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Caddy Multihost Agent

Extend caddy-docker-proxy to support containers across multiple hosts.

Why This Exists

caddy-docker-proxy is excellent for automatic reverse proxy configuration via Docker labels on a single host. However, it doesn't support multi-host setups - you can't route traffic to containers running on other machines.

caddy-multihost-agent solves this by:

  • Running agents on remote hosts that watch local Docker containers
  • Pushing route configurations to a central Caddy server via the Admin API
  • Using the same Docker label syntax as caddy-docker-proxy

This gives you caddy-docker-proxy's simplicity across your entire infrastructure.

Quick Start

Docker Image

docker pull mcsdodo/caddy-agent:latest

Single Host Setup

For a single host, just use caddy-docker-proxy directly. This agent is only needed for multi-host setups.

Multi-Host Setup

Host 1 (Server) - runs Caddy with caddy-docker-proxy:

# docker-compose.yml on host1
services:
  caddy:
    image: lucaslorentz/caddy-docker-proxy:latest
    container_name: caddy-server
    network_mode: host
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - caddy_data:/data
    labels:
      # Expose Admin API for remote agents
      caddy_0: ":2020"
      caddy_0.reverse_proxy: "localhost:2019"

  # Optional: agent for testing server mode
  caddy-agent:
    image: mcsdodo/caddy-agent:latest
    container_name: caddy-agent-server
    network_mode: host
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
    environment:
      - CADDY_URL=http://localhost:2019
      - AGENT_ID=host1-server
      - DOCKER_LABEL_PREFIX=agent  # Use different prefix to avoid conflict
      - SNIPPET_API_PORT=8567      # Serve snippets to remote agents

volumes:
  caddy_data:

Host 2 (Remote Agent) - watches containers and pushes routes to host1:

# docker-compose.yml on host2
services:
  caddy-agent:
    image: mcsdodo/caddy-agent:latest
    container_name: caddy-agent-remote
    network_mode: host
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
    environment:
      - CADDY_URL=http://192.168.1.10:2020   # Point to host1
      - AGENT_ID=host2-remote
      - DOCKER_LABEL_PREFIX=caddy
      - SNIPPET_SOURCES=http://192.168.1.10:8567  # Fetch snippets from host1

  # Example app - routes automatically configured
  myapp:
    image: nginx
    network_mode: host
    labels:
      caddy: myapp.example.com
      caddy.reverse_proxy: "{{upstreams 80}}"

Test it:

# From host1
curl -sk --resolve myapp.example.com:443:127.0.0.1 https://myapp.example.com

Architecture

host1 (192.168.1.10) - SERVER
├── Caddy (caddy-docker-proxy)
│   ├── Port 80/443 (routes)
│   └── Port 2020 (Admin API proxy → 2019)
└── Local containers (via caddy.* labels)
        ▲
        │ HTTP POST /load
    ┌───┴───┐
host2       host3
Agent       Agent
└── Local   └── Local
    containers  containers

Each agent:

  1. Watches its local Docker daemon for container events
  2. Extracts route config from caddy.* labels
  3. Pushes routes to the central Caddy server via Admin API
  4. Uses unique AGENT_ID prefixes to prevent route conflicts

Configuration

Environment Variables

Variable Default Description
CADDY_URL http://localhost:2019 Caddy Admin API URL
AGENT_ID hostname Unique identifier (prefixes route IDs)
DOCKER_LABEL_PREFIX caddy Label prefix to watch
DOCKER_PROXY_URL empty Docker API proxy URL (overrides DOCKER_HOST)
HOST_IP auto-detected IP for upstream addresses
SNIPPET_API_PORT 0 (disabled) Port to serve snippet API
SNIPPET_SOURCES empty Comma-separated URLs to fetch snippets
SNIPPET_CACHE_TTL 300 Snippet cache duration in seconds
HEALTH_CHECK_INTERVAL 5 Seconds between health checks
RESYNC_INTERVAL 300 Seconds between full resyncs
CONFIG_PUSH_ENABLED true Set to false for snippet-only mode
LOG_LEVEL INFO Logging verbosity: DEBUG, INFO, WARNING, ERROR

Docker Labels

Standard caddy-docker-proxy labels work:

labels:
  # Basic reverse proxy
  caddy: example.com
  caddy.reverse_proxy: "{{upstreams 8080}}"

  # Multiple domains
  caddy: app1.example.com, app2.example.com
  caddy.reverse_proxy: "{{upstreams 8080}}"

  # HTTP-only (no TLS)
  caddy: http://internal.lan
  caddy.reverse_proxy: "{{upstreams 8080}}"

  # With snippet import
  caddy: secure.example.com
  caddy.import: internal
  caddy.reverse_proxy: "{{upstreams 8080}}"

  # Numbered labels for complex configs
  caddy_0: api.example.com
  caddy_0.reverse_proxy: "{{upstreams 8080}}"
  caddy_0.reverse_proxy.header_up: "X-Real-IP {remote_host}"

  # Port-based server (creates separate listener)
  caddy: ":3000"
  caddy.reverse_proxy: "{{upstreams 8080}}"

Layer4 (TCP/UDP) Routing

Route raw TCP/UDP traffic (not HTTP) through Caddy. Requires Caddy built with caddy-l4 module.

labels:
  # Simple TCP proxy (e.g., MQTT broker)
  caddy.layer4: ":1883"
  caddy.layer4.route.proxy: "{{upstreams 11883}}"

  # With SNI matching (TLS passthrough)
  caddy.layer4: ":443"
  caddy.layer4.@sni: "tls sni *.example.com"
  caddy.layer4.route.proxy: "192.168.1.50:443"

  # Multiple L4 routes (numbered)
  caddy.layer4_0: ":1883"
  caddy.layer4_0.route.proxy: "{{upstreams 1883}}"
  caddy.layer4_1: ":8883"
  caddy.layer4_1.route.proxy: "{{upstreams 8883}}"

Note: If Caddy doesn't have the Layer4 module, the agent logs a warning and skips L4 routes (HTTP routes still work).

