Tired of editing /etc/hosts when testing docker containers from the host?
For docker containers to resolve each other using (service and container) names, an embedded Docker DNS can be utilized.
You can set up dnsmasq or dnsdock and use those to resolve names from your host machine where the docker daemon runs.
This repository contains two directories demonstrating two approaches for how to do this (using dnsmasq and dnsdock respectively).For name resolution, the classic solution is dnsmasq but there is also a more Docker-tailored variant that can be used; dnsdock, which seems to be popular as an alternative to "skydns" and other heavier service discovery solutions.
This setup works also using the convenient docker-compose wrapper, if the .yml file starts with "version: '2'". In this project, you will see that each directory has a docker-compose.yml file defining docker micro-services and a Makefile that manages the services (start, stop) and runs tests etc.
For making sure that the host running the docker daemon also can resolve a name in the same way as names are resolved internally between containers, the host machine running the containers need to be setup to resolve names not only using its default configured DNS resolution, but also in addition, using a separate Docker DNS container running locally on the host.
In practice, on a Linux host, these are possible configs to achieve what was described above:
- The docker daemon needs to be configured to run on the
docker0bridge interface (with a static ip: 172.17.0.1) and thendnsmasqwould bind to port 53 udp of this ip. For details, see the next section! - The host machine needs to be configured to use
resolvconfand to add the Docker internal DNS (/etc/resolvconf/resolv.conf.d/head needs to havenameserver 172.17.0.1). You may need to do "sudo dpkg-reconfigure resolvconf". - Possibly, you may want to edit "/etc/dhcp/dhclient.conf" and add "prepend domain-name-servers 172.17.0.1;". But possibly not, seems to work without it.
Instructions below are taken from Docker Hub pages for the "dnsdock" image: https://hub.docker.com/r/tonistiigi/dnsdock/:
- If you use systemd (present on Fedora and recent Ubuntu versions including latest Mint editions), edit /lib/systemd/system/docker.service and add the options to the command you will see in the ExecStart section, then run sudo systemctl daemon-reload. Or even better, follow the official instructions from https://docs.docker.com/engine/admin/systemd/ and add your own .conf file in the recommended location, then system updates will not be able to overwrite your config settings.
- If you do not, Open file /etc/default/docker and add --bip=172.17.0.1/24 --dns=172.17.0.1 to DOCKER_OPTS variable.
Restart docker daemon after you have done that (sudo service docker restart or sudo systemctl restart docker.service respectively depending on your linux distro).
To figure out which nameservers you use currently, you can use some tool suitable for your platform. On Linux it could for example be a command like: nm-tool | grep DNS or for other ways to do it, see http://askubuntu.com/questions/152593/command-line-to-list-dns-servers-used-by-my-system.
For your host machine to be able to use the Docker DNS server a few changes has to be made for the docker daemon and the IP of the Docker DNS server needs to be added to the nameserver list used by your host - on Linux this is done through the resolvconf configurations. To configure the Docker Daemon, create a file /etc/docker/daemon.json with the following content:
{
"dns": ["172.17.0.1", "your.corporate.network.nameserver1", "your.corporate.network.nameserver2", "8.8.8.8", 8.8.4.4"],
"bip": "172.17.0.1/24"
}
The first IP is the IP of the dnsdock container. The second and third are nameserver used in your corporate network. The last two are public DNS server IPs offered by Google. To add the DNSdock's IP in the first line of resolv.conf in your host
machine, do NOT edit that file, instead ensure you have 'resolvconf' package installed and add the following line to /etc/resolvconf/resolv.conf.d/head:
nameserver 172.17.0.1
Then restart the service resolvconf
sudo service resolvconf restart # or equiv depending on your distro
The DNSdock container IP is then added to resolv.conf, you can verify it with:
cat /etc/resolv.conf
This should enable other services to identify each other using domain name besides container name and also be able to reach the Internet from within the containers.
Using this approach, the key is to set "container_name" in docker-compose.yml!
Look at the docker-compose.yml file for details.
Use the Makefile to test that you have DNS working:
cd dnsmasq
make up
make test-dns-internal
make test-dns-external
make test-dns-debug
make clean
Further reading:
- http://stackoverflow.com/questions/23012273/setting-up-docker-dnsmasq
- https://hub.docker.com/r/andyshinn/dnsmasq/
Unlike with dnsmasq, with dnsdock, the container_name is not everything and you have more at your disposal...
You also get service discovery using a REST API:
curl -s http://dnsdock.docker/services | json_pp
And you get aliases and other stuff too, ie the same container can be known under several different names.
The format for a request matching a container is:
<anything>.<container-name>.<image-name>.<environment>.<domain>.
Details for components in that formula:
- environment (defaults to "") and domain (defaults to "docker") are static suffixes that are set on startup.
- image-name is last part of the image tag used when starting the container.
- container-name alphanumerical part of container name.
You can always leave out parts from the left side. If multiple containers match then they are all returned. Wildcard requests are also supported, i.e "dig *.docker +short" will list all ips matching that DNS query.
NB: The "tonistiigi/dnsdock:latest" docker image is no good.
You need something like this in the docker-compose.yml file:
dnsdock:
image: tonistiigi/dnsdock:amd64-1.13.1
ports:
- 172.17.0.1:53:53/udp
volumes:
- /var/run/docker.sock:/var/run/docker.sock
Do this to test that you have it working:
cd dnsdock
make up
make test-dns-internal
make test-dns-external
make test-dns-debug
make clean
Various links with reading materials