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Accepted
Tom Klino
Asked: 2017-12-26 12:10:37 +0800 CST

Docker service discovery (or dns resolution) from the host

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I have a recipe that sets up a docker instance with a specific mysql image I made for my app. As part of that recipe I would like to have it so, that after the container comes up with mysql running, a procedure for restoring data from backup will begin automatically.

For that I need to have the IP of the container (or a way to get it from DNS). However, Since I bring the container up automatically from a recipe, I have no way of knowing what that IP will be, so I'm trying to have docker set it with a hostname that it can later resolve.

Oddly, if I link a container to another, I will be able to resolve the host of the other container from within the one linked to it. But, I have found no way resolving that hostname from the host.

Here is the part of the recipe (chef) I use to bring up the container:

docker_container 'imhere-mysql' do
  hostname mysqlHost
  repo 'lutraman/imhere'
  tag 'mysql'
  env ["MYSQL_ROOT_PASSWORD=#{mysql_password}"]
  volumes [ '/var/imhere/mysql:/var/lib/mysql' ]
  action :run
end

The command I would like to use to restore data is something like this (constructed from the recipe):

"gunzip -c #{parent_data_dir}/#{db_restore_filename} | mysql -h #{mysqlHost} -u root -p'#{mysql_password}'"

only that running from the host, the name in mysqlHost (in the case at hand - imhere-mysql) is not resolved from the host.

There is a note at the bottom of this article, saying there should be a docker dns server running on 127.0.0.11 - but I fear this is internal to containers.

What I'm hoping I'll be able to do, is query docker (in the ideal situation, as a DNS, but any way will work) for the ip of the container named 'imhere-mysql'. What's the best way to do this in a way that will hopefully hold for future versions on docker and regardless of the host OS?

docker

2 Answers

  1. Best Answer

    You can obtain the ip address of the container using docker inspect command with the filter:

    docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' <container name>

    Your command will be:

    "gunzip -c #{parent_data_dir}/#{db_restore_filename} | mysql -h $(docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' #{mysqlHost}) -u root -p'#{mysql_password}'"

    Also you can use static ip addresses for containers, declaring own networks.

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  2. My solution to this would be to run the restore commands in a container on the same network (or with a link). It sounds like you already have the folder information available so those can be passed to a temporary container as a volume.

    Using the legacy Docker links it would look something like:

    docker run --link #{mysqlHost} --rm -v #{parent_data_dir}/#{db_restore_filename}:/tmp/dbrestore.sql lutraman/imhere bash -c "gunzip -c /tmp/dbrestore.sql | mysql -h #{mysqlHost} -u root -p'#{mysql_password}'"

    This would use the same image (assuming mysql and gunzip binaries are available and there is no ENTRYPOINT) to mount the host volume to a temporary location in the container and then unzip from that temporary location to the mysql binary connecting to the link.

    You could probably avoid the legacy link by creating a custom Docker network bridge. In this case, the container names in that custom network should be available by DNS without the need for a link. But this would add additional parts to your recipe that may be more work than worth.

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