reverse-proxy/docs/architecture/tls.md

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TLS Termination

What It Is

The TLS termination component handles all aspects of encrypted connections: certificate provisioning (ACME and manual), TLS handshake, SNI-based certificate selection, and connection wrapping for the axum router.

Why It Exists

TLS termination is the security boundary between the public internet and our upstream services. It replaces nginx's ssl_certificate, ssl_protocols, and ssl_ciphers configuration with a memory-safe Rust implementation using rustls.

Architecture

The proxy supports multiple independent TLS listeners, each with its own bind address, TLS configuration, and site routing. See ADR-019 for the rationale.

                    ┌──────────────────────────────────────────┐
                    │          TLS Termination                   │
                    │                                            │
                    │  ┌─ Listener 1 ─────────────────────────┐ │
                    │  │  bind_addr_1:443                       │ │
                    │  │  TcpListener::bind(bind_addr_1)       │ │
                    │  │       │                               │ │
                    │  │       ▼                               │ │
                    │  │  tokio-rustls::TlsAcceptor            │ │
                    │  │       │                               │ │
                    │  │  ACME or Manual TLS config            │ │
                    │  │  (per-listener TLS mode)              │ │
                    │  │       │                               │ │
                    │  │       ▼                               │ │
                    │  │  TlsStream<TcpStream>                 │ │
                    │  │       │                               │ │
                    │  │       ▼                               │ │
                    │  │  axum router (per-listener sites)     │ │
                    │  └───────────────────────────────────────┘ │
                    │                                            │
                    │  ┌─ Listener N ─────────────────────────┐ │
                    │  │  bind_addr_N:443                       │ │
                    │  │  ... (same structure)                  │ │
                    │  └───────────────────────────────────────┘ │
                    └──────────────────────────────────────────┘

  bind_addr:80  ──►  HTTP listener (redirect to HTTPS, no TLS)

Each listener is independently configured. This supports two deployment models:

1. Shared-IP multi-domain: One listener with multiple domains in acme_domains, using a single SAN certificate and SNI routing.
2. Dedicated-IP single-domain: Multiple listeners, each with its own IP, its own TLS certificate, and its own site. No SNI needed.

Certificate Provisioning

Each listener has its own TLS mode (ACME or manual), configured independently.

ACME Mode (Primary)

Uses rustls-acme for automatic certificate provisioning and renewal through Let's Encrypt. This is the primary mode — no certbot dependency, no cron jobs, no deploy hooks.

How it works:

1. Each listener in ACME mode creates its own AcmeCertProvider with the listener's domain list, cache directory, and Let's Encrypt directory.
2. AcmeConfig::new(domains) creates an ACME configuration for the domains listed in that listener's acme_domains. Let's Encrypt will issue a certificate covering those domains (a single SAN certificate or a single-domain certificate, depending on how many domains are listed).
3. The acme_contact field provides a contact email address (as a mailto: URI) required by Let's Encrypt for production certificate requests. Without a contact email, Let's Encrypt production API returns a 400-level error.
4. The ACME state machine runs as a background tokio task per listener, handling:
   • Account registration with Let's Encrypt
   • Certificate ordering
   • TLS-ALPN-01 challenge (or HTTP-01 challenge)
   • Certificate issuance
   • Certificate renewal (automatic, ~30 days before expiry)
5. ResolvesServerCertAcme is a rustls ResolvesServerCert implementation that automatically serves the ACME-provisioned certificate.
6. When a new certificate is issued, the resolver updates atomically — no restart or signal handling needed.

Configuration (within a [[listeners]] entry):

[[listeners]]
bind_addr = "203.0.113.10"

[listeners.tls]
mode = "acme"
acme_domains = ["git.alk.dev", "alk.dev"]
acme_cache_dir = "/var/lib/reverse-proxy/acme-cache"
acme_directory = "production"  # or "staging" for testing
acme_contact = "mailto:admin@alk.dev"  # Required for Let's Encrypt production

Cache directory: The DirCache from rustls-acme persists ACME account data, private keys, and certificates between restarts. This avoids re-provisioning on every restart. Each listener should use its own cache directory to avoid conflicts between separate ACME state machines.

Manual Mode (Fallback)

For environments where ACME is not desired (testing, self-signed certs, corporate CAs, or BYO certificates), the proxy loads certificates from file paths at startup.

