Traefik and my web app inside a container - docker

Question

I am trying to do something I thought was simple, but looks like I’m missing a thing.
I have a web app, which is package in a docker image I manage. It starts a server listening on port 9000. It does have an endpoint publishing metrics, on /admin/metrics.
The application is deployed on a system that requires me to publish those metrics on port 9100, with the path /metrics. I could change the application, run a second server, etc, but for fun I tried something quicker (I thought): running a companion reverse proxy.
I chose traefik, and I managed to configure it properly using a file provider: when running on my machine (no container), it does redirect properly calls from /metrics on port 9100 to my app’s /admin/metrics. But when inside the container, it only gives 404 errors, although the configuration is ok. I also tried to run the app only and have traefik on my machine route to the app inside the container, but it fails too.
This is my configuration:
#/app/traefik.toml
[entryPoints]
[entryPoints.MetricsProxy]
address = ":9100"
[providers]
providersThrottleDuration = 42
[providers.file]
directory = "/app"
watch = false
[api]
insecure = false
dashboard = false
debug = false
[log]
level = "TRACE"
#/app/metrics.toml
[http]
[http.routers]
[http.routers.Router0]
entryPoints = ["MetricsProxy"]
middlewares = ["PathConvert"]
service = "MetricsService"
rule = "Path(`/metrics`)"
[http.services]
[http.services.MetricsService]
[http.services.MetricsService.loadbalancer]
[[http.services.MetricsService.loadBalancer.servers]]
url = "http://0.0.0.0:9000"
[http.middlewares]
[http.middlewares.PathConvert]
[http.middlewares.PathConvert.addPrefix]
prefix = "/admin"
Please note that I tried to replace 0.0.0.0 with 127.0.0.1 or localhost, neither works.
Finally, the Dockerfile:
FROM openjdk:8-jre-slim
WORKDIR /app
RUN \
apt-get update -qq && apt-get install -y -qq curl && \
curl -sSL https://github.com/containous/traefik/releases/download/v2.0.4/traefik_v2.0.4_linux_amd64.tar.gz | tar -xz
COPY bin/myapp.sh .
COPY target/universal/bluevalet-server.zip .
COPY deploy/traefik/traefik.toml .
COPY deploy/traefik/metrics.toml .
COPY deploy/nginx.conf .
COPY deploy/run.sh .
#run.sh ~~> ./traefik --configfile /app/traefik.toml & ./myapp.sh
CMD [ "/app/run.sh" ]
EXPOSE 9000
EXPOSE 9100
I guess there is something with "localhost" in the service definition, but cannot understand what.
Anyone has an idea?

Answer 1

Not sure why it does work this way, but I succeeded using another configuration for traefik:
[http]
[http.routers]
[http.routers.Router0]
entryPoints = ["MetricsProxy"]
middlewares = ["PathConvert"]
service = "MetricsService"
rule = "Path(`/metrics`)"
[http.services]
[http.services.MetricsService]
[http.services.MetricsService.loadbalancer]
[[http.services.MetricsService.loadBalancer.servers]]
url = "http://localhost:9000/"
[http.middlewares]
[http.middlewares.PathConvert]
[http.middlewares.PathConvert.replacePathRegex]
regex = "^/metrics"
replacement = "/admin/metrics/prometheus"