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DNS

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The DNS server is embedded in the simplecontainer agent. It is a fully functional DNS server. The port it uses is 53 and is exposed in the Docker network where the simplecontainer is running.

All containers started with the simplecontainer are configured to use DNS server from the simplecontainer agent.

It is aware of all the containers running on the single Docker host and makes service discovery enabled.

Format of the DNS:

network.group.generatedName.cluster

Where:

  • network is the network name of the Docker network where the container is deployed
  • group is the group of the container defined in the meta
  • generatedName is the name generated using container specification group-name-index
  • .cluster is the suffix inside the simplecontainer network

When the container is created the respective DNS name is added as the A record in the server. Also, when the container is removed, stopped, or dead the DNS recorded is removed hence the name resolution will fail.

Example single node mode

Let's examine the next minimal container definition:

kind: container
meta:
  group: example
  name: test
spec:
  container:
    image: busybox
    tag: latest
    replicas: 2
    entrypoint: ["sleep"]
    args: ["3600"]

If no network block is specified container will be added to the bridge network.

This will create three DNS records:

  • bridge.example.example-test-1.cluster.private
  • bridge.example.example-test-2.cluster.private
  • bridge.example.test.cluster.private

The first two DNS records will return the IP of the specific container. The last record will return the IP addresses of both containers.

The records will be live in the DNS server only when the container reaches the RUNNING state otherwise, the DNS record will be discarded.

Example cluster mode

This example assumes there are two nodes in the cluster.

kind: container
meta:
  group: example
  name: test
spec:
  container:
    image: busybox
    tag: latest
    replicas: 2
    entrypoint: ["sleep"]
    args: ["3600"]
    spread:
      spread: uniform
    networks:
      - group: smr
        name: cluster
      - gorup: docker
        name: bridge

This will deploy one container on Node 1 and one container on Node 2.

This will create three DNS records:

  • cluster.example.example-test-1.cluster.private
  • cluster.example.example-test-2.cluster.private
  • cluster.example.test.cluster.private

The first two DNS records will return the IP of the specific container. The last record will return the IP addresses of both containers.

The records will be live in the DNS server only when the container reaches the RUNNING state otherwise, the DNS record will be discarded.

Even if containers are not on the same node DNS will be able to resolve and communication is done via overlay network between two containers.