# Architecture

This project runs Traefik Proxy as a Dockerized TCP load balancer.

## Components

- `compose.yaml`: defines the Traefik container, published port, host-based configuration volume, container hardening, and healthcheck.
- `config/traefik.yml`: static Traefik configuration for HTTP, TCP TLS, ping, file provider, and logging.
- `config/dynamic/tcp-services.yml`: dynamic HTTP challenge routers, TCP routers, and backend load-balancer services.

## Data flow

1. A client opens a TLS connection to the Docker host on `443/tcp`.
2. Docker forwards the connection to Traefik on container port `8443/tcp`.
3. Traefik matches the TCP router using `HostSNI(*)`.
4. Traefik forwards the unchanged TCP stream to one configured backend IP address and port.
5. The selected backend terminates TLS and presents its own SSL certificate.

For HTTP-01 certificate validation:

1. Let's Encrypt opens an HTTP connection to the Docker host on `80/tcp`.
2. Docker forwards the request to Traefik on container port `8080/tcp`.
3. Traefik matches the HTTP `Host(...)` and `PathPrefix('/.well-known/acme-challenge/')` router.
4. Traefik forwards the request to the configured local challenge responder.

## Persistence

Traefik configuration is stored in the host `./config` directory and mounted read-only at `/etc/traefik`.

No certificates, private keys, application data, or Docker socket are mounted into the Traefik container.

## Deployment topology

Traefik runs as one Docker container on the load-balancer host. Backends are external to this Compose stack and must be reachable from the container through their configured IP addresses and ports. The Compose service defines `host.docker.internal` through Docker's `host-gateway` feature for backends that listen on the Docker host itself.

See `docs/adr/0001-traefik-tcp-tls-passthrough.md` for the design decision.