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
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?
2
Answers
Seems like the issue is that all the containers were connecting to the default bridge network, which only allows resolution by IP address. Documentation:
For resolution by service name, I need to create my own network first, and then connect all the containers to the network.
The commands should have been
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:
Then, run your containers with the
--net
argument:Using this user-created network will then allow you to use the container names:
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.