Sunday, November 22, 2015

MySQL and Docker on a Mac: networking oddity

This is a quick post only indirectly related to the series of articles about Docker that I have written recently.

Yesterday I was giving a presentation about Docker in Buenos Aires, and as usual I included a long live demo. Almost all went as expected. There was one step that I tried some time ago, and had always worked well, but when I tried to repeat it on stage, it failed miserably:

  • Step 1: run the container
$ docker run  -d --name mybox -e MYSQL_ROOT_PASSWORD=secret mysql/mysql-server
  • Step 2: get the IP address
$ docker inspect --format '{{ .NetworkSettings.IPAddress}}'  mybox
  • Step 3: connect to the container from a local client.

$ mysql -h -u root -psecret
Enter Password:

And here the connection hangs.

That was odd. This operation was working fine at home. I have it pasted into my first article of the series. Why it did not work here?

I could not solve the issue on stage. I had to go on with my presentation. The installation of three nodes in replication went well. Everything that I tried at home was working, except connecting from the external client to the server inside a container.

When I came back to the hotel, I realised what was different: I had done all the tests for the first article on Linux only, and I started testing on Mac when using the material for the second article. Thus I hadn't tried this particular example on a Mac until I went on stage. And of course it did not work: I was trying to connect from my Mac to a container inside a virtual machine which uses a different subnet and cannot be reached without indirection.

The only way of connecting a local client to the container is by exposing explicitly the MySQL port to the external (and in this case the "external" is the virtual machine itself) and then connect to the virtual machine.

This approach works:

$ docker rm -v -f mybox

$ docker run -p 3306:3306 -d --name mybox -e MYSQL_ROOT_PASSWORD=secret mysql/mysql-server

Here the port 3306 of the container is attached to the port 3306 to the host (= the virtual machine).

$ docker-machine ip default

Now we get the address of the virtual machine, so that we can connect to its MySQL port.

$ mysql -h -u root -psecret
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.9-log MySQL Community Server (GPL)

Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.


And the connection now works!

I owe it to the attendees to my talk to explain the oddity, but I believe it could be useful for others out there as well.

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