Each time Docker starts a container, the IP and hosts specified operations

Preface

Every time you use Docker to start a Hadoop cluster, you need to rebind the network card, fix the IP, and modify the /etc/hosts file. This is very troublesome, so I want to explore the cause and optimization.

1. Reasons

/etc/hosts, /etc/resolv.conf and /etc/hostname, these three files in the container do not exist in the image. When starting the container, these files are mounted into the container in the form of mount. Therefore, if these files are modified in the container, the modified parts will not exist in the top layer of the container, but will be written directly to these three physical files.

Why do the changes disappear after restart? The reason is: every time Docker starts a container, it rebuilds a new /etc/hosts file. Why is this? The reason is: the container is restarted, the IP address is changed, and the original IP address in the hosts file is invalid. Therefore, the hosts file should be modified, otherwise dirty data will be generated.

2. Solution

Specify the IP and hostname each time you start the container, and add hosts to /etc/hosts. The command is as follows:

docker run -itd --name hadoop0 --hostname hadoop0 --net network_my --ip 192.168.10.30 --add-host hadoop1:192.168.10.31 --add-host hadoop2:192.168.10.32 -d -P -p 50070:50070 -p 8088:8088 hadoop:master

Docker network mode and configuration operations

--hostname : specify hostname;
--net: Specify network mode --ip: Specify IP
--add-host : Specify the host to be added to /etc/hosts

The above commands require docker version 1.9 or above;

After starting the container, enter the container to view /etc/hosts

[root@centos-linux-7 /]# docker exec -it hadoop0 bash
[root@hadoop0 /]# cat /etc/hosts
127.0.0.1 localhost
::1 localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
192.168.10.31 hadoop1
192.168.10.32 hadoop2
192.168.10.30 hadoop0
[root@hadoop0 /]# 

The above command is too long and can be written as a shell script. There are several other ways mentioned online:

1. Use Dockerfile to build an image

2. Start using docker-compose

3. Modify the configuration file of the environment variables that the docker container starts to load publicly (I didn't find the configuration file for my version)

The first method should not work. Although Dockerfile can set relevant environment variables when building an image, as mentioned earlier, IP, hostname, and /etc/hosts are all reloaded, so they must be specified when the Docker container is started. Those in the image are not acceptable.

The second one, I am not familiar with docker-compose yet, and I will not discuss it in depth for the time being;

The third one is inconvenient for me, because it is public and cannot be personalized for different containers;

To sum up, I think it is most convenient to specify through the command line when the container is started, but the disadvantage is that the command line is too long. But you can just write it as a shell script!

Supplement: Linux Docker sets a fixed container IP (Docker default container IP will change)

1. Create your own network type and specify the network segment

Order

docker network create --subnet=172.18.0.0/16 mynetwork

2. Specify your own network IP when the image is started

Order

docker run -itd -p 5001:5001 --name image name --net mynetwork --ip 172.18.0.2 --privileged=true --restart=always -d image name

3. Restart Docker and view container IP

Restart dockers

Order

service docker restart

View All Containers

Order

docker ps -a

View container information

Order

docker inspect container name

This way the container IP will not change

The above is my personal experience. I hope it can give you a reference. I also hope that you will support 123WORDPRESS.COM. If there are any mistakes or incomplete considerations, please feel free to correct me.