How to dynamically modify container port mapping in Docker

Preface: Docker port mapping is often done by mapping the internal port of the container to the specified port of the host machine through -p during the Docker Run command. Generally speaking, the port corresponding to the container port is determined in advance to be mapped. However, in some cases, you have to temporarily map ports. For example, when running a MySQL container in Docker, the default port is not open. So is there any way to expose the specified port in the running container? Please read below--->

Method 1: Change the Docker configuration file (risky)

To achieve our goal, we need to modify the Docker configuration file. Generally speaking, we need to modify the following files: config.v2.json and hostconfig.json. The default path is /var/lib/docker/containers/<容器名稱> .

First, shut down the Docker service through systemctl stop docker . Then modify the ExposedPorts configuration information in the config.v2.json file to add the container internal port, such as "8080/tcp":{}. Then modify the PortBindings configuration information in hostconfig.json as shown below.

After completing the above configuration, restart the Docker service systemctl restart docker , and restart the specified container to access the corresponding port. (This method is risky and is not recommended for frequent use)

 "Config": {
  "ExposedPorts": {
   // Add internal port 5432 mapping "5432/tcp": {},
   "8080/tcp": {}
  },s
  ...
 },

"PortBindings":{
  // Add internal port and external port 15432
  "5432/tcp":[
   {
    "HostIp":"",
    "HostPort":"15432"
   }
  ],
  "8080/tcp":[
   {
    "HostIp":"",
    "HostPort":"28080"
   }
  ]
 },

Method 2: Iptables port forwarding

The principle of Docker's network port mapping is to achieve port forwarding through Iptables. Based on this principle, we can directly use iptables to forward the port to the target container IP. Port forwarding can be achieved by using the following command. This method relies on Iptables rules. In some scenarios, it may cause Iptables rule conflicts and affect the effective startup of the container.

# Port mapping iptables -t nat -A DOCKER -p tcp --dport <container external port> -j DNAT --to-destination <container ip>:<container internal port>
# Cancel the port mapping rule iptables -t nat -D DOCKER -p tcp -d 0/0 --dport <container external port> -j DNAT --to-destination <container ip>:<container internal port>

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