I try to set up Traefik in version 2 but I only get "404 Page not found" or DNS_PROBE_FINISHED_NXDOMAIN errors in my browser. 
When I check the API endpoints for routers I can see that my two containers are enabled in Traefik and that the rules are correct.
curl http://localhost:8080/api/http/routers
[{"entryPoints":["web","secure"],"service":"gotify-gotify","rule":"Host(`sub2.example.org`)","tls":{"certResolver":"letsencrypt"},"status":"enabled","using":["secure","web"],"name":"gotify@docker","provider":"docker"},{"entryPoints":["web","secure"],"service":"nextcloud-cloud","rule":"Host(`sub.example.org`)","tls":{"certResolver":"letsencrypt"},"status":"enabled","using":["secure","web"],"name":"nextcloud@docker","provider":"docker"}]
But on "sub2" I get no website at all and on "sub" I get "404 page not found". I have set a DNS entry for "*" so all subdomains go to the same server.
I have set the following labels for the docker containers
labels:
traefik.enable: true
traefik.http.routers.nextcloud.rule: "Host(`sub.example.org`)"
traefik.http.routers.nextcloud.entrypoints: "web, secure"
traefik.http.routers.nextcloud.tls.certresolver: "letsencrypt"
And this is my Traefik configuration traefik.toml
[entryPoints]
  [entryPoints.web]
    address = ":80"
  [entryPoints.secure]
    address = ":443"
[providers.docker]
  exposedByDefault = false
  network = "traefik"
[certificatesResolvers.letsencrypt.acme]
  email = "[email protected]"
  storage = "acme.json"
  [certificatesResolvers.letsencrypt.acme.httpChallenge]
    entryPoint = "web"
[api]
  insecure = true
  debug = true
  dashboard = false
Traefik itself is running as a docker container.
version: "3.7"
services:
  traefik:
    image: traefik:v2.0
    container_name: traefik
    restart: unless-stopped
    volumes:
    - "./traefik.toml:/etc/traefik/traefik.toml"
    - "./acme:/etc/traefik/acme"
    - "/var/run/docker.sock:/var/run/docker.sock"
    ports:
    - "80:80"
    - "127.0.0.1:8080:8080"
    - "443:443"
    networks:
    - traefik
networks:
  traefik:
    driver: bridge
    name: traefik
I use ufw to manage firewall rules and opened port 22, 80 and 443
Status: active
To                         Action      From
--                         ------      ----
22/tcp                     ALLOW       Anywhere
80/tcp                     ALLOW       Anywhere
443/tcp                    ALLOW       Anywhere
22/tcp (v6)                ALLOW       Anywhere (v6)
80/tcp (v6)                ALLOW       Anywhere (v6)
443/tcp (v6)               ALLOW       Anywhere (v6)
Go back to your browser ( http://localhost:8080/api/rawdata ) and see that Traefik has automatically detected the new instance of the container. The output will show alternatively one of the followings: Hostname: a656c8ddca6c IP: 172.27.
Traefik is an open-source Edge Router that makes publishing your services a fun and easy experience. It receives requests on behalf of your system and finds out which components are responsible for handling them.
Airbnb, Uber Technologies, and Spotify are some of the popular companies that use nginx, whereas Traefik is used by Docplanner, Viadeo, and Condé Nast.
You can find a working example for traefik 2.2.1. Also, you can check full setup gist: https://gist.github.com/fatihyildizhan/8f124039a9bd3801f0caf3c01c3601fb
I prefer to use traefik.yml with version 2.0. It looks simple and many people are familiar with the YAML files.
[Traefik v2.0] - docker-compose.yml  with httpChallenge
    version: '3.7'
    services:
      traefik:
        image: traefik:v2.2.1
        container_name: traefik
        restart: unless-stopped
        security_opt:
          - no-new-privileges:true
        networks:
          - proxy
        ports:
          - 80:80
          - 443:443
        volumes:
          - /etc/localtime:/etc/localtime:ro
          - /var/run/docker.sock:/var/run/docker.sock:ro
          - ./traefik.yml:/traefik.yml:ro
          - ./acme.json:/acme.json
        labels:
          - "traefik.enable=true"
          - "traefik.http.routers.traefik.entrypoints=http"
          - "traefik.http.routers.traefik.rule=Host(`traefik.your-domain.com`)"
          - "traefik.http.middlewares.traefik-auth.basicauth.users=username:hashed-password"
          - "traefik.http.middlewares.traefik-https-redirect.redirectscheme.scheme=https"
          - "traefik.http.routers.traefik.middlewares=traefik-https-redirect"
          - "traefik.http.routers.traefik-secure.entrypoints=https"
          - "traefik.http.routers.traefik-secure.rule=Host(`traefik.your-domain.com`)"
          - "traefik.http.routers.traefik-secure.middlewares=traefik-auth"
          - "traefik.http.routers.traefik-secure.tls=true"
          - "traefik.http.routers.traefik-secure.tls.certresolver=http"
          - "traefik.http.routers.traefik-secure.service=api@internal"
          - "traefik.http.services.traefik.loadbalancer.server.port=8080"
    networks:
      proxy:
        external: true
    [Traefik v2.0] - traefik.yml with httpChallenge
    api:
      dashboard: true
    # Writing Logs to a File, in JSON
    log:
      level: DEBUG
      filePath: "log-file.log"
      format: json
    # Configuring a buffer of 100 lines
    accessLog:
      filePath: "log-access.log"
      bufferingSize: 100  
    entryPoints:
      http:
        address: ":80"
      https:
        address: ":443"
    providers:
      docker:
        endpoint: "unix:///var/run/docker.sock"
        exposedByDefault: false
    certificatesResolvers:
      http:
        acme:
          email: your-email.com
          storage: acme.json
          httpChallenge:
            entryPoint: http    
    [Traefik v2.0] - your-container docker-compose.yml
    version: '3.7'
    services:
        your-container-name:
          image: docker.pkg.github.com/username/repo-name/image-name:latest
          container_name: your-container-name
          restart: unless-stopped
          security_opt:
            - no-new-privileges:true
          networks:
            - proxy
          volumes:
            - /etc/localtime:/etc/localtime:ro
            - /var/run/docker.sock:/var/run/docker.sock:ro
            - ./data:/data
          labels:
            - "traefik.enable=true"
            - "traefik.http.routers.your-container-name.entrypoints=http"
            - "traefik.http.routers.your-container-name.rule=Host(`your-container-name.your-domain.com`)"
            - "traefik.http.middlewares.your-container-name-https-redirect.redirectscheme.scheme=https"
            - "traefik.http.routers.your-container-name.middlewares=your-container-name-https-redirect"
            - "traefik.http.routers.your-container-name-secure.entrypoints=https"
            - "traefik.http.routers.your-container-name-secure.rule=Host(`your-container-name.your-domain.com`)"
            - "traefik.http.routers.your-container-name-secure.tls=true"
            - "traefik.http.routers.your-container-name-secure.tls.certresolver=http"
            - "traefik.http.routers.your-container-name-secure.service=your-container-name"
            - "traefik.http.services.your-container-name.loadbalancer.server.port=80"
            - "traefik.docker.network=proxy"
    networks:
      proxy:
        external: true
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