reverse-proxy/docs/architecture/tls.md

status, last_updated

status	last_updated
draft	2026-06-11

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

                    ┌──────────────────────────────────────────┐
                    │          TLS Termination                   │
                    │                                            │
  bind_addr:443 ──► │  TcpListener::bind(bind_addr)             │
                    │       │                                    │
                    │       ▼                                    │
                    │  tokio-rustls::TlsAcceptor                 │
                    │       │                                    │
                    │       ├─ ACME mode:                        │
                    │       │  rustls-acme::ResolvesServerCertAcme │
                    │       │  (auto-provisions & renews certs)   │
                    │       │                                    │
                    │       └─ Manual mode:                        │
                    │          rustls::ServerConfig               │
                    │          .with_single_cert(cert_chain, key) │
                    │                                            │
                    │       │                                    │
                    │       ▼                                    │
                    │  TlsStream<TcpStream>                      │
                    │       │                                    │
                    │       ▼                                    │
                    │  hyper::service_fn → axum router            │
                    └──────────────────────────────────────────┘

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

Certificate Provisioning

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. AcmeCertProvider configures the ACME client with the domain list, cache directory, and Let's Encrypt directory (staging or production).
2. AcmeConfig::new(domains) creates an ACME configuration for all listed domains. Let's Encrypt will issue a single SAN certificate covering all domains.
3. The ACME state machine runs as a background tokio task, 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)
4. ResolvesServerCertAcme is a rustls ResolvesServerCert implementation that automatically serves the ACME-provisioned certificate.
5. When a new certificate is issued, the resolver updates atomically — no restart or signal handling needed.

Configuration:

[server.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

Cache directory: The DirCache from rustls-acme persists ACME account data, private keys, and certificates between restarts. This avoids re-provisioning on every restart.

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.

[tls]
mode = "manual"
cert_path = "/etc/letsencrypt/live/git.alk.dev/fullchain.pem"
key_path = "/etc/letsencrypt/live/git.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

For manual mode, the ServerConfig is built with with_no_client_auth() and a custom ResolvesServerCert implementation that 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 all domains listed in acme_domains, and the resolver manages a single SAN certificate covering all of them. The ACME TLS-ALPN-01 protocol identifier (acme-tls/1) must be registered in the alpn_protocols list so the server can respond to TLS-ALPN-01 challenges.

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

ACME Mode (Multi-Domain)

In ACME mode, rustls-acme manages a single SAN certificate covering all configured domains. 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 (Multi-Domain)

In manual mode, a custom ResolvesServerCert implementation is required to map 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).

See open-questions.md OQ-07 for per-site TLS overrides.

HTTP Listener (Port 80)

The HTTP listener on port 80 is a plain TCP listener with no TLS. It has one job: redirect all requests to the HTTPS equivalent.

The listener binds to the same IP address as the 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.

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

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? (open)