What should I do if docker containers cannot communicate with each other?

Docker is a popular open source containerization platform that makes it easier for developers and operators to create, deploy and run applications. Docker achieves isolation by running multiple virtual containers on a single host, each of which can run a different application or service.

However, sometimes we encounter such a problem: multiple Docker containers are deployed on the same host, and the containers cannot access each other. This is very troublesome for development and operation and maintenance personnel, because communication between containers is the key to the normal operation of applications.

This article will discuss the reasons why Docker containers cannot communicate with each other and provide solutions.

1. Container network configuration issues

Each Docker container is assigned an IP address by default, and this IP address can only be accessed on the host machine. If multiple containers are assigned the same IP address, the containers cannot access each other. At this time, we need to reconfigure the network address of the container.

First, we can use the docker network ls command to view Docker’s network configuration. For example, we can use the following command to view the default network information:

$ docker network ls

Next, we can use the docker network inspect command to view the network configuration of the container, for example:

$ docker network inspect bridge

If the container If there is a problem with the network configuration, we can use the following command to restart the container and specify the IP address:

$ docker run -itd --name mycontainer1 --network mynetwork --ip 172.18.0.10 myimage

In this command, we specify the name, network name and IP address of the container.

2. Security group configuration issues

Another reason why Docker containers cannot access each other is that the security group is not configured correctly. A security group is a virtual firewall that controls network traffic entering and leaving a container. If the security group rules of a container do not allow other containers to access, then the containers cannot access each other.

We can use the network settings in Docker to configure security group rules. For example, we can use the following command to create a new network and specify the communication rules between containers:

$ docker network create --subnet 172.18.0.0/16 mynetwork

In this command, we create a new network named mynetwork and specify The subnet of this network.

Next, we can use the docker run command to start the containers and add them to the network. For example, we can use the following command to start two containers and add them to the mynetwork network:

$ docker run -d --name mycontainer1 --network mynetwork --ip 172.18.0.2 myimage
$ docker run -d --name mycontainer2 --network mynetwork --ip 172.18.0.3 myimage

In this command, we start two containers respectively and assign them to the mynetwork network . Note that we specify the IP address of each container to ensure they can communicate with each other.

Finally, we can use the iptables command to configure security group rules, for example:

$ iptables -I DOCKER-USER -i mynetwork ! -s 172.18.0.0/16 -d 172.18.0.0/16 -j DROP

In this command, we specify a rule that prevents other networks from accessing the container of the mynetwork network. This way, we can ensure that communication between containers is secure.

3. DNS configuration issue

We know that Docker containers can access each other through container names and IP addresses. However, if the container name cannot be resolved to an IP address, the containers cannot access each other.

At this time, we need to configure Docker’s DNS server. Docker uses the default Google DNS server to resolve domain names, but we can also use other DNS servers.

To configure Docker’s DNS server, we can edit Docker’s daemon.json file. For example, in a Linux system, we can use the following command to open this file:

$ sudo vi /etc/docker/daemon.json

In this file, we can specify the DNS server of Docker. For example:

{
  "dns": ["8.8.8.8", "8.8.4.4"]
}

In this configuration, we specify two DNS servers, which are Google's DNS servers.

After completion, we can restart the Docker service to make the configuration take effect:

$ sudo systemctl restart docker

Now, we can use the name of the Docker container to access other containers.

Conclusion

The problem that Docker containers cannot communicate with each other may be a problem with the container network configuration, security group configuration or DNS configuration. By checking these configurations and making necessary changes, we can easily resolve this issue.

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