Docker: How to fix "could not translate host name "postgres" to address" when connecting from one container to another using container name?

user_1367204 0
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I start 3 container:

docker run -d --rm --name redis    --publish 6379:6379 redis 
docker run -d --rm --name postgres --publish 5432:5432 -e POSTGRES_HOST_AUTH_METHOD=trust -e POSTGRES_DB=hi postgres:9.6
docker run -d --rm --name web      --publish 8000:8000 python:3.6.8-stretch sleep 1000

Doing docker ps shows: enter image description here

Then I exec into web:

docker exec -it web bash

Then I install psycopg2:

$ pip install psycopg2

Then I try to connect to the host: postgres, port:5432, database:hi:

I start python:

$ python

Then I run these commands:

import psycopg2
conn = psycopg2.connect(host="postgres", port = 5432, database="hi", user="postgres", password="<passwd>")

And I get the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/psycopg2/__init__.py", line 127, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: could not translate host name "postgres" to address: Name or service not known

I thought it might be a docker network issue, so I went to inspect it, and all the hostnames are there:

{
    "Name": "bridge",
    "Id": "8e37606d20ae80189da4d7ecdb7673274fa2e0c9dd2b5c2b4c3db4e545c1ef20",
    "Created": "2020-05-14T19:10:43.532158344Z",
    "Scope": "local",
    "Driver": "bridge",
    "EnableIPv6": false,
    "IPAM": {
        "Driver": "default",
        "Options": null,
        "Config": [
            {
                "Subnet": "172.17.0.0/16",
                "Gateway": "172.17.0.1"
            }
        ]
    },
    "Internal": false,
    "Attachable": false,
    "Ingress": false,
    "ConfigFrom": {
        "Network": ""
    },
    "ConfigOnly": false,
    "Containers": {
        "3bb2a6cf4fbb235324d335f238c20fe026e37bccc717d1cd0bd1df43e70dbec6": {
            "Name": "redis",
            "EndpointID": "f927fc2a199d033bbbc8ecc8ff1bccf508e607384e4820884f3236f11a961c04",
            "MacAddress": "02:42:ac:11:00:02",
            "IPv4Address": "172.17.0.2/16",
            "IPv6Address": ""
        },
        "545065d96fb6f867724ece2cf11881af26be9885f16212bef4260bccba00a2be": {
            "Name": "web",
            "EndpointID": "750133a6bb72e23929c57f32aa7776bc85d3dd255a16473a4afe230d0c6cb9c2",
            "MacAddress": "02:42:ac:11:00:04",
            "IPv4Address": "172.17.0.4/16",
            "IPv6Address": ""
        },
        "cfb2e556ca9f26ff8c7063f2347b81b2a90365cfa476cab8611edf1586404a88": {
            "Name": "postgres",
            "EndpointID": "069411d5c42b3b968ce38b92b7122c608f1dca696517122728f23086fdce53c7",
            "MacAddress": "02:42:ac:11:00:03",
            "IPv4Address": "172.17.0.3/16",
            "IPv6Address": ""
        }
    },
    "Options": {
        "com.docker.network.bridge.default_bridge": "true",
        "com.docker.network.bridge.enable_icc": "true",
        "com.docker.network.bridge.enable_ip_masquerade": "true",
        "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
        "com.docker.network.bridge.name": "docker0",
        "com.docker.network.driver.mtu": "1500"
    },
    "Labels": {}
}

It seems that the hostname postgres should be resolved to the ip of 172.17.0.3.

When I do that in python, it works:

import psycopg2
conn = psycopg2.connect(host="172.17.0.3", port = 5432, database="hi", user="postgres", password="")

How can I connect to the container named postgres using the its name as a host?

docker networking postgresql python
2 Answers

You are right in suspecting that this is a docker network issue.

As you discovered, name resolution does not work with the default network being used. To allow for name resolution to work, you'll need to use a separate network like so:

docker network create my-network

Then, run your containers with the --net argument:

docker run -d --rm --name postgres --publish 5432:5432 -e POSTGRES_HOST_AUTH_METHOD=trust -e POSTGRES_DB=hi --net my-network postgres:9.6
docker run -d --rm --name redis --publish 6379:6379 --net my-network redis
docker run -d --rm --name web --publish 8000:8000 --net my-network python:3.6.8-stretch sleep 1000

Using this user-created network will then allow you to use the container names:

>>> import psycopg2
>>> conn = psycopg2.connect(host="postgres", port = 5432, database="hi", user="postgres", password="")
>>> conn.info.user
'postgres'

Note that for setups using multiple containers I would recommend to use docker-compose instead of just docker commands, where this network is set up in the background for you.

user_2804730

Seems like the issue is that all the containers were connecting to the default bridge network, which only allows resolution by IP address. Documentation:

Containers on the default bridge network can only access each other by IP addresses, unless you use the --link option, which is considered legacy.

For resolution by service name, I need to create my own network first, and then connect all the containers to the network.

On a user-defined bridge network, containers can resolve each other by name or alias

The commands should have been

docker create network my-network

docker run -d --rm --network my-network --name redis    --publish 6379:6379 redis 
docker run -d --rm --network my-network --name postgres --publish 5432:5432 -e POSTGRES_HOST_AUTH_METHOD=trust -e POSTGRES_DB=hi postgres:9.6
docker run -d --rm --network my-network --name web      --publish 8000:8000 python:3.6.8-stretch sleep 1000
user_13672040
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