Hello everyone,
I am trying to understand the difference between docker-compose and docker run when attempting to resolve the hostname of a host on the network at my work from inside a container. In the example below, why does docker run work only when I specify my work’s DNS address whereas docker-compose works without any explicit DNS info? In this example, myhost is another PC on the network at my work.
Minimal example
Dockerfile
FROM ubuntu:16.04
MAINTAINER Kyle M. Douglass <myemail@mywork.com>
RUN apt-get update && apt-get install -y wget
CMD wget myhost > index.html
docker-compose.yml
version: "3"
services:
my_service:
build: .
image: kmdouglass/my_image:latest
command: wget myhost > index.html
Versions
Client:
Version: 18.09.0
API version: 1.39
Go version: go1.10.4
Git commit: 4d60db4
Built: Wed Nov 7 00:48:57 2018
OS/Arch: linux/amd64
Experimental: false
Server: Docker Engine - Community
Engine:
Version: 18.09.0
API version: 1.39 (minimum version 1.12)
Go version: go1.10.4
Git commit: 4d60db4
Built: Wed Nov 7 00:16:44 2018
OS/Arch: linux/amd64
Experimental: false
Network
$docker network ls
NETWORK ID NAME DRIVER SCOPE
4e829bd60a47 bridge bridge local
bdf7a8489980 host host local
8d15410c6cc1 none null local
Experiment 1
Build the container and use docker-run to fetch the web page from the host. In this case, it fails because I do not pass the DNS address:
$ docker build -t kmdouglass/my_image:latest .
$ docker run -t --rm kmdouglass/my_image
--2018-12-06 08:00:23-- http://myhost/
Resolving myhost (myhost)... failed: Name or service not known.
wget: unable to resolve host address 'myhost'
Experiment 2
The same as before, but specify the DNS address at my work. In this experiment the hostname resolution succeeds:
$ docker run -t --rm --dns=x.x.x.x kmdouglass/my_image
--2018-12-06 08:02:27-- http://myhost/
Resolving myhost (myhost)... y.y.y.y
Connecting to myhost (myhost)|y.y.y.y|:80... connected.
HTTP request sent, awaiting response... 302 Found
Location: /graphs/ [following]
--2018-12-06 08:02:27-- http://myhost/graphs/
Reusing existing connection to myhost:80.
HTTP request sent, awaiting response... 200 OK
[...]
Experiment 3
Now, remove the image to have a clean working environment and run the container with docker-compose. This should have the same result as Experiment 1 because I do not specify my local DNS in the docker-compose.yml file. However, the hostname resolution succeeds without any problems:
$ docker image rm kmdouglass/my_image:latest
$ docker-compose up
Creating network "docker_question_default" with the default driver
Creating docker_question_my_service_1 ... done
Attaching to docker_question_my_service_1
my_service_1 | --2018-12-06 08:11:54-- http://myhost/
my_service_1 | Resolving myhost (myhost)... y.y.y.y
my_service_1 | Connecting to myhost (myhost)|y.y.y.y|:80... connected.
my_service_1 | HTTP request sent, awaiting response... 302 Found
my_service_1 | Location: /graphs/ [following]
my_service_1 | --2018-12-06 08:11:54-- http://myhost/graphs/
my_service_1 | Reusing existing connection to myhost:80.
my_service_1 | HTTP request sent, awaiting response... 200 OK
Thanks for the help!
-kmd