How Docker implements communication between containers with different domain names

In the process of using Docker for container deployment, a common problem is how to communicate between different containers. Especially when the domain names of these containers are different, how to let them find each other and interact becomes a problem that needs to be solved.

This article will introduce how to implement communication between containers with different domain names in Docker. First, we need to understand Docker’s network model.

Docker’s network model

Docker supports multiple network models, including Bridge mode, Host mode, Overlay mode, MACVLAN mode, etc. Among them, the most widely used in containerized deployment is the Bridge mode.

In Bridge mode, Docker will create a virtual bridge (bridge) for each container and assign an IP address to each container. These containers can communicate through this virtual bridge. At the same time, Docker also supports connecting containers to the same user-defined network bridge, so that they can communicate directly through this user-defined network bridge.

However, in practical applications, we found that the problem that needs to be solved is how to let the containers find each other and interact when their domain names are different.

Solution

In order to solve the problem of different domain names between containers, we can use DNS service.

Docker’s own DNS service

Docker comes with a DNS service that can perform domain name resolution for containers. Each container sends a DNS request to a Docker-assigned DNS server and uses the container's name as the hostname in DNS resolution.

For example, in a Docker network, there are two containers web1 and web2. Their IP addresses are 172.17.0.2 and 172.17.0.3 respectively. If we use the default Bridge mode, Docker will assign a domain name to the two containers, and this domain name can be used to communicate when accessed within the container. The format of this domain name is a combination of the container name and the Docker network name, similar to web1_default and web2_default. Here default is the name of the Docker network.

So, if we use the ping command in web1 to test the connectivity of web2, we can use the following command:

ping web2_default

This command will send a DNS request to the DNS server assigned by Docker and use web2_default is resolved as a hostname.

Customized DNS service

If you want to customize the DNS service, you can use Docker's --dns option to specify the address of the DNS server. For example, if we want to use Google's public DNS server 8.8.8.8, we can start the container with the following command:

docker run --dns 8.8.8.8 myimage

This will use 8.8.8.8 as the default DNS server for the container. Inside the container, this DNS server can be used to resolve domain names.

If we want to use a custom DNS in a Docker container, we can create a Dockerfile and add the custom DNS configuration to the container. For example, in Ubuntu, you can use the following command to modify the DNS configuration file:

RUN echo 'nameserver 8.8.8.8' > /etc/resolv.conf

This will add the address of the DNS server to the /etc/resolv.conf file in the container. Inside the container, this DNS server can be used to resolve domain names.

Summary

Implementing communication between containers with different domain names in Docker is a problem that must be solved. By using Docker's own DNS service or a custom DNS service, we can easily solve this problem and allow smooth communication between containers. At the same time, for network security, we can also customize DNS services to enhance security and protect sensitive data in containers.

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