If you are running a postgreSQL database on your host machine and want to connect it to a Docker container running on the same host machine, there are a few steps you need to follow. In this article, we will go through the process of connecting a Docker container running on a host machine to a database running on the same host machine.

Prerequisite

Before we start, make sure you have the following:

• Docker installed on your host machine

• A database running on your host machine

• Basic knowledge of Docker and databases

I write the article using the following.

• OS: Debian 11 (bullseye)

• PostgreSQL: 15 (Hosted on Debian)

• Docker: Server Application (which connects to PostgreSQL)

• I am using docker-compose.yml to build application.

STEP 1:

Please add host.docker.internal:

version: '3'
services:
  bank-server:
    ...
    depends_on:
      ....
    restart: on-failure
    ports:
      - 9090:9090
    extra_hosts:
      - "host.docker.internal:<docker0 IP>"

To find IP of docker i.e. you can use:
$> ifconfig docker0

docker0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 172.17.0.1  netmask 255.255.0.0  broadcast 172.17.255.255

OR

$> ip a

1: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default
    inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
       valid_lft forever preferred_lft forever

STEP 2:

Update the listen_addresses setting in postgresql.conf.

This will allow PostgreSQL to listen on all available network interfaces.

In postgresql.conf, change listen_addresses to listen_addresses = '*'. Use the command bellow to edit postgres conf

sudo nano /etc/postgresql/<your_postgres_version>/main/postgresql.conf

Step 3:

Add a new entry to pg_hba.conf

In the pg_hba.conf file, add a new entry that allows connections from all IP addresses.

host    all             all             0.0.0.0/0               md5

Step 4:

Restart PostgreSQL

After making the changes, restart the PostgreSQL service using the following command:

sudo service postgresql restart

STEP 5:

Use host.docker.internal hostname to connect database from Server Application.

Ex: jdbc:postgresql://host.docker.internal:5432/bankDB

If you are encountering issues with the host.docker.internal hostname not being recognized on Ubuntu, you can try replacing it with the local Docker IP address, which is typically 172.17.0.1. This should allow your container to connect to the database running on the host machine.

It's always a good idea to restrict access to your database to only the necessary IP addresses for security purposes. Using the Docker bridge IP as the allowed IP address can help ensure that only the container connected to the same network can access the database.

you can use the docker bridge IP (use ip -h -c a to find that IP)

There may be other solutions to this problem, but this solution has been tested and verified to work with The technologies and their respective versions I highlighted above.

Thanks 😀.