How to resolve the host operating system 'localhost' address from within a container?

Question

I have two docker containers running:

Postgres 9.4: 0.0.0.0:5431->5432/tcp
Postgres 11: 0.0.0.0:5432->5432/tcp

I can connect to both of them via clients on my computer, but I am unable to access one container from within another. I would like to use Postgres's dblink feature - i.e. (in case it matters), I am running a query on one server that looks like this:

select
dp.*
from dblink(
    'dbname=mydb
    port=5431
    host=localhost
    user=postgres
    password=password', 'select 1'
) as dp (i int)

This results in an error along that says:

Could not connect to server: Connection refused
  Is the server running on host "localhost" (127.0.0.1) and accepting
  TCP/IP connections on port 5431?

And I guess this is because localhost is the loopback address inside the container. What is the way around this? (I'm super new to Docker by the way)

Answer 1

You are correct in that using localhost within the container is referencing the container, and not your network. You could use inspect, find the ip, and hardcode the ip. However, a simpler and cleaner solution would be to utilize Docker's network features. When you run the containers you can put the containers inside a defined network. Containers in this network can resolve requests to other containers using their container name instead of an ip (e.g. Server=container2).

Docker Network Documentation

Answer 2

Ah. Well a high level answer that allows for me to just 'get by' for now is this:

Docker will automatically create a network when running one or more containers. You can check what a container's IP address is on this virtual network by running:

docker container inspect <container name>

I was able to find the gateway address of the postgres container that I want to query from and use it that way.

Answer 3

you can use

"docker network create "

to create your own network. Once you do that when you are creating a container use the

"--network "

to create the 2 containers in the same network.The containers in the same network can interact with each other.