What should I do if centos cannot access the docker host?

For developers who need to use Docker to build a container environment, using Docker on the Linux operating system is a good choice. However, some users may encounter some problems when using Docker on CentOS, one of which is the inability to access Docker through the host machine. This article will explain why this problem occurs and provide some possible solutions.

1. Cause of the problem

After installing Docker on CentOS, it will create a default virtual bridge named "docker0". Each Docker container will connect to this bridge and use an IP address from the "docker0" IP address range. In this way, the host and Docker containers can communicate through the shared "docker0" bridge. However, because the CentOS firewall is not configured correctly by default, access to the Docker host is not possible. This is because the CentOS firewall blocks incoming packets from other IP address ranges.

2. Solution

1. Configure the firewall
   First, you need to configure the firewall on CentOS to allow packets from the Docker bridge address range. Here are the commands to configure the firewall:

sudo firewall-cmd --permanent --zone=trusted --add-interface=docker0
sudo firewall-cmd --permanent --zone=trusted --add-source=172.17.0.0/16
sudo firewall-cmd --permanent --zone=trusted --add-source=172.18.0.0/16
sudo firewall-cmd --reload

In the above command, we defined a "trusted" zone and added the "docker0" bridge. Additionally, we allow packets from the "172.17.0.0/16" and "172.18.0.0/16" IP address ranges to enter this zone. Finally, use the "firewall-cmd --reload" command to ensure the new firewall rules take effect immediately.

2. Set up Docker DNS
   Similar to firewall issues, Docker containers cannot resolve DNS names correctly. This way, even though the container is connected to the host, it still cannot access the external network. One way to solve this problem is to manually configure DNS in the Docker container.

To do this, you need to edit the Docker configuration file located under the path "/etc/docker/daemon.json" and add the following content:

"dns": ["8.8.8.8", "8.8.4.4"]

This will be inside the container Set up Google DNS to help Docker resolve DNS names.

3. Check Docker version and update
   In addition to the above two methods, you should also ensure that the Docker version you are using has been upgraded to the latest version. If your Docker version is too old, you may experience unpredictable behavior or errors.

To check your Docker version, type the following command:

$ docker version

If you are running an older version of Docker, you can upgrade using the following command:

$ sudo yum update docker

Or, if you install it using the Yum repository officially recommended by Docker, the following command can be used to upgrade Docker:

$ sudo yum install docker-ce

4. Check the Docker network configuration
   Finally, you should check the Docker network configuration to ensure All settings are correct. Please type the following command:

$ docker network ls

This will list all Docker networks. Please make sure that all networks are not related to "docker0" and are configured correctly.

Summary

Using Docker on CentOS can provide a convenient container environment. However, some users may encounter some different problems. One of the most common ones is the inability to pass the host Visit Docker. This article discusses solutions to this problem, including firewall configuration, Docker DNS settings, Docker version updates, and inspection of Docker network settings. If you are facing this issue on CentOS, you can try any of the above methods to resolve it.

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