Make a global variable od "extra_hosts" in docker-compose

Question

Lets suppose that I have got a ten services in docker-compose and every this service need the same extra_hosts with a four records. I would like to define extra_hosts once and only include it to every service.

Is it possible?

version: '3.7'
services:

  web:
    build:
      context: ./apache
      dockerfile: dockerfile_apache2
    image: debian:latest
    container_name: hsthttp1
    extra_hosts:
     - "somehost1:162.242.195.82"
     - "somehost2:162.242.195.83"
     - "somehost3:162.242.195.84"
     - "somehost4:162.242.195.85"
  web2:
    build:
      context: ./apache
      dockerfile: dockerfile_apache2
    image: debian:latest
    container_name: hsthttp2
    extra_hosts:
     - "somehost1:162.242.195.82"
     - "somehost2:162.242.195.83"
     - "somehost3:162.242.195.84"
     - "somehost4:162.242.195.85"
  web3:
    build:
      context: ./apache
      dockerfile: dockerfile_apache2
    image: debian:latest
    container_name: hsthttp3
    extra_hosts:
     - "somehost1:162.242.195.82"
     - "somehost2:162.242.195.83"
     - "somehost3:162.242.195.84"
     - "somehost4:162.242.195.85"

Are you perhaps looking like a DNS service (BIND, Consul, AWS Route 53, ...)? That would let you define these host names centrally in one place and not manually copy them around everywhere. — David Maze
– David Maze, Commented Jul 7, 2019 at 17:15

atline · Accepted Answer · 2019-07-10 06:36:47Z

Yes, it's possible to use Extension fields to define reusable fragments since compose version 3.4:

For your situation, you can use next:

docker-compose.yaml:

version: '3.7'

x-extra_hosts:
  &default-extra_hosts
  - "somehost1:162.242.195.82"
  - "somehost2:162.242.195.83"
  - "somehost3:162.242.195.84"
  - "somehost4:162.242.195.85"

services:
  web:
    image: debian:latest
    container_name: hsthttp1
    extra_hosts: *default-extra_hosts
  web2:
    image: debian:latest
    container_name: hsthttp2
    extra_hosts: *default-extra_hosts
  web3:
    image: debian:latest
    container_name: hsthttp3
    extra_hosts: *default-extra_hosts

Above, we define a global &default-extra_hosts which later in every service we can reference it with *default-extra_hosts.

You can use docker-compose config to quick check the effect as next:

shubuntu1@shubuntu1:~/try$ docker-compose config
services:
  web:
    container_name: hsthttp1
    extra_hosts:
    - somehost1:162.242.195.82
    - somehost2:162.242.195.83
    - somehost3:162.242.195.84
    - somehost4:162.242.195.85
    image: debian:latest
  web2:
    container_name: hsthttp2
    extra_hosts:
    - somehost1:162.242.195.82
    - somehost2:162.242.195.83
    - somehost3:162.242.195.84
    - somehost4:162.242.195.85
    image: debian:latest
  web3:
    container_name: hsthttp3
    extra_hosts:
    - somehost1:162.242.195.82
    - somehost2:162.242.195.83
    - somehost3:162.242.195.84
    - somehost4:162.242.195.85
    image: debian:latest
version: '3.7'

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