Could not translate host name "db" to address using Postgres, Docker Compose and Psycopg2

Question

In one folder I have 3 files: base.py, Dockerfile and docker-compose.yml.

base.py:

import psycopg2

conn = psycopg2.connect("dbname='base123' user='postgres' host='db' password='pw1234'")

Dockerfile:

FROM ubuntu:16.04

RUN apt-get update
RUN apt-get -y install python-pip
RUN apt-get update
RUN pip install --upgrade pip
RUN pip install psycopg2-binary

COPY base.py base.py

RUN python base.py

docker-compose.yml:

version: '3'
services:
  db:
    image: 'postgres:latest'
    expose:
      - "5432"
    environment:
      POSTGRES_PASSWORD: pw1234
      POSTGRES_DB: base123
  aprrka:
    build: .    
    depends_on:
      - db

After I ran docker-compose up, I got the following error:

Traceback (most recent call last):
  File "base.py", line 5, in <module>
conn = psycopg2.connect("dbname='base123' user='postgres' host='db' password='pw1234'")
   File "/usr/local/lib/python2.7/dist-packages/psycopg2/__init__.py", line 130, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: could not translate host name "db" to address: Name or service not known

ERROR: Service 'aprrka' failed to build: The command '/bin/sh -c python base.py' returned a non-zero code: 1

I don't know why I have this error. I exposed port 5432. By default Compose sets up a single network for app. Each service joins the default network, I think that my app with postgres should work together. Did I write incorrect docker-compose.yml?

Squander

jack-gore's answer addresses your question. One approach for your diagnosis could have been to use a known good 2nd container to access the db. See the image's documentation on docs.docker.com and the use of the adminer image (docs.docker.com/samples/library/postgres/…). Second, the Dockerfile would benefit from reducing the number of layers created by reducing the number of RUN commands. For example RUN apt-get update && apt-get -y install python-pip (you don't need the final apt-get update). – Carlycarlye

I had the same issue, and what helped was: 1) stopping docker containers in the docker Desktop interface; 2) closing the terminal where the python script runs; 3) opening the new terminal window where the script was re-run. – Undersea

Answer 1

The problem is you should not be running python base.py as part of the RUN directive.

The RUN directive is executed only when you are building the image. The postgres container is not running at this point, nor has the network been created. Instead you want to use the CMD directive.

Change the Dockerfile to this:

FROM ubuntu:16.04

RUN apt-get update
RUN apt-get -y install python-pip
RUN apt-get update
RUN pip install --upgrade pip
RUN pip install psycopg2-binary

COPY base.py base.py

CMD ["python", "base.py"]

The above should result in the hostname db to be resolved. However if your python code doesn't have any reconnection logic for connecting to the database the container will likely still error out. This because the postgres container will be running but the database won't be ready to accept connections.

This can be temporarily fixed by adding restart: always to your docker-compose.yml.

version: '3'
services:
  db:
    image: 'postgres:latest'
    expose:
      - "5432"
    environment:
      POSTGRES_PASSWORD: pw1234
      POSTGRES_DB: base123
  aprrka:
    restart: always
    build: .    
    depends_on:
      - db

Hopefully this will get you up and running.

Thionate

Thank You! It is sad but I still don't know how to fix next error. aprrka_1 | Traceback (most recent call last): File "base.py", line 5, in <module> conn = psycopg2.connect("dbname='base123' user='postgres' host='db' password='pw1234'") File "/usr/local/lib/python2.7/dist-packages/psycopg2/__init__.py", line 130, in connect conn = _connect(dsn, connection_factory=connection_factory, **kwasync) psycopg2.OperationalError: could not connect to server: Connection refused Is the server running on host "db" (172.20.0.2) and accepting TCP/IP connections on port 5432? – Squander

This error is what the second part of my answer tried to fix. You either need to add reconnection logic to your code, add the restart: always to your compose-file. Have you tried that? – Thionate

Yes I did. It works, but firsts of tryies to connect to db are unsuccessful. Do I need to type in my base.py file some 'wait time' to make connection correctly without errors? I suppose that it won't be 'better' for my whole Docker image, but it will be nicer to see how it runs. Is my sollution correct? – Squander

Yes, this is a seperate question, but you want to implement some sort of waiting mechanism in your code. – Thionate

Answer 2

Add database and web service on same network in docker compose file. Also link db service and start web service after starting db service.

After properly adding network, link and depends_on configuration in docker compose file issue will be fixed.

Configuration example:

  services:
      db:
          container_name: db
          networks:
              - djangonetwork
      web:
          depends_on:
             - db
          links:
             - db:db
          networks:
             - djangonetwork

  networks:
      djangonetwork:
          driver: bridge

In my docker compose file, I have used network name as 'djangonetwork', you can use any other name.

the above configuration helped me to resolve the issue 'Could not translate host name db'.

Nevanevada

This worked for me. Keep on forgetting to add a network whenever I create a new service. – Mount

Setting a links entry of db:db seems to be superfluous, judging from the docs. What's your intention behind it? – Mac

You don't have to set the network explicitly. By default, a network is already created for multi container applications and each container join the network with its container name. docs.docker.com/compose/networking – Bitten

Answer 3

If you're using docker-compose first add this line to your .yml:

environment:
  - "POSTGRES_HOST_AUTH_METHOD=trust"

After that you start only you db docker:

docker-compose up db

It should start normally with a message like:

Recreating db... done
Attaching to db
db_1           | ********************************************************************************
db_1           | WARNING: POSTGRES_HOST_AUTH_METHOD has been set to "trust". This will allow
db_1           |          anyone with access to the Postgres port to access your database without
db_1           |          a password, even if POSTGRES_PASSWORD is set. See PostgreSQL
db_1           |          documentation about "trust":
db_1           |          https://www.postgresql.org/docs/current/auth-trust.html
db_1           |          In Docker's default configuration, this is effectively any other
db_1           |          container on the same system.
db_1           | 
db_1           |          It is not recommended to use POSTGRES_HOST_AUTH_METHOD=trust. Replace
db_1           |          it with "-e POSTGRES_PASSWORD=password" instead to set a password in
db_1           |          "docker run".
db_1           | ********************************************************************************

ADVISE: This is NOT SAFE to do. If You decide to use this solution, make sure to list your docker file in .gitignore so you won't push this to production.

Frangible

I posted this in another answer, but this is not a good idea. Its like begging to be robbed. – Sabelle

You completly right, however I don't see any problem if your docker won't be sent to production. Juts put the config file in gitnore and you will be fine. – Frangible

This should be added as a warning in the answer then, otherwise, newcomers will think this is a perfectly fine way to do this. – Sabelle

@Sabelle Agreed and done. – Frangible

Answer 4

if you add this to your db container in your docker-compose.yml it should resolve the issue

environment:
  - "POSTGRES_HOST_AUTH_METHOD=trust"

Hamer

It will also make your database unsecure. – Parliament

This is like begging to be robbed. Don't do this, not even in dev env. – Sabelle

Answer 5

Another possible scenario,

Check if ports have been used or not by other docker container. Use command:

$ docker container ls --format "table {{.ID}}\t{{.Names}}\t{{.Ports}}" -a

Then change your ports/expose in docker-compose file

Sect

Answer 6

Updating the docker compose version to 3.8 helped me

Angus

Answer 7

I thought I'd give an updated answer as I recently found a similar issue.

I had a similar setup where the app would connect correctly from docker running locally, but would fail when running in Jenkins.

Changing image: postgres:latest to image: postgres:13.4-alpine solved the issue.

Shedevil