Master Docker Container Networking: IP, Ports, and Link Mechanism Explained

System Environment

Server: CentOS Linux release 7.4.1708 (Core); Docker version 20.10.12; CPU architecture x86_64.

Docker Container Interconnection Overview

2.1 Three Ways to Connect Containers on the Same Host

When multiple Docker containers run on the same host, they can communicate via:

Using the container's IP address directly. This approach hard‑codes IPs, making migration difficult, and IPs may change after a restart unless a static IP is assigned.

Using the host's IP address combined with the container's exposed port. This method is simple but limited to services listening on the published ports.

Using Docker's --link mechanism, which allows one container to discover another by name and securely passes connection information.

2.2 Using docker --link : Precautions

When employing --link, keep in mind:

The linked container must be running.

The container that creates the link also needs the target container to be running.

The host IP of the linked container does not need to be fixed; Docker updates /etc/hosts with a new alias each time the container is created.

2.3 How --link Works

The --link option adds an entry to the target container's /etc/hosts file, mapping a chosen name to the container's IP address, enabling name‑based communication.

Practical Examples

3.1 Communicating via Container IP

Create a MySQL container:

docker run -dit --restart=always --name=mysql5 -e MYSQL_ROOT_PASSWORD=123456 -p 3306:3306 hub.c.163.com/library/mysql:latest

Inspect its IP address:

docker inspect mysql5 | grep -i ipaddr
# Output includes "IPAddress": "172.17.0.4"

Launch a CentOS container and ping the MySQL container:

docker run -it --restart=always --name=centos7 hub.c.163.com/library/centos:latest ping 172.17.0.4

3.2 Communicating via --link

Docker allows inter‑container communication by default; it can be disabled with -icc=false. When disabled, --link name:alias is required.

Create a CentOS container linked to the MySQL container:

docker run -it --restart=always --name=c7 --link mysql5:mysql hub.c.163.com/library/centos:latest ping mysql

Ping results show successful communication using the alias mysql.

Enter the linked container and test connectivity:

docker exec -it c7 /bin/bash
ping mysql

Install a MySQL client (MariaDB) inside the CentOS container: yum -y install mariadb Connect to the MySQL server using the container IP: mysql -uroot -p123456 -h 172.17.0.4 After logging in, you can list databases, e.g., show databases;.