Traefik Catch-All to return HTTP421 for unresolved routes

app.go

// inspired by https://github.com/mergermarket/404
package main

import "net/http"

func healthcheck_handler(res http.ResponseWriter, req *http.Request) {
    res.WriteHeader(http.StatusOK)
}

func default_handler(res http.ResponseWriter, req *http.Request) {
    res.WriteHeader(http.StatusMisdirectedRequest)
}

func main() {
    http.HandleFunc("/internal/healthcheck", healthcheck_handler)
    http.HandleFunc("/", default_handler)
    http.ListenAndServe(":80", nil)
}

docker-compose.yml

version: "2.1"

services:
  catchall:
    restart: always
    build:
      context: .
    networks:
      - web
    labels:
      - "traefik.enable=true"
      - "traefik.docker.network=web"
      - "traefik.http.routers.catchall_router.entrypoints=web"
      - "traefik.http.routers.catchall_router.rule=HostRegexp:(`{catchall:.*}`)"       # <<---
      - "traefik.http.routers.catchall_router.priority=1"                              # <<---
      - "traefik.http.routers.catchall_router.service=catchall_service@docker"
      - "traefik.http.routers.catchall_router_ssl.entrypoints=web-secure"
      - "traefik.http.routers.catchall_router_ssl.rule=HostRegexp:(`{catchall:.*}`)"   # <<---
      - "traefik.http.routers.catchall_router_ssl.priority=1"                          # <<---
      - "traefik.http.routers.catchall_router_ssl.tls=true"
      - "traefik.http.routers.catchall_router_ssl.service=catchall_service@docker"
      - "traefik.http.services.catchall_service.loadbalancer.server.port=80"

networks:
  web:
    external: true

Dockerfile

FROM golang:1.14-alpine
COPY . /go/src/app
WORKDIR /go/src/app
RUN go get -v -d
RUN go install
CMD ["app"]