networking - Docker-compose container using host DNS server

Question

I'm running several containers on my "Ubuntu 16.10 Server" in a "custom" bridge network with compose 2.9 (in a yml version 2.1). Most of my containers are internally using the same ports, so there is no way for me to use the "host" network driver. My containers are all links together, using the dedicated links attribute.

But, I also need to access services exposed outside of my containers. These services have dedicated URL with names registered in my company's DNS server. While I have no problem to use public DNS and reach any public service from within my containers, I just can't reach my private DNS.

Do you know a working solution to use private DNS from a container? Or even better, use host's network DNS configuration?

PS: Of course, I can link to my company's services using the extra_hosts attribute in my services in my docker-compose.yml file. But... that's definitively not the goal of having a DNS. I don't want to register all my services in my YML file, and I don't want to update it each time services' IP are updated in my company.

Context :

• Host: Ubuntu 16.10 server
• Docker Engine: 1.12.6
• Docker Compose: 1.9.0
• docker-compose.yml: 2.1
• Network: Own bridge.

docker-compose.yml file (extract):

version: '2.1'
  services:
    nexus:
    image: sonatype/nexus3:$NEXUS_VERSION
    container_name: nexus
    restart: always
    hostname: nexus.$URL
    ports:
      - "$NEXUS_81:8081"
      - "$NEXUS_443:8443"
    extra_hosts:
      - "repos.private.network:192.168.200.200"
    dns:
      - 192.168.3.7
      - 192.168.111.1
      - 192.168.10.5
      - 192.168.10.15
    volumes_from:
      - nexus-data
    networks:
      - pic

  networks:
    pic:
      driver: bridge
      ipam:
        driver: default
        config:
          - subnet: 172.18.0.0/16
            gateway: 172.18.0.1

I tried with and without the ipam configuration for the pic network, without any luck.

Tests & Results: docker exec -ti nexus curl repos.private.network returns properly the HTML page served by this service

docker exec -ti nexus curl another-service.private.network Returns curl: (6) Could not resolve host: another-service.private.network; Name or service not known While curl another-service.private.network from the host returns the appropriate HTML page.

And "of course" another-service.private.network is known in my 4 DNS servers (192.168.3.7, 192.168.111.1, 192.168.10.5, 192.168.10.15).

Answer 1

You don't specify which environment you're running docker-compose in e.g Mac, Windows or Unix, so it will depend a little bit on what changes are needed. You also don't specify if you're using the default bridge network in docker on a user created bridge network.

In either case, by default, Docker should try and map DNS resolution from the Docker Host into your containers. So if your Docker Host can resolve the private DNS addresses, then in theory your containers should be able to as well.

I'd recommend reading this official Docker DNS documentation as it is pretty reasonable. Here for the default Docker bridge network, here for user created bridge networks.

A slight gotcha is if you're running using Docker for Mac, Docker Machine or Docker for Windows you need to remember that your Docker Host is actually the VM running on your machine and not the physical box itself, so you need to ensure that the VM has the correct DNS resolution options set. You will need to restart your containers for changes to DNS resolution to be picked up by them.

You can of course override all the default settings using docker-compose. It has full options for explicitly setting DNS servers, DNS search options etc. As an example:

version: 2
services:
 application:
  dns:
   - 8.8.8.8
   - 4.4.4.4
   - 192.168.9.45

You'll find the documentation for those features here.