[[listeners]]
bind_addr = "203.0.113.11"

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

Certificate files are loaded once at startup using rustls_pemfile. Manual mode requires a restart to pick up new certificates. See ADR-004 for the rationale behind making ACME the primary mode and manual mode restart-dependent.

TLS Configuration

Protocol Versions

The proxy supports TLS 1.2 and TLS 1.3 only, matching the minimum security level of the current nginx configuration. The aws_lc_rs crypto provider defaults to these protocol versions; explicit configuration ensures no regression if defaults change in future rustls releases.

Cipher Suites

Cipher suites are explicitly restricted to match the scope of our current nginx configuration. See ADR-012 for the full rationale.

TLS 1.2 (explicitly selected):

• TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
• TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
• TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
• TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384

TLS 1.3 (all default suites):

• TLS_AES_128_GCM_SHA256
• TLS_AES_256_GCM_SHA384
• TLS_CHACHA20_POLY1305_SHA256

This is configured by building a CryptoProvider with a custom cipher_suite list and passing it to ServerConfig::builder_with_provider(). The cipher list matches our current nginx configuration's scope, providing behavioral parity during migration.

ServerConfig Construction

Each listener constructs its own ServerConfig based on its TLS mode.

For manual mode, the ServerConfig is built with with_no_client_auth() and the loaded certificate chain and private key. If the listener serves multiple domains from a single listener, a custom ResolvesServerCert implementation maps SNI hostnames to certificate/key pairs loaded from disk.

For ACME mode, the ServerConfig is built with with_cert_resolver(), passing the ResolvesServerCertAcme resolver. The ACME configuration includes the domains listed in that listener's acme_domains, and the resolver manages the certificate.

The TLS ServerConfig advertises ALPN protocols to enable HTTP/2 negotiation. The ALPN configuration differs by TLS mode:

• ACME mode: h2, http/1.1, and acme-tls/1. The acme-tls/1 entry is required for TLS-ALPN-01 challenge verification during certificate provisioning.
• Manual mode (single-cert and multi-domain/SNI): h2 and http/1.1 only. The acme-tls/1 entry is not included because manual mode does not use ACME challenges.

After the TLS handshake, the proxy inspects the negotiated ALPN protocol to select the appropriate HTTP server: h2 triggers hyper::server::conn::http2::Builder, while http/1.1 (or no ALPN) triggers hyper_util::server::conn::auto::Builder. See ADR-023 for details.

Both modes use the aws_lc_rs crypto provider with safe default protocol versions (TLS 1.2 and TLS 1.3).

SNI-Based Certificate Selection

After the TLS handshake, the proxy inspects the negotiated ALPN protocol to determine whether to serve the connection as HTTP/2 or HTTP/1.1. If the client negotiated h2 via ALPN, the proxy uses hyper::server::conn::http2::Builder; otherwise, it uses hyper_util::server::conn::auto::Builder with HTTP/1.1 and upgrade support. See ADR-023 for details.

Dedicated-IP Single-Domain (Multi-Config)

In the dedicated-IP model, each listener binds to its own IP address and serves exactly one domain with one certificate. SNI is not required for certificate selection — the listener's TLS config already has the correct certificate.

This is the simplest case: one IP, one listener, one certificate, one domain. No SNI resolution logic is needed.

Shared-IP Multi-Domain (SAN Certificate)

In the shared-IP model, a single listener serves multiple domains using a SAN certificate. SNI-based certificate selection is required.

In ACME mode, rustls-acme manages a single SAN certificate covering all configured domains for that listener. The ResolvesServerCertAcme resolver automatically serves the correct certificate during the TLS handshake.

1. TLS handshake: The client sends the SNI extension indicating which hostname it's connecting to.
2. Certificate resolution: ResolvesServerCertAcme matches the SNI hostname against the provisioned certificate's Subject Alternative Names and serves the certificate.
3. HTTP routing: After the TLS handshake, axum's Host extractor routes the request to the correct site handler based on the Host header.

This is the same pattern nginx uses — SNI selects the cert during TLS, then Host header selects the server block. ACME mode handles this automatically through the cert resolver.

Manual Mode with Multiple Domains

In manual mode on a shared-IP listener, a custom ResolvesServerCert implementation maps SNI hostnames to the correct CertifiedKey. This implementation:

1. Loads certificate files at startup (or on SIGHUP for reload)
2. Maps each domain name to its certificate chain and private key
3. During the TLS handshake, looks up the SNI hostname and returns the matching CertifiedKey

The custom resolver must handle the case where no matching certificate exists for the SNI hostname — in this case, the handshake fails, which is the correct behavior (we don't serve a default certificate for unknown domains).

