MDNS, avahi and Docker non-root containers

The reason why I have chosen Redis Enterprise docker image is that it aligns well with the previous cluster example. With Redis Enterprise container, we can create a redis database cluster. And the cluster will have a domain name with .local (i.e., my-awesome-cluster.local ) and each node in the cluster receives a FQDN (i.e., node1.my-awesome-cluster.local ).

To sum up the above paragraph, we get a cluster which has a <my-cluster>.local address and nodes with <my-node>.<my-cluster>.local .

Now let's start the example.

You can start the container as follows:

docker run -it --cap-add sys_resource -p 12000:12000 -p 8443:8443 -p 9443:9443 redislabs/redis

Ports 8443 and 9443 should be exposed compulsorily for the container to do all its magic for services.

In the logs, you can find out that the container starts a mDNS server.

...more logs...
2020-05-30 05:10:03,048 INFO success: mdns_server entered RUNNING state, process has stayed up for > than 1 seconds (startsecs)
...more logs...

The port 12000 we have exposed can be any free port. This is the port where the database we create will be listening on. In the current example, I'm creating just one node. But it can be many.

Once the container is up and running, you can navigate to localhost:8443 .

You don't have to edit anything and under "Cluster configuration" you can give your cluster FQDN like this.

You can skip entering the certificates to which the container creates the certificates with default options. And then you can set some credentials the redis enterprise container requires you to. Once you set the credentials your page will be refreshed and you'll again be told to enter the credentials. Once you sign in, you are ready to create a database.

You can select "Redis" as your database, and you'll be given a form to enter the configuration of the database.

I'm giving the name as node1 and most importantly, you must set the port to 12000 since that's the only port that we have exposed. Click on "show advanced options" and you can find the field "Endpoint port number" where you can enter the port. Enter it as 12000 (since that's the port I have exposed). Click "Activate" to create the database. You'll then be redirected to your created DB's configuration page.

You can find the .local address and IP address of the DB in the Endpoint field.

Let us now create our sample application that simply connects to node1 and pings the database.

FROM python:3.8-buster

RUN pip3 install redis

CMD ["python3"]

We have taken the python buster image which is Debian 10 that has the mdns packages which we will install later.

Let's build it the application.

$ docker build -t py-mdns .
Sending build context to Docker daemon  3.072kB
Step 1/3 : FROM python:3.8-buster
3.8-buster: Pulling from library/python
376057ac6fa1: Pull complete 
5a63a0a859d8: Pull complete 
496548a8c952: Pull complete 
2adae3950d4d: Pull complete 
0ed5a9824906: Pull complete 
bb94ffe72389: Pull complete 
241ada007777: Pull complete 
be68aa7d1eeb: Pull complete 
820ffc2e28ca: Pull complete 
Digest: sha256:ebe8df5c3e2e10a7aab04f478226979e3b8754ee6cd30358379b393ef8b5321e
Status: Downloaded newer image for python:3.8-buster
 ---> 659f826fabf4
Step 2/3 : RUN pip3 install redis
 ---> Running in 1d31f5312e3e
Collecting redis
  Downloading redis-3.5.2-py2.py3-none-any.whl (71 kB)
Installing collected packages: redis
Successfully installed redis-3.5.2
Removing intermediate container 1d31f5312e3e
 ---> c21a8bc34782
Step 3/3 : CMD ["python3"]
 ---> Running in 711a17a97211
Removing intermediate container 711a17a97211
 ---> 27b0d68b69fc
Successfully built 27b0d68b69fc
Successfully tagged py-mdns:latest
$ docker run --it py-mdns

Now lets start the application as follows:

$ docker  run -it py-mdns

Remember that the .local address of node1 is redis-12000.mycluster.local , IP address is 172.17.0.2 and the port is 12000 .

Now lets try to ping the database using the .local address.

$ docker  run -it py-mdns
Python 3.8.3 (default, May 16 2020, 07:08:28) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import redis
>>> redis.Redis(host='redis-12000.mycluster.local', port=12000).ping()
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/redis/connection.py", line 550, in connect
    sock = self._connect()
  File "/usr/local/lib/python3.8/site-packages/redis/connection.py", line 575, in _connect
    for res in socket.getaddrinfo(self.host, self.port, self.socket_type,
  File "/usr/local/lib/python3.8/socket.py", line 918, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno -2] Name or service not known

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.8/site-packages/redis/client.py", line 1378, in ping
    return self.execute_command('PING')
  File "/usr/local/lib/python3.8/site-packages/redis/client.py", line 898, in execute_command
    conn = self.connection or pool.get_connection(command_name, **options)
  File "/usr/local/lib/python3.8/site-packages/redis/connection.py", line 1183, in get_connection
    connection.connect()
  File "/usr/local/lib/python3.8/site-packages/redis/connection.py", line 554, in connect
    raise ConnectionError(self._error_message(e))
redis.exceptions.ConnectionError: Error -2 connecting to redis-12000.mycluster.local:12000. Name or service not known.

What happened is that your application lacks an mDNS service discovery such that it doesn't know whom to ask and how to resolve the .local hostname to IP address.

Connecting using the IP address works.

>>> redis.Redis(host='172.17.0.2', port=12000).ping()
True

Now lets try to add mDNS.