Host resolution with docker and docker-compose

Question

I'm wondering why, with docker-compose, the container cannot resole host-name while it's working with docker ? The host is on different physical machine but on the same network.

My Dockerfile

FROM openjdk:8-jre-alpine
CMD ping -c 2 myhost

My docker-compose.yml

version: '3.3'
services:
    net:
        build: .
        image: test/myimage:1.0
        container_name: mycontainer

After a docker-compose build I tried

docker run -it test/myimage:1.0
PING myhost (10.20.78.13): 56 data bytes
64 bytes from 10.20.78.13: seq=0 ttl=250 time=0.720 ms
64 bytes from 10.20.78.13: seq=1 ttl=250 time=0.515 ms

but

docker-compose up
Recreating mycontainer ...
Recreating mycontainer ... done
Attaching to mycontainer
mycontainer | ping: bad address 'myhost'
mycontainer exited with code 1

What can I do to have it working ?

Edit1

Using cat /etc/resolv.conf instead of ping -c 2 myhost

docker-compose up
Recreating mycontainer ...
Recreating mycontainer ... done
Attaching to mycontainer
mycontainer | search myorg.intra
mycontainer | nameserver 127.0.0.11
mycontainer | options ndots:0
mycontainer exited with code 0

and

docker run -it test/myimage:1.0
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 10.19.128.55
nameserver 10.19.142.23
nameserver 10.19.128.56
search myorg.intra

Answer 1

You need use the dns entries in your compose also

version: '3.3'
services:
    net:
        build: .
        image: test/myimage:1.0
        container_name: mycontainer
        dns:
          - 10.19.128.55
          - 10.19.142.23
          - 10.19.128.56
        dns_search: myorg.intra

If you don't want to specify them per container then you can even set them directly at the daemon level.

Create or edit /etc/docker/daemon.json and below entries in the JSON file

{
    "dns": ["10.19.128.55", "10.19.142.23", "10.19.128.56"],
    "dns-search": ["myorg.intra"],
}

Restart the docker service using service docker restart . Then you should not need the entries in the docker-compose . Your original docker-compose should work

Answer 2

There is a subtle difference in network settings when running docker commands directly vs. performing some docker-compose runs. With docker-compose the containers are attached to some user-defined network which is a bridged network but not with the same configuration as the default bridged network . The latter one does some special configuration to be backwards compatible to the versions where these networking features didn't exist like today.

I assume that when running from compose, you are missing some dns settings that are available on your host machine and thus you aren't able to resolve the other host on your network. Please have a look at the differences here and here . For a first check, you could compare the output when running cat /etc/resolv.conf instead of your ping command for both ways and see what you need to add to make it work from compose.