hostname attribute in docker-compose

Question

In the below docker-compose file,

dbc:
  image: mysql:5.6
  hostname: db
  expose:
    - "3386"
  environment:
    MYSQL_DATABASE: somebackenddb
    MYSQL_USER: user1
    MYSQL_PASSWORD: pswd
    MYSQL_ROOT_PASSWORD: pswd

agent:
  image: somedockerhub/ansible
  volumes:
    - ../../whatever/x.yml:/whatever/y.yml
  links:
    - dbc
  environment:
    PROBE_HOST: "db"
    PROBE_PORT: "3306"

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we are using hostname attribute in dbc service.

agent service is linked to that hostname whose value is db

Generally, hostnames are given to virtual machines.

---

Every container is given an IP address, but,

For a container launched by dbc service, what does hostname mean for a container?

Answer 1

By default hostnames of a docker container is equal to their container id.

Defining attribute hostname: db in docker-compose file will set hostname of dbc service container to be db .

If you enter the created container with docker exec -it [container_id] bash

and run command hostname you should see db .

Running hostname -i would show virtual ip address.

cat /etc/hosts would show: ... 172.17.0.4 db

Env. variable HOSTNAME insider container would equal db