Ubuntu – Correct way to fix name resolution from Docker container

16.04containerdnsdockernetworking

I have clean Ubuntu MATE 16.04.6 LTS installed.
I installed Docker from repository here with:

sudo apt-get install docker.io

and added my user to docker group with

sudo usermod -a -G docker $USER

then I downloaded Ubuntu 19.04 container with

docker pull ubuntu:19.04

and tried to run apt update inside it

docker run -it ubuntu:19.04 apt update

but got network errors:

$ docker run -it ubuntu:19.04 apt update
Err:1 http://archive.ubuntu.com/ubuntu disco InRelease                   
  Temporary failure resolving 'archive.ubuntu.com'
Err:2 http://security.ubuntu.com/ubuntu disco-security InRelease         
  Temporary failure resolving 'security.ubuntu.com'
Err:3 http://archive.ubuntu.com/ubuntu disco-updates InRelease           
  Temporary failure resolving 'archive.ubuntu.com'
Err:4 http://archive.ubuntu.com/ubuntu disco-backports InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Reading package lists... Done        
Building dependency tree       
Reading state information... Done
All packages are up to date.
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/disco/InRelease  Temporary
failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/disco-updates/InRelease 
Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/disco-backports/InRelease 
Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/disco-security/InRelease 
Temporary failure resolving 'security.ubuntu.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.

The /etc/resolv.conf inside container is the following:

$ docker run -it  ubuntu:19.04 cat /etc/resolv.conf
# 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 8.8.8.8
nameserver 8.8.4.4

The host system has internet, the NetworkManager is used with default settings:

$ cat /etc/NetworkManager/NetworkManager.conf 
[main]
plugins=ifupdown,keyfile,ofono
dns=dnsmasq

[ifupdown]
managed=false

all files on the system are unchanged – the debsums --changed --all --silent command returns nothing.

The requested output of iptables -S is below:

$ sudo iptables -S
-P INPUT ACCEPT
-P FORWARD DROP
-P OUTPUT ACCEPT
-N DOCKER
-N DOCKER-ISOLATION-STAGE-1
-N DOCKER-ISOLATION-STAGE-2
-N DOCKER-USER
-A FORWARD -j DOCKER-USER
-A FORWARD -j DOCKER-ISOLATION-STAGE-1
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -o docker0 -j DOCKER
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT
-A DOCKER-ISOLATION-STAGE-1 -i docker0 ! -o docker0 -j DOCKER-ISOLATION-STAGE-2
-A DOCKER-ISOLATION-STAGE-1 -j RETURN
-A DOCKER-ISOLATION-STAGE-2 -o docker0 -j DROP
-A DOCKER-ISOLATION-STAGE-2 -j RETURN
-A DOCKER-USER -j RETURN

Which settings should I change to fix DNS (name resolution) from Docker container?

Notes:

  1. I know about disabling DNS masquerading (#dns=dnsmasq), but I do not want to use this option.
  2. I got the same result while using Docker-CE from docker.com.
  3. The system installed from netinstall mini.iso does not suffer from aforementioned problem. It uses ifupdown. So the NetworkManager is the root of this problem.

Best Answer

  • The solution for docker run is very simple - we need to choose host network with --network host option:

    docker run -it --network host ubuntu:19.04 apt update
    

    and then the problem will be solved.


    But it also should be noted that docker build has --network option only starting only from 18.04 LTS. So if we need docker build --network host for 16.04 LTS we need to use docker-ce package from docker.com.