Docker cannot resolve DNS on private network [closed] - docker

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Docker is a software tool primarily used by programmers as it is the mechanism programmers use to produce container images.
My machine is on a private network with private DNS servers, and a private zone for DNS resolution. I can resolve hosts on this zone from my host machine, but I cannot resolve them from containers running on my host machine.
Host:
root#host:~# 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 127.0.1.1
root#host:~# ping privatedomain.io
PING privatedomain.io (192.168.0.101) 56(84) bytes of data.
Container:
root#container:~# 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
root#container:~# ping privatedomain.io
ping: unknown host privatedomain.io
It's fairly obvious that Google's public DNS servers won't resolve my private DNS requests. I know I can force it with docker --dns 192.168.0.1, or set DOCKER_OPTS="--dns 192.168.0.1" in /etc/default/docker, but my laptop frequently switches networks. It seems like there should be a systematic way of solving this problem.

Answer 1

Docker populates /etc/resolv.conf by copying the host's /etc/resolv.conf, and filtering out any local nameservers such as 127.0.1.1. If there are no nameservers left after that, Docker will add Google's public DNS servers (8.8.8.8 and 8.8.4.4).
According to the Docker documentation:
Note: If you need access to a host’s localhost resolver, you must modify your DNS service on the host to listen on a non-localhost address that is reachable from within the container.
The DNS service on the host is dnsmasq, so if you make dnsmasq listen on your docker IP and add that to resolv.conf, docker will configure the containers to use that as the nameserver.
1 . Create/edit /etc/dnsmasq.conf† and add these lines:
interface=lo
interface=docker0
2 . Find your docker IP (in this case, 172.17.0.1):
root#host:~# ifconfig | grep -A2 docker0
docker0 Link encap:Ethernet HWaddr 02:42:bb:b4:4a:50
inet addr:172.17.0.1 Bcast:0.0.0.0 Mask:255.255.0.0
3 . Create/edit /etc/resolvconf/resolv.conf.d/tail and add this line:
nameserver 172.17.0.1
4 . Restart networking, update resolv.conf, restart docker:
sudo service network-manager restart
sudo resolvconf -u
sudo service docker restart
Your containers will now be able to resolve DNS from whatever DNS servers the host machine is using.
† The path may be /etc/dnsmasq.conf, /etc/dnsmasq.conf.d/docker.conf, /etc/NetworkManager/dnsmasq.conf, or /etc/NetworkManager/dnsmasq.d/docker.conf depending on your system and personal preferences.

Answer 2

For Ubuntu 18.04, and other systems that use systemd-resolved, it may be necessary to install dnsmasq and resolvconf. systemd-resolved is hard-coded to listen on 127.0.0.53, and Docker filters out any loopback address when reading resolv.conf.
1 . Install dnsmasq and resolvconf.
sudo apt update
sudo apt install dnsmasq resolvconf
2 . Find your docker IP (in this case, 172.17.0.1):
root#host:~# ifconfig | grep -A2 docker0
docker0 Link encap:Ethernet HWaddr 02:42:bb:b4:4a:50
inet addr:172.17.0.1 Bcast:0.0.0.0 Mask:255.255.0.0
3 . Edit /etc/dnsmasq.conf and add these lines:
interface=docker0
bind-interfaces
listen-address=172.17.0.1
4 . Create/edit /etc/resolvconf/resolv.conf.d/tail and add this line:
nameserver 172.17.0.1
5 . Restart networking, update resolv.conf, restart docker:
sudo service network-manager restart
sudo resolvconf -u
sudo service dnsmasq restart
sudo service docker restart
Your containers will now be able to resolve DNS from whatever DNS servers the host machine is using.

Answer 3

As you know, Docker copy host /etc/resolv.conf file to containers but removing any local nameserver.
My solution to this problem is to keep using systemd-resolvd and NetworkManager but add dnsmasq and use it to "forward" Docker containers DNS queries to systemd-resolvd.
Step by step guide:
Make /etc/resolv.conf a "real" file
sudo rm /etc/resolv.conf
sudo touch /etc/resolv.conf
Create file /etc/NetworkManager/conf.d/systemd-resolved-for-docker.conf to tell NetworkManager to inform systemd-resolvd but to not touch /etc/resolv.conf
[main]
# NetworkManager will push the DNS configuration to systemd-resolved
dns=systemd-resolved
# NetworkManager won’t ever write anything to /etc/resolv.conf
rc-manager=unmanaged
Install dnsmasq
sudo apt-get -y install dnsmasq
Configure dnsmasq in /etc/dnsmasq.conf for listening DNS queries comming from Docker and using systemd-resolvd name server
# Use interface docker0
interface=docker0
# Explicitly specify the address to listen on
listen-address=172.17.0.1
# Looks like docker0 interface is not available when dnsmasq service starts so it fails. This option makes dynamically created interfaces work in the same way as the default.
bind-dynamic
# Set systemd-resolved DNS server
server=127.0.0.53
Edit /etc/resolv.conf to use systemd-resolvd nameserver (127.0.0.53) and the host IP (172.17.0.1) in Docker network
# systemd-resolvd name server
nameserver 127.0.0.53
# docker host ip
nameserver 172.17.0.1
Restart services
sudo service network-manager restart
sudo service dnsmasq restart
sudo service docker restart
For more info see my post (in spanish) https://rubensa.wordpress.com/2020/02/07/docker-no-usa-los-mismos-dns-que-el-host/

Answer 4

I had problems with the DNS resolver in our docker containers. I tried a lot of different things, and in the end, I just figured that my CentOS VPS in Hostgator didn't have installed by default NetworkManager-tui (nmtui), I just installed and reboot it.
sudo yum install NetworkManager-tui
And reconfigured my resolv.conf with default DNS as 8.8.8.8.
nano /etc/resolv.conf

Answer 5

My case with many images from docker hub (nodered, syncthing and another):
container is running under not-root user
/etc/resolv.conf inside container has permissions 600 and owned by root
So my solution is very simple
root#container:~# chmod 644 /etc/resolv.conf
Profit! :))

Answer 6

If you are using a VPN, the VPN protocol might be appending to outbound packets beyond the configured MTU on your private network.
A typical MTU is 1500.
Try adding this content to /etc/docker/daemon.json
{
"mtu": 1300,
"dns": ["<whatever DNS server you need in your private network>"]
}
Then systemctl restart docker.

Answer 7

I have the same error message in my systemctl status docker.
I run a Nextcloud and a nextcloud nginx proxy container and used docker compose to install it. It worked for multiple months without big hickups, but on Friday it wasn't accessable. The server had shut down.
I restarted it, my icecast2 instance is working fine and was used this sunday for the service in our church. But the docker containers are gone. docker ps -a doesn't show any, I can't access the nextcloud via docker exec like I would do normally. And I have the error message:
No non-localhost DNS nameservers are left in resolv.conf. Using default external servers: [nameserver 8.8.8.8 nameserver 8.8.4.4]"
Feb 06 19:04:58 ncxxxxxxxxx dockerd[21551]: time="2022-02-06T19:04:58.894366765Z" level=info msg="IPv6 enabled; Adding default IPv6 external servers: [nameserver 2001:4860:4860::8888 nameserver 2001:4860:4860::8844]
My resolv.conf looks like this:
GNU nano 4.8
/etc/resolv.conf
nameserver 127.0.0.53
options edns0 trust-ad
search fritz.box

Answer 8

Based on answer from #rubensa, but simpler and more integrated IMHO:
Install dnsmasq
sudo apt-get -y install dnsmasq
Configure dnsmasq in /etc/dnsmasq.d/docker-dns-fix.conf for listening to DNS queries coming from Docker and using systemd-resolvd name server
# Use interface docker0
interface=docker0
# Explicitly specify the address to listen on
listen-address=172.17.0.1
# Looks like docker0 interface is not available when dnsmasq service starts so it fails. This option makes dynamically created interfaces work in the same way as the default.
bind-dynamic
# Set systemd-resolved DNS server
server=127.0.0.53
Tell Docker to use dnsmasq by editing/creating /etc/docker/daemon.json
{
"dns": ["172.17.0.1"]
}
Restart services
sudo service dnsmasq restart
sudo service docker restart

Answer 9

It was enough for Ubuntu 18.04 LTS:
sudo service network-manager restart
sudo resolvconf -u
sudo service dnsmasq restart
sudo service docker restart