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Exposing a Port

Run a simple, static python server that:

  • runs its service on port 8000
  • this port should be mapped on 9090 of the docker host
$ docker run -it -v $(pwd)/message:/message.html -p 9090:8000 python:3-alpine python -m http.server
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...
  • -p configuration flag for mapping a port: :

Access it from Host System:

$ curl localhost:9090/message.html

Note:

  • Containers cannot have public IPv4 addresses by default
  • they only have private addresses
  • services have to be exposed port by port
  • port have to be mapped to avoid conflicts

Port mapping allows us to run services that run on the same port:

$ docker run -d -v $(pwd)/message:/message.html -p 9091:8000 python:3-alpine python -m http.server
$ docker run -d -v $(pwd)/message:/message.html -p 9092:8000 python:3-alpine python -m http.server
$ docker run -d -v $(pwd)/message:/message.html -p 9093:8000 python:3-alpine python -m http.server

To know which ports are mapped you can use docker port:

$ docker port <container-id>
8000/tcp -> 0.0.0.0:9093

Entering a container at runtime

$ docker exec -it

Communication between containers

Create a network:

$ docker network create testnetwork

Start a container within this network, exposing a server:

$ docker run --name server --net testnetwork -d -v $(pwd)/message:/message.html python:3-alpine python -m http.server

Ping that Server from within another container:

$ docker run --name pinger --net testnetwork -it nicolaka/netshoot ping server

Congratulations!

Docker inspect:

docker inspectis an advanced command, that can retrieve a ton of information about our containers

$ docker inspect server

# Get the IP Address of a Container
$ docker inspect server --format='{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}'

More examples: https://docs.docker.com/engine/reference/commandline/inspect/

Bonus:

Starting a container in a containers network namespace

$ docker run -it --net container:<container-id> nicolaka/netshoot curl http://server:8000/message.html
$ docker run -it --net container:ac1120c15b8e nicolaka/netshoot netstat -tulpn