Host.docker.internal resolves to nothing in linux

Could you post your config.<env>.exs that containing the database connection configuration?

I am using

HOST=host.docker.internal
PORT=5432
DATABASE_URL=ecto://postgres:postgres@host.docker.internal/demo_dev

in .env file

and in docker-compose.yml file calling the .env variable as

version: '3.2'

networks:
  net:

services:
  api:
    image: .............
    working_dir: ............
    networks:
      - net
    environment:
          - DATABASE_URL=${DATABASE_URL}
          - HOST=${HOST}
          - PORT=${PORT}

Seems the host.docker.internal hostname is a Mac Docker Desktop exclusive feature. Are you using Docker Desktop on Mac?

No.. I am using Docker setup on Ubuntu.. Are there any alternative for Ubuntu?

You can use any of the following approaches:

Using host network drive

In the docker-compose.yml.

...
services:
  api:
    network_mode: host
    ...

In the .env file,

DATABASE_URL=ecto://postgres:postgres@localhost/demo_dev

This approach makes your docker container directly use the network adapters on the host without creating subnet or NAT.

The network throughput of this approach is about 20% higher than the NAT approach, but it prevents the container to join any docker networks, and can expose ports that you don’t want to expose (for example, the epmd port 4369).

Add extra hosts to the container

In the docker-compose.yml,

...
services:
  api:
    extra_hosts:
      - "host.docker.internal:<the IP address of the host>"

This approach is less performant and less flexible, but is more secure.

Thank you so much.