Snippet Sharing

Define snippets on host1, use them on all hosts:

Host1 (serves snippets):

labels:
  caddy_1: (internal)
  caddy_1.tls: internal

  caddy_2: (https)
  caddy_2.reverse_proxy.transport: http
  caddy_2.reverse_proxy.transport.tls: ""
  caddy_2.reverse_proxy.transport.tls_insecure_skip_verify: ""

Remote agents fetch automatically when SNIPPET_SOURCES is configured.

Snippet compatibility with reverse_proxy:

Snippet Safe to import? Reason
internal Yes Only sets TLS cert type
https Yes Forces TLS to backend (for self-signed certs)
wildcard No Contains handle.abort - terminates request

Note: Wildcard domains (e.g., *.example.com) should import wildcard to set up TLS certs. Individual services should NOT import wildcard - just declare their domain and the cert is used automatically.

Snippet-only mode: To run an agent that only serves snippets without pushing any routes to Caddy, set CONFIG_PUSH_ENABLED=false:

caddy-agent:
  environment:
    - SNIPPET_API_PORT=8567
    - CONFIG_PUSH_ENABLED=false  # Disable route pushing, only serve snippets

Route Recovery

When Caddy restarts, routes are lost. Agents automatically recover:

  1. Health check (every 5s) - detects missing routes quickly
  2. Periodic resync (every 5 min) - fallback for network issues
  3. Exponential backoff - reduces load when Caddy is unreachable

Troubleshooting

Routes not appearing

# Check agent logs
docker logs caddy-agent-remote

# Check Caddy API is accessible from agent host
curl http://192.168.1.10:2020/config/

# Check container labels
docker inspect myapp | jq '.[0].Config.Labels'

Routes disappear after restart

Routes are pushed to Caddy's in-memory config. After Caddy restarts, agents resync within ~15 seconds (10s startup delay + 5s health check).

Connection refused to Admin API

Make sure port 2020 is exposed and accessible:

# Test from agent host
curl http://192.168.1.10:2020/config/

Duplicate AGENT_ID conflicts

Each agent must have a unique AGENT_ID. Routes are prefixed with this ID to prevent conflicts:

host1-server_myapp
host2-remote_webapp

Complete Example

Three-host setup with Cloudflare DNS:

Host1 (Server):

services:
  caddy:
    # Option A: Use caddy-docker-proxy (recommended for single-host)
    image: lucaslorentz/caddy-docker-proxy:latest
    # Option B: Use regular Caddy + caddy-agent (full control)
    # image: caddy:latest  # Requires caddy-agent with CONFIG_PUSH_ENABLED=true
    network_mode: host
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - caddy_data:/data
    labels:
      # Global config
      caddy_0.email: admin@example.com
      caddy_0.auto_https: prefer_wildcard

      # Cloudflare DNS for wildcard certs
      caddy_1: (wildcard)
      caddy_1.tls.dns: "cloudflare {env.CF_API_TOKEN}"
      caddy_1.handle.abort: ""

      caddy_10: "*.example.com"
      caddy_10.import: wildcard

      # Admin API proxy for agents
      caddy_20: ":2020"
      caddy_20.reverse_proxy: "localhost:2019"

  caddy-agent:
    image: mcsdodo/caddy-agent:latest
    network_mode: host
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
    environment:
      - CADDY_URL=http://localhost:2019
      - AGENT_ID=host1-server
      - SNIPPET_API_PORT=8567
      - CONFIG_PUSH_ENABLED=false  # Snippet-only mode

volumes:
  caddy_data:

Host2/Host3 (Agents):

services:
  caddy-agent:
    image: mcsdodo/caddy-agent:latest
    network_mode: host
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
    environment:
      - CADDY_URL=http://192.168.1.10:2020
      - AGENT_ID=host2-remote  # Change for each host
      - SNIPPET_SOURCES=http://192.168.1.10:8567

  myapp:
    image: myapp:latest
    network_mode: host
    labels:
      caddy: myapp.example.com
      caddy.reverse_proxy: "{{upstreams 8080}}"

FAQ

Q: Can I use this without caddy-docker-proxy? A: Yes, but you'll need a Caddyfile that enables the Admin API and defines your base config.

Q: What happens if two containers have the same domain? A: The agent overwrites with the latest one. Use unique domains.

Q: Does it work with Docker Swarm or Kubernetes? A: Not directly. For orchestrated environments, use native ingress controllers.

Q: How do I secure the Admin API? A: Use firewall rules to restrict access to port 2020 to agent IPs only.

License

MIT

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