What should I do if docker cannot resolve the domain name?

Solution: 1. Use "cat /proc/sys/net/ipv4/ip_forward" to check the "/proc" file system and confirm whether the system has turned on the ip forwarding function; 2. Use "echo 1 > / proc/sys/net/ipv4/ip_forward" and modify the content in the file from 0 to 1 to turn on the ip forwarding function; 3. Modify the value of "ip_forward" in the "/etc/sysctl.conf" file to permanently modify ip forwarding .

The operating environment of this tutorial: linux7.3 system, docker version 19.03, Dell G3 computer.

What should I do if docker cannot resolve the domain name?

It keeps reporting unknown host and cannot resolve the domain name. Let’s take a look at the detailed solution:

Solution

1. The IP forwarding function is not turned on by default in the Linux system. To confirm the status of the IP forwarding function,

You can view the /proc file system and use the following command:

For security reasons, Linux systems prohibit data packet forwarding by default.

The so-called forwarding means that when the host has more than one network card, one of them receives the data packet and sends the packet to another network card of the local machine according to the destination IP address of the data packet. The network card continues to send the data according to the routing table. data pack. Configure the ip forwarding function of the Linux system. First ensure the hardware connectivity, and then turn on the system's forwarding function.

cat /proc/sys/net/ipv4/ip_forward. The content of this file is 0, which means that packet forwarding is prohibited. 1 Indicates permission.

2. If the value in the above file is 0, it means that IP forwarding is prohibited;

If it is 1, it means that the IP forwarding function has been turned on. If you want to turn it on With the IP forwarding function, you can directly modify the above file:

echo 1 > /proc/sys/net/ipv4/ip_forward

Modify the content of the file from 0 to 1. Disabling IP forwarding changes 1 to 0.

3. The above command does not save the changes to the IP forwarding configuration. The original value will still be used the next time the system starts.

If you want to permanently modify the IP forwarding , you need to modify the /etc/sysctl.conf file and modify the value of the following line:

net.ipv4.ip_forward = 1

After modification, you can restart the system to make the modification effective, or execute the following command to make the modification effective:

sysctl -p /etc/sysctl.conf

After the above configuration, the IP forwarding function is permanently enabled

Extended knowledge:

After entering the Docker container, configure domain name resolution in the hosts file .

# 进入 docker 容器
docker exec -it  my_web /bin/bash
# 修改 hosts 文件
vi /etc/hosts
# 添加域名解析规则
192.168.99.100  example.test
# 利用 ping 命令验证一下，是否可以 ping 通
ping example.test

If your Docker host does not have the ping command, you can install it; of course, it doesn’t matter if you don’t want to install it. You can use other methods to verify whether the domain name resolution has been resolved.

It should be noted that every time the container (service) is restarted, the newly added configuration in the /etc/hosts file in the container will become invalid, so it needs to be reconfigured.

Recommended learning: "docker video tutorial"

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