HTTP Listener (Port 80)

Each listener has its own HTTP listener on port 80 (or the configured http_port). It is a plain TCP listener with no TLS. It has one job: redirect all requests to the HTTPS equivalent.

Each HTTP listener binds to the same IP address as its corresponding TLS listener, but on port 80.

ACME Challenge Type

The default ACME challenge type is TLS-ALPN-01, since the proxy already listens on port 443. This avoids requiring a separate HTTP-01 challenge server. HTTP-01 is available as a fallback for environments where TLS-ALPN-01 is not suitable (e.g., behind a CDN that terminates TLS). When using HTTP-01, the port 80 listener serves /.well-known/acme-challenge/{token} paths for challenge verification.

Certificate Failure Behavior

ACME certificate provisioning and renewal can fail for various reasons (network outages, Let's Encrypt unavailability, DNS issues, rate limiting). The proxy's behavior depends on the scenario:

Scenario	Behavior
First start, no cached cert, ACME unreachable	Fail to start with clear error message. The proxy cannot serve TLS without a certificate.
First start, no cached cert, ACME succeeds	Normal startup. Certificate is provisioned and cached.
Start with cached cert, ACME unreachable for renewal	Start normally with cached cert. Log error at warn level. rustls-acme retries per its built-in schedule.
Renewal failure after startup	Continue serving existing cert. Log error at warn level. rustls-acme retries per its built-in schedule.
Cached cert expired, renewal fails at startup	Fail to start if cert is expired at startup. An expired certificate cannot serve valid TLS.
Cached cert expires during runtime	Continue serving expired cert. Clients will receive certificate errors. Log at error level. This is the correct behavior — silently dropping TLS would be worse.

The key principle: never start without a valid TLS certificate, but always continue serving if a valid cert exists, even if renewal fails.

TLS Error Handling

TLS handshake failures are logged and the connection is closed. The proxy does not serve a default certificate for unknown hostnames — connections that don't match any configured certificate fail.

Scenario	Behavior
SNI hostname doesn't match any certificate (manual mode)	TLS handshake fails with unrecognized_name alert. Log at warn level with client IP and SNI hostname.
No SNI extension sent by client	TLS handshake fails with handshake_failure alert. Log at warn level with client IP.
Unsupported TLS version (1.0/1.1)	TLS handshake fails with protocol_version alert. Log at info level.
Cipher suite negotiation fails	TLS handshake fails with handshake_failure alert. Log at info level with client IP.
Certificate expired (manual mode)	Connection fails during TLS handshake. Log at error level. Other listeners/connections continue serving.

In ACME mode, the ResolvesServerCertAcme resolver handles certificate selection automatically — there is no SNI mismatch scenario because the resolver serves the ACME-provisioned certificate for all valid domains.

Key Files and Crates

Component	Crate	Purpose
TLS acceptor	tokio-rustls 0.26	Async TLS handshake over TCP streams
TLS config	rustls 0.23	ServerConfig, CryptoProvider, cipher suites
ACME client	rustls-acme 0.12	Automatic cert provisioning and renewal
PEM parsing	rustls-pemfile 2	Load cert/key from PEM files (manual mode)
PKI types	rustls-pki-types 1	CertificateDer, PrivateKeyDer

Design Decisions

All design decisions are documented as ADRs in decisions/.

ADR	Decision	Summary
004	ACME-primary cert management	Eliminates certbot; automatic provisioning and renewal
005	tokio-rustls directly	Full control over TLS config and ACME resolver integration
010	Multi-site in Phase 1	Multiple domains from initial release
011	Multi-domain TLS config	Single SAN certificate covering all domains via rustls-acme
012	Restrict cipher suites	Match nginx scope: four ECDHE-AES-GCM suites for TLS 1.2, all TLS 1.3 suites
019	Multi-config listeners	[[listeners]] supporting both dedicated-IP and shared-IP deployment models
023	HTTP/2 client-facing support	ALPN-based protocol detection; h2 and http/1.1 advertised

Open Questions

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

• OQ-01: Should cipher suites be restricted beyond rustls defaults? (resolved — ADR-012: restrict to nginx scope)
• OQ-07: Should per-site TLS overrides be supported for mixed ACME/manual domains? (resolved — ADR-019: [[listeners]] with per-listener TLS config)