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)
allow_wildcard_bind	bool	Allow 0.0.0.0 as a bind address. Required for container deployments. Default: false (see ADR-016, ADR-020)
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)
shutdown_timeout_secs	u64	Maximum seconds to wait for in-flight requests during graceful shutdown (default: 30)
logging	LoggingConfig	Logging configuration (see below)

LoggingConfig (nested in [logging] TOML section):

Field	Type	Description
level	"trace", "debug", "info", "warn", "error"	Logging verbosity
format	"text" or "json"	Log output format
log_file_path	String	Path to log file. When set, structured logs are written to this file in addition to stdout/stderr. Strongly recommended for fail2ban integration in container deployments (see ADR-020). Default: not set (file logging disabled)

Note: The entire LoggingConfig (including log_file_path) is static and requires a process restart to change. Log file path changes require reopening file handles, which is complex and low-value for Phase 1. Log rotation (Phase 2) will be handled via signal-based or built-in rotation.

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. Supports Docker DNS (gitea:3000), loopback (127.0.0.1:3000), LAN IPs, and tunnel endpoints. No assumption about upstream locality (see ADR-020)
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 for organizational purposes, but at runtime they are collected into a single global routing table in DynamicConfig. The proxy looks up the Host header in this global table to route requests. Hostnames must be unique across all listeners — a Host header can only match one site definition, regardless of which listener received the request. See ADR-019 for the rationale behind the [[listeners]] configuration format.

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.

Default Values

Field	Type	Default	Required
allow_wildcard_bind	bool	false	No
health_check_port	u16	9900	No
admin_socket_path	String	/run/reverse-proxy/admin.sock	No
shutdown_timeout_secs	u64	30	No
logging.level	String	"info"	No
logging.format	String	"text"	No
logging.log_file_path	String	(not set)	No
listeners[].http_port	u16	80	No
listeners[].https_port	u16	443	No
listeners[].tls.acme_directory	String	"production"	No
sites[].upstream_scheme	String	"http"	No
sites[].upstream_connect_timeout_secs	u64	5	No
sites[].upstream_request_timeout_secs	u64	60	No
rate_limit.requests_per_second	u32	—	Yes
rate_limit.burst	u32	—	Yes
body.limit_bytes	u64	—	Yes

Fields without defaults are required and must be specified in the config file.

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)

Static Config Changes During Reload

When the config file is reloaded (via SIGHUP or admin socket), the entire file is read and validated — both static and dynamic portions. This provides early error detection for misconfigurations that would prevent a restart from succeeding.

If the full config fails validation, the reload is rejected and the old DynamicConfig remains active.

If the full config passes validation but static fields have changed, the DynamicConfig is swapped normally and a warning is logged listing the changed static fields and noting that a restart is required for those changes to take effect. This gives operators early feedback about config drift.

Only the DynamicConfig portion is swapped via ArcSwap. StaticConfig changes require a process restart to take effect.

Reload Serialization

Reload operations are serialized using a tokio::sync::Mutex on the reload code path. If a reload is in progress (triggered by SIGHUP or admin socket) and a second reload is requested, the second request waits for the first to complete, then re-reads the config file (getting the latest version) and proceeds. This prevents race conditions where two concurrent reloads could apply an older config over a newer one.

Out of Scope: File Watching

Automatic file watching (inotify, fsnotify, etc.) is out of scope for Phase 1. Config reload is triggered explicitly by SIGHUP or admin socket command. File watching adds complexity (debouncing, handling atomic renames, handling editor swap files) that is not justified for a single-instance proxy with infrequent config changes.

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"
# log_file_path = "/var/log/reverse-proxy/access.log"  # Optional; always-on when set

[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"
# log_file_path = "/var/log/reverse-proxy/access.log"  # Optional; always-on when set

[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 unless allow_wildcard_bind is enabled. This can be enabled via config (allow_wildcard_bind = true) or CLI flag (--allow-wildcard-bind). Either source enables it — it is an OR relationship, not AND. The CLI flag does not override the config value; if either is set, wildcard binding is allowed.
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
10. Each listener's bind_addr and http_port combination must be unique (prevents bind-time errors, same as rule 3 for https_port)
11. Within a listener, http_port and https_port must differ
12. https_port must be 1–65535 (required — TLS needs a port)
13. http_port must be 0 (disabled) or 1–65535
14. health_check_port must not conflict with any listener's http_port or https_port on the same bind address
15. Site host values must not include a port number (e.g., git.alk.dev, not git.alk.dev:443)
16. Site host values must be valid hostnames (not IP addresses, not including ports). Hostnames are normalized to lowercase during validation.
17. upstream must be in host:port format where port is a required integer 1–65535. Examples: gitea:3000, 127.0.0.1:3000, [::1]:3000. Invalid examples: gitea (missing port), http://gitea:3000 (includes scheme), 10.0.0.5 (missing port). The upstream_scheme field handles the protocol.
18. upstream_scheme values are case-sensitive: only "http" or "https" (lowercase). Default is "http".

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
020	Container deployment model	Flexible upstream addressing; allow_wildcard_bind override for containers

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)