Docker – CentOS 7 container on Ubuntu

Create base image from current CentOS 7 system with one command:

tar --numeric-owner --exclude=/proc --exclude=/sys --exclude=/boot --exclude=/usr/lib/firmware --exclude=/usr/lib/modules --exclude=/usr/lib/udev -cvf centos7.tar /

You will have file size 565M from CentOS 7 minimal install.

More optimal way: https://github.com/docker/docker/blob/master/contrib/mkimage-yum.sh
As result you'll have 247M file.

Copy centos7.tar to Ubuntu and install docker:

apt-get install docker.io

CentOS 7 has issues if Storage Driver AUFS. Recommended driver: devicemapper.

# docker info | grep Storage
Storage Driver: aufs

# echo 'DOCKER_OPTS="--storage-driver=devicemapper"' >> /etc/default/docker

# service docker restart

Import new docker image:

cat centos7.tar | docker import - c7-v0

Check:

# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
c7-v0               latest              29a8eab387cf        11 seconds ago      571.3 MB

# docker run -i -t --rm c7-v0 cat /etc/redhat-release
CentOS Linux release 7.2.1511 (Core) 

Options:

    --rm Automatically remove the container when it exits.
    -t Allocate a pseudo-TTY. You cannot run bash without this option for example. 
    -i Keep STDIN open even if not attached.

Understanding how commit works.

Create 3 files. And 3 containers will be created:

docker run -i -t c7-v0 touch /root/1.txt
docker run -i -t c7-v0 touch /root/2.txt
docker run -i -t c7-v0 touch /root/3.txt

List containers: docker ps -a

# docker ps -a
CONTAINER ID        IMAGE               COMMAND               CREATED              STATUS                          PORTS               NAMES
6e5123730f7b        c7-v0:latest        "touch /root/3.txt"   59 seconds ago       Exited (0) 58 seconds ago                           berserk_colden        
313e04322a2f        c7-v0:latest        "touch /root/2.txt"   About a minute ago   Exited (0) About a minute ago                       romantic_colden       
8f48f50290cb        c7-v0:latest        "touch /root/1.txt"   About a minute ago   Exited (0) About a minute ago                       suspicious_stallman   

Create a new image from a container's changes: docker commit

# docker commit 313e04322a2f

# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
<none>              <none>              268ec44b328b        8 seconds ago       571.3 MB
c7-v0               latest              29a8eab387cf        4 minutes ago       571.3 MB

# docker tag 268ec44b328b c7-v1

# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED              VIRTUAL SIZE
c7-v1               latest              268ec44b328b        About a minute ago   571.3 MB
c7-v0               latest              29a8eab387cf        5 minutes ago        571.3 MB

Check new image:

# docker run -i -t --rm c7-v1 ls /root/
2.txt  anaconda-ks.cfg

Only file 2.txt was commited.

Cleaning

Now we can delete unused containes and image.

Remove one or more containers: docker rm
Remove one or more images: docker rmi

# docker rm 6e5123730f7b
# docker rm 313e04322a2f
# docker rm 8f48f50290cb

# docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

# docker rmi c7-v0

# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
c7-v1               latest              268ec44b328b        6 minutes ago       571.3 MB

Install Apache HTTPD:

docker run -i -t c7-v1 bash
yum -y install httpd
exit

# docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                     PORTS               NAMES
ce2c30eae819        c7-v1:latest        "bash"              10 minutes ago      Exited (1) 8 seconds ago                       focused_thompson    

If you want to start the same container again and install some thing in addition:

docker start ce2c30eae819
docker attach ce2c30eae819

Let's create next image release:

# docker ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS                      PORTS               NAMES
ce2c30eae819        c7-v1:latest        "bash"              24 minutes ago      Exited (0) 30 seconds ago                       focused_thompson    
# docker commit ce2c30eae819
# docker tag 8d8cc96460c1 c7-v2
# docker rm ce2c30eae819
# docker rmi c7-v1

Run HTTPD and SSHD:

docker run -i -t -p 2200:22 -p 8000:80 c7-v2
/usr/sbin/sshd
/usr/sbin/httpd
# ps aux 
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0 115384  3324 ?        Ss   13:16   0:00 bash
root        19  0.0  0.0  82552  2664 ?        Ss   13:17   0:00 /usr/sbin/sshd
root        42  0.0  0.0 221908  6036 ?        Ss   13:18   0:00 /usr/sbin/httpd
apache      43  0.0  0.0 221908  6012 ?        S    13:18   0:00 /usr/sbin/httpd
apache      44  0.0  0.0 221908  6012 ?        S    13:18   0:00 /usr/sbin/httpd
apache      45  0.0  0.0 221908  6012 ?        S    13:18   0:00 /usr/sbin/httpd
apache      46  0.0  0.0 221908  6012 ?        S    13:18   0:00 /usr/sbin/httpd
apache      47  0.0  0.0 221908  6012 ?        S    13:18   0:00 /usr/sbin/httpd
root        48  0.0  0.0 139500  3376 ?        R+   13:18   0:00 ps aux

To detach the tty without exiting the shell, use the escape sequence Ctrl+p + Ctrl+q.

Test SSHD:

ssh 127.0.0.1 -p 2200

Test HTTPD:

# curl http://127.0.0.1:8000 | grep Test
. . .
<title>Apache HTTP Server Test Page powered by CentOS</title>
. . .

Network

Ports 2200 and 8000 available outside. By default docker adds iptables rules:

# iptables -t nat -L -n
Chain PREROUTING (policy ACCEPT)
target prot opt source destination 
DOCKER all -- 0.0.0.0/0 0.0.0.0/0 ADDRTYPE match dst-type LOCAL

Chain INPUT (policy ACCEPT)
target prot opt source destination 

Chain OUTPUT (policy