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HOWTO: Enable Docker API through firewalld on CentOS 7.x (el7)

centos-dockerPlaying more and more with Docker across multiple Linux distributions has taught me that not all Linux distributions are treated the same.

There’s a discord right now in the Linux community about systemd vs. SysV init. In our example, CentOS 7.x uses systemd, where all system services are spawned and started.

I am using this version of Linux to set up my own Docker lab host for tire-kicking, but it needs some tweaks.

I also wanted to see if I could use the Docker API from my Android phone, using DockerDroid, which (after configuring this) works famously!

Here’s what you need to do:

  1. Log into your CentOS machine and update to the most-current Docker version. The version shipped with CentOS 7 in the repo as I write this post, is “docker-1.3.2-4.el7.centos.x86_64”. You want to be using something more current, and 1.4 is the latest. To fetch that (and preserve your existing version), run the following:
    $ su -
    # cd /bin && mv /bin/docker /bin/docker.el7
    # wget https://get.docker.com/builds/Linux/x86_64/docker-latest -O docker
    # systemctl restart docker
    # exit
    $ 
    

    Now you should have a working Docker with the right version (current). You can verify that:

    $ sudo docker version
    Client version: 1.4.1
    Client API version: 1.16
    Go version (client): go1.3.3
    Git commit (client): 5bc2ff8
    OS/Arch (client): linux/amd64
    Server version: 1.4.1
    Server API version: 1.16
    Go version (server): go1.3.3
    Git commit (server): 5bc2ff8
  2. So far, so good! Now we need to make sure firewalld has a rule to permit this port to be exposed for external connections:
    $ sudo firewall-cmd --zone=public --add-port=4243/tcp --permanent
    $ sudo firewall-cmd --reload
    success
    

    You can verify that this new rule was added, by looking at /etc/firewalld/zones/public.xml, which should now have a line that looks like this:

    <port protocol="tcp" port="4243"/>
  3. Now let’s reconfigure Docker to expose the API to external client connections, by making sure the OPTIONS line in /etc/sysconfig/docker looks like this (note the portion in bold):
    OPTIONS=--selinux-enabled -H fd:// -H tcp://0.0.0.0:4243
    
  4. Restart the Docker service to enact the API on that port (if successful, you will not see any output):
    sudo systemctl restart docker
  5. To test the port locally, install telnet and then try telnet’ing to the port on localhost:
    $ sudo telnet localhost 4243
    Trying ::1...
    Connected to localhost.
    Escape character is '^]'.
    
    HTTP/1.1 400 Bad Request
    
    Connection closed by foreign host.

    All looks good so far!

  6. Lastly, install DockerDroid and configure it to talk to your server on this port:

    DockerDroid connecting to CentOS via API

  7. Now you should be able to use DockerDroid to navigate your Images, Containers and API.

    Good luck!

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