How to solve the problem that docker’s new port is blocked

Docker is an open source containerization platform that allows developers to easily create, deploy and run applications for fast and reliable delivery. It enables applications to be quickly deployed and run in different environments by packaging the application and required components into an independent container. However, when using Docker, some problems will also arise, such as the problem of new ports being blocked.

Docker allows us to map the port inside the container to the port on the host when running the container, so that we can access the service on that port. When we change the internal port of the container, sometimes we may encounter the problem that the new port is unreachable. Below, let’s explore the possible causes and solutions to this problem.

1. The service in the container is not started correctly

When we change the internal port of the container, we need to ensure that the service in the container has been started correctly and is listening on the new port. You can check whether the service has started successfully by running the following command:

docker ps  # 查看容器是否处于运行状态

docker logs <container_name>  # 查看容器日志，看是否有服务启动的相关信息

If you find that the service does not start properly, you may need to check the Dockerfile or startup script for errors or missing, or run some other debugging commands.

2. In-container firewall or security group settings

Sometimes firewall or security group settings inside a container may prevent access to services within the container from ports on the host. In this case, we need to manually open the port inside the container or change the configuration file.

You can view the network settings of the container through the following command:

docker exec -it <container_name> bash
ip addr

Inside the container, you can use the iptables command to open the port:

iptables -I INPUT -p tcp --dport <container_port> -j ACCEPT

Or directly modify the port in the configuration file information and then restart the container.

3. Host Firewall or Network Proxy Settings

Sometimes the firewall or proxy settings on the host may prevent access to services inside the container from ports on the host. We need to check the host's firewall settings to make sure the new port is allowed through.

If you are using a network proxy, you may need to change the proxy settings to allow access to services inside the container from the new port.

4. The host port is occupied

When we change the internal port of the container, we need to ensure that the port on the host is not occupied. You can check the port occupancy on the host through the following command:

netstat -tlnp | grep <host_port>

If you find that the port is occupied, you may need to stop the process occupying the port or change the mapped port of the container.

Summary

The new port being blocked is a common problem encountered when using Docker. There may be many reasons for this problem, including the service in the container not starting correctly, the firewall or security group settings in the container, the host firewall or network proxy settings, the host port is occupied, etc. By analyzing the cause of the problem and taking corresponding solutions, we can easily solve this problem, allowing us to use the Docker platform more efficiently.

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