docker-compose not adding hostname to /etc/hosts

Question

I'm working on a docker-compose config which should spin up an openvpn container attached to a dnsmasq container. The openvpn server should automatically discover the dnsmasq container and use it as the dns server. Discovery is done by searching for an entry "dnsmasq" in the /etc/hosts file.

I have the following docker-compose.yml file:

version: '2'
services:
  data:
    build: ./
  dnsmasq:
    cap_add:
      - NET_ADMIN
    hostname: dnsmasq
    image: <dnsmasq image>
    ports:
      - 53:53/udp
    restart: always
  openvpn:
    cap_add:
      - NET_ADMIN
    depends_on:
      - data
      - dnsmasq
    image: <openvpn image>
    ports:
      - 1194:1194/udp
    restart: always
    volumes_from:
      - data

I've specified "dnsmasq" as hostname for the dnsmasq container and expecting it to appear in "/etc/hosts" in the openvpn container. This doesn't seem to happen.

This is a dump from /etc/hosts from within the openvpn container:

openvpn_1  | 127.0.0.1  localhost
openvpn_1  | ::1    localhost ip6-localhost ip6-loopback
openvpn_1  | fe00::0    ip6-localnet
openvpn_1  | ff00::0    ip6-mcastprefix
openvpn_1  | ff02::1    ip6-allnodes
openvpn_1  | ff02::2    ip6-allrouters
openvpn_1  | 172.19.0.4 d44a72f42ef9

I expect d44a72f42ef9 to be "dnsmasq".

What am I doing wrong?

I'm running docker-compose 1.8.1.

Answer 1

Older versions of linking in docker worked by adding to /etc/hosts. It's probable your system was setup initially under this paradigm where the way to retrieve the container resolution was built into /etc/hosts.

Newer versions of docker however do not require this. They automatically allow service name resolution, as long as a container is on the same network. So you can access it via:

http://dnsmasq:port

Answer 2

First you need to add a "links" key to your docker-compose.yml like so:

services:
  dnsmasq:
    ...
  openvpn:
    ...
    links:
      - dnsmasq

The DNS resolution for for this hostname is not handled through /etc/hosts but rather the Docker Embedded DNS . You can query it using normal DNS tools like so:

$ getent hosts dnsmasq
$ nslookup dnsmasq
$ dig dnsmasq A
$ # etc...

Answer 3

Apparently, the hosts files doesnt get update. The host "dnsmasq" is just available using some other different mechanism. This can be verified using:

ping -c1 dnsmasq