Docker Networking and Port Access

In Docker, containers are isolated by default, meaning:

  • Containers cannot access the host machine’s ports directly.
  • Containers cannot access the ports of other containers directly.
  • Container ports are not accessible from outside the host machine unless configured.

🛠️ Accessing Container Ports from Outside

To access a container’s port from outside (e.g., from the host or another machine), you can use port mapping:

1. Port Mapping

docker run -p host_port:container_port imagename
  • This command maps a container’s port to a port on the host machine.

2. Host Network

To share the host’s network namespace with the container:

docker run --network host imagename

🌐 Accessing Container Ports from Another Container

To allow inter-container communication, you need to create and use a custom Docker network.

Steps:

1. Create a Docker Network

docker network create my_network

2. Run Containers in the Same Network

docker run --network my_network --name container1 imagename
docker run --network my_network --name container2 imagename

3. Access One Container from Another

Use the container name and port to access one container from another:

container1:port

Example Scenario

  1. Create a Network
docker network create my_network
  1. Run First Container
docker run --network my_network --name webserver -d nginx
  1. Run Second Container
docker run --network my_network --name client -it alpine /bin/sh
  1. Access Web Server from Client Inside the client container, use the following commands to access the web server:
wget -qO- http://webserver

or

curl http://webserver

📝 Important Notes

  • Container-to-Container Access: Containers in the same custom network can communicate using their names as hostnames.
  • Host Machine Access: Containers cannot directly access the host machine’s ports.
  • External Access: Use port mapping or host networking to access container ports from outside the host machine.

List Docker Networks

To list all available Docker networks and verify the existence of custom networks:

docker network ls