↓Skip to main content
Hugo’s IT journal

Docker

·
docker
Hugo
Author
Hugo
DevOps Engineer based in London
Table of Contents

Dockerfile
#

Most containers are not well-documented, making it hard to find essential information about them on Docker Hub or GitHub’s README. Some of the critical information to know includes:

  • Which user does the container run as?
  • Which ports are used by the application?
  • Where are the config files/directory?
  • Where will the persistent data be stored?

The best way to find these answers is by examining the Dockerfile. For example:

Prometheus

  • User: nobody
  • Port: 9090
  • Config path: /etc/prometheus/prometheus.yml
  • Data path: /prometheus

php:8-apache-buster

  • Port: 80
  • PHP config: /usr/local/etc/php/php.ini
  • Apache config: /etc/apache2/conf-available/docker-php.conf
  • Document root: /var/www/html/

Disk Usage
#

Analyse the disk usage of docker

Live Restore
#

This will keep containers running during the docker upgrade and systemctl stop dockerd

  • Edit the file /etc/docker/daemon.json
  • Add the following content to the file:
{
  "live-restore": true,
}

Address pool
#

  • To override the default address pool, add the following content to the daemon.json file:
{
  "default-address-pools": [
    {
      "base": "10.1.0.0/16",
      "size": 24
    }
  ]
}

Logging
#

  • Docker logging does not rotate by default.
  • To change the logging driver to JSON, you need to modify the Docker daemon configuration file (/etc/docker/daemon.json) and add the following line:
{
  "log-driver": "json-file"
}
  • To set the retention policy for the JSON logging driver, you can add the following configuration options to the same file:
{
  "log-opts": { 
    "max-size": "10m", 
    "max-file": "3"
  }
}
  • This will retain a maximum of 3 log files, each with a maximum size of 10 MB.
  • After making changes to the configuration file, you need to restart the Docker daemon for the changes to take effect:
sudo systemctl restart docker

Docker Swarm
#

Service Management
#

  • To reference a service, use tasks.<service-name> instead of a specific replica.
  • Publishing a port will expose it on all Swarm nodes, requiring a load balancer to balance the service.
  • A load balancer distributes incoming requests to nodes running the service, which will handle the request using one of its replicas.
  • Horizontal scaling of the service is achieved by adding additional replicas to handle increased traffic.

Volume Management
#

  • Volumes/local mount points are independent on each node.
  • When creating a volume/local mount point for a service, it is created on the node where the task is assigned.
  • Changes made to a volume/local mount point on one node will not be reflected on other nodes.
  • To ensure data consistency across nodes, use a distributed file system like GlusterFS or NFS, or a cloud-based storage service like Amazon EFS or Google Cloud Storage.
  • When using a distributed file system or cloud-based storage service, the volume/local mount point is shared across all nodes in the Swarm, ensuring that all nodes have access to the same data.

iptables
#

  • Docker uses iptables for traffic routing

  • Be cautious when restarting iptables service to avoid overriding Docker’s rules

  • Services like firewalld or ufw may interfere with Docker’s iptables rules

  • Turning off iptables may have side effects

  • Routing in iptables forwards published ports/container subnets to NAT table

  • NAT table routes traffic between containers

  • Docker host defaults to IPv4 forwarding enabled, making containers public on subnets

  • Docker host can be used as router to access container directly

  • Example: Limiting access to specific IPs for Node.js container with port 80 and published port 8080 on Docker host

    • Dropping port 8080 in firewalld won’t work; traffic is forwarded to NAT table
    • Dropping container port 80 in Docker’s custom chain (DOCKER_USER) will work but it will also block all traffic into the container, not just from outside Docker host

Useful links#