How to connect to a host Postgres database from a Docker container?

Published 2019-10-05

Let’s assume we have a web service running inside a Docker container. We also have a Postgres database running on a host system (i.e., outside of Docker as you probably don’t want to run a database in Docker in production). Assuming you don’t want the container to share the network with the host (using --network host parameter when running it), the database and the container need to be configured in order to communicate.

Postgres configuration changes

The database needs configuration changes to allow connections from Docker containers. First, find Postgres configuration files. They’re usually available in /etc/postgresql/<postgres version>/main.

File postgresql.conf

Change the following line:

listen_addresses = 'localhost'

to

listen_addresses = '*'

This will allow Postgres to listen to all addresses, not just to localhost.

File pg_hba.conf

In this file, add the following line

host	all	all	172.17.0.1/16	md5

This line will allow connections to Postgres from 172.17.0.1/16 range of addresses. They belong to Docker and when running a container Docker will assign an IP to a container from this range.

Docker

When running your Docker container you need to configure it use the host system.

Tell the container about the host’s IP address

Pass the host’s IP using --add-host option, e.g., --add-host=host:<ip>. In this case host is a name of the host and it will be added to container’s /etc/hosts file. Inside the Docker container, you will be able to use the name host to connect to the host system.

Remember to publish container’s ports

For instance, if your dockerised app is running a server on port 8000, you can pass -p 8000:8000 to the docker run command to make your app available on the host on port 8000. The full docker run command can look similar to this:

docker run -dt --rm --add-host=host:<ip> --name container_name -p 8000:8000 image_name

Those changes should be enough to make a Postgres database running on a host system work with a containerised app.