Ubuntu, NetworkManager and Docker DNS workaround

ubunut_network_manager_docker_dns_workaround.md

Docker issues are frequently logged for DNS resolution in containers because it doens't inhert or get values for DNS from NetworkManager which leverages a built in dnsmasq to inteligently manage DNS.

Perminant workarround

sudo bash -c "echo listen-address=$(ip -4 addr show dev docker0 | grep -oP '(?<=inet\s)\d+(\.\d+){3}') > /etc/NetworkManager/dnsmasq.d/docker-bridge"
sudo systemctl reload NetworkManager
sudo bash -c 'echo -e "{\n\t\"dns\": [\"$(ip -4 addr show dev docker0 | grep -oP "(?<=inet\s)\d+(\.\d+){3}")\"]\n}" > /etc/docker/daemon.json'
sudo systemctl restart docker

Note:

• makes dnsmasq plugin for network manager listen on the host's docker bridge interface
• adds (clobbers!) the daemon.json - take care, could overwrite other customisations you already have there...

Per run workarround

The bash one liner below generates the dns attributes needed for docker

nm_dns=$(for d in $(nmcli device show | grep -E "^IP4.DNS" | grep -oP '(\d{1,3}\.){3}\d{1,3}'); do echo -n " --dns $d"; done)
sudo -E docker run -it --rm -e http_proxy -e https_proxy -e no_proxy $nm_dns ubuntu

Ain't pretty, but works... (until they change the nmcli output format or something)

Related Issues

Related issues with docker DNS:

• Docker should use the host network DNS server
• Fix Docker's networking DNS config

Great ! but that's not sufficient for Fedora with firewalld.

What we need to do is to apply rules to let docker0 be able to accept dns requests. I'm using "internal" zone:

sudo firewall-cmd --add-interface=docker0 --zone=internal
sudo firewall-cmd --add-service=dns --zone=internal

# To make that permanent
sudo firewall-cmd --add-interface=docker0 --zone=internal --permanent
sudo firewall-cmd --add-service=dns --zone=internal --permanent
sudo firewall-cmd --reload

If you're interessed, I did a little service that add DNS entries in dnsmasq from docker hostnames, and a documentation to configure the system:
https://github.com/metal3d/docker-auto-dnsmasq

This works! Many thanks. I do not get how dnsmasq work locally at the host level at the ip address 127.0.0.1 if I specify to listen only at 172.17.0.1 !
...
Just found in the doc that Dnsmasq add the loopback interface by default. Great!