domains for ACME SAN certificate (ACME mode only)
tls.acme_cache_dir	String	ACME state cache directory
tls.acme_directory	"production" or "staging"	Let's Encrypt directory
tls.cert_path	String	Certificate file path (manual mode only)
tls.key_path	String	Private key file path (manual mode only)

Why listeners are static: Each listener requires binding a TCP socket and constructing a TLS acceptor — operations that fundamentally require a restart. Changing a listener's bind address, TLS mode, or certificate configuration cannot be done without creating new listeners. See ADR-008 and ADR-019.

DynamicConfig

Hot-reloadable at runtime via ArcSwap. Changes take effect for new connections immediately.

Field	Type	Description
sites	Vec<SiteConfig>	Site definitions (hostname → upstream mapping)
rate_limit.requests_per_second	u32	Rate limit per IP (global in Phase 1)
rate_limit.burst	u32	Burst capacity (global in Phase 1)
body_limit_bytes	u64	Max request body size in bytes (global in Phase 1)

SiteConfig:

Field	Type	Description
host	String	Hostname to match (e.g., "git.alk.dev")
upstream	String	Upstream address (e.g., "127.0.0.1:3000")
upstream_scheme	"http" or "https"	Protocol for upstream connection (default: "http")
upstream_connect_timeout_secs	u64	TCP connect timeout in seconds (default: 5; see ADR-015, ADR-017)
upstream_request_timeout_secs	u64	Full request timeout in seconds (default: 60; see ADR-015, ADR-017)

Sites are defined per listener in the [[listeners]] entries. Each listener routes its own sites independently. The DynamicConfig collects all sites across all listeners for hot-reload via ArcSwap. When a config reload occurs, all listener site mappings are updated atomically.

Why these are dynamic: See ADR-008 for the rationale. Site definitions and rate limits are per-request concerns that should not require restarting the proxy or dropping active connections. Rate limits and body limits are global settings in Phase 1; per-site configuration for these is deferred to Phase 2.

Config Reload

ArcSwap Pattern

DynamicConfig is wrapped in Arc<ArcSwap<DynamicConfig>>. This provides:

• Lock-free reads: Every handler reads the current config via a single Arc dereference — no lock contention on the request hot path.
• Atomic writes: ConfigReloadHandle::reload(new_config) swaps the entire config atomically. All new requests see the new config immediately.
• No partial updates: The entire config is swapped at once. There's no risk of reading a half-updated config.

See ADR-008 for the rationale behind this split.

Reload Trigger

Config reload is triggered by two mechanisms:

1. SIGHUP: Re-reads the config file, validates, and swaps DynamicConfig if valid. Simple and well-understood, but provides no feedback on success or failure.

2. Admin socket: The reload command via the admin Unix domain socket performs the same action as SIGHUP but returns a structured response indicating success or failure with an error message. See ADR-014 for details.

Both mechanisms converge on the same code path:

1. Re-read the config file from disk
2. Deserialize into DynamicConfig
3. Validate (check upstream reachability is optional)
4. Call ConfigReloadHandle::reload(new_config)

TOML Config Format

Multi-Config (Dedicated-IP Per Domain)

The primary deployment model — each listener on its own IP with its own TLS certificate:

# reverse-proxy config

# Global settings
health_check_port = 9900     # Local health check (0 to disable)
admin_socket_path = "/run/reverse-proxy/admin.sock"  # Empty string to disable

[logging]
level = "info"
format = "text"                  # "text" or "json"

[rate_limit]
requests_per_second = 10
burst = 20

[body]
limit_bytes = 104857600          # 100 MB

# Listener 1: git.alk.dev on its own IP
[[listeners]]
bind_addr = "203.0.113.10"
http_port = 80
https_port = 443

[listeners.tls]
mode = "acme"
acme_domains = ["git.alk.dev"]
acme_cache_dir = "/var/lib/reverse-proxy/acme-cache-git"
acme_directory = "production"

[[listeners.sites]]
host = "git.alk.dev"
upstream = "127.0.0.1:3000"
upstream_scheme = "http"
# upstream_connect_timeout_secs = 5    # Default: 5s
# upstream_request_timeout_secs = 60    # Default: 60s

# Listener 2: alk.dev on its own IP with a manual certificate
[[listeners]]
bind_addr = "203.0.113.11"
http_port = 80
https_port = 443

[listeners.tls]
mode = "manual"
cert_path = "/etc/ssl/alk.dev/fullchain.pem"
key_path = "/etc/ssl/alk.dev/privkey.pem"

[[listeners.sites]]
host = "alk.dev"
upstream = "127.0.0.1:8080"
upstream_scheme = "http"

Shared-IP Multi-Domain (SAN Certificate)

A single listener serving multiple domains with one SAN certificate:

# Global settings
health_check_port = 9900
admin_socket_path = "/run/reverse-proxy/admin.sock"

[logging]
level = "info"
format = "text"

[rate_limit]
requests_per_second = 10
burst = 20

[body]
limit_bytes = 104857600

# Single listener with multi-domain SAN certificate
[[listeners]]
bind_addr = "203.0.113.10"
http_port = 80
https_port = 443

[listeners.tls]
mode = "acme"
acme_domains = ["git.alk.dev", "alk.dev"]
acme_cache_dir = "/var/lib/reverse-proxy/acme-cache"
acme_directory = "production"

[[listeners.sites]]
host = "git.alk.dev"
upstream = "127.0.0.1:3000"

[[listeners.sites]]
host = "alk.dev"
upstream = "127.0.0.1:8080"

Validation

On startup, the config is validated:

1. At least one [[listeners]] entry must exist
2. Each listener's bind_addr is not 0.0.0.0 (must be explicit; see ADR-016)
3. Each listener's bind_addr and https_port combination must be unique
4. In ACME mode, acme_domains must be non-empty
5. In manual mode, cert_path and key_path must both be set and the files must be readable
6. Each site must have a host and upstream
7. Site host values must be unique across all listeners (no duplicate hostnames, even across different listeners). Duplicate hostnames would create ambiguous routing — the proxy would not know which listener's upstream to route a request to when the Host header matches multiple sites.
8. rate_limit.requests_per_second must be > 0
9. body.limit_bytes must be > 0

On SIGHUP reload, the same validation applies. If the new config fails validation, the reload is rejected and the old config remains active. An error is logged.

On startup: If config validation fails, the process exits with a non-zero code and logs the validation errors. The proxy will not start with an invalid configuration.

Design Decisions

All design decisions are documented as ADRs in decisions/.

ADR	Decision	Summary
003	TOML configuration format	Rust-native, unambiguous, excellent serde support
008	Static/dynamic config split	Immutable StaticConfig, hot-reloadable DynamicConfig via ArcSwap
010	Multi-site in Phase 1	Multiple domains from initial release
011	Multi-domain TLS config	Single SAN certificate covering all domains
013	Health check on separate local port	Localhost-only HTTP health check, configurable port
014	Unix domain socket config reload API	Programmatic reload with success/failure feedback
015	Per-site upstream timeouts with defaults	5s connect / 60s request defaults, per-site overrides
016	Explicit bind address required	Rejects 0.0.0.0 to prevent accidental exposure
019	Multi-config listeners	[[listeners]] supporting both dedicated-IP and shared-IP deployment models

Open Questions

Open questions are tracked in open-questions.md. Key questions affecting this document:

• OQ-04: Should config reload support a Unix domain socket API in addition to SIGHUP? (resolved — ADR-014: Unix domain socket admin API added)
• OQ-07: Should per-site TLS overrides be supported for mixed ACME/manual domains? (resolved — ADR-019: [[listeners]] with per-listener TLS config)