Reasons and solutions why docker cannot resolve domain names

Docker is an open source application container engine that enables developers to build, deploy and run applications. Containerized applications are often made up of multiple components that need to communicate with each other to complete tasks. However, when using Docker, sometimes you encounter the problem that the domain name cannot be resolved, which results in the inability to communicate between containers or the inability to connect to external services. In this article, we'll cover some common causes and solutions to help you resolve this issue quickly.

1. DNS resolution failed

In Docker containers, domain name resolution is usually completed through the DNS server. When Docker cannot resolve a domain name, it may be due to a DNS server failure or network instability. In order to solve this problem, you can do the following:

1.1 Check the DNS server configuration

First, you can check whether the DNS configuration of Docker is correct. You can run the following command in the container:

cat /etc/resolv.conf

to check whether the DNS server is configured correctly.

If the DNS server is misconfigured, you can specify the correct DNS server address by appending the --dns option when the Docker daemon is started. For example:

dockerd --dns 8.8.8.8

2.2 Modify the container DNS configuration

Docker also provides a method to set the container DNS configuration. You can specify the DNS server address when creating the container using the --dns option. For example:

docker run --dns 8.8.8.8 nginx

In addition, you can also configure the DNS server address in the /etc/docker/daemon.json file. For example:

{
  "dns": ["8.8.8.8", "8.8.4.4"]
}

After restarting the Docker daemon, these configurations will take effect.

2. Network connection problem

Docker containers run on the host. If the host network connection is unstable, the Docker container may not be able to access external services or communicate with other containers. communication. The following are several possible solutions:

2.1 Check the host network connection

You can run the ping command on the host to test the network connection. For example:

ping www.baidu.com

If you cannot connect, you can try to restart the host or contact the network administrator to solve the problem.

2.2 Check the Docker bridge configuration

Docker has a default network mode called bridge (bridge), and the container communicates with the host and other containers through this bridge. If the bridge configuration is incorrect, the container may not be able to resolve the domain name, so you can view the Docker bridge configuration on the host machine and make adjustments. You can check it with the following command:

docker network inspect bridge

If there is a problem, you can use the following command to re-create the bridge:

docker network create bridge

3. Domain name resolution cache problem

In In some cases, problems with domain name resolution caching may occur when communicating between containers and external services. This may cause the domain name to fail to be resolved or the DNS resolution results to be delayed. If such a problem occurs, you can perform the following operations:

3.1 Clear the DNS cache

You can execute the following command in the container to clear the DNS cache:

/etc/init.d/nscd restart

If in the host If you encounter this problem on the host, you can try the following command:

sudo service dns-clean restart

3.2 Modify the DNS cache threshold

If there is a delay in the DNS resolution results, you can try to modify the DNS cache threshold. You can run the following command in the container:

echo 2000 > /proc/sys/net/ipv4/neigh/default/gc_stale_time

This will set the DNS cache threshold to 2000 milliseconds.

Summary

When using Docker, it is very common to encounter the problem of being unable to resolve domain names. This problem may be caused by various factors such as DNS server configuration, network connection, or DNS resolution cache. There are many ways to solve this problem, which can be solved by checking the DNS server configuration, modifying the container DNS configuration, checking the host network connection, checking the Docker bridge configuration, clearing the DNS cache, or modifying the DNS cache threshold. I hope the solutions provided in this article can help you solve the problem quickly and make better use of the Docker container engine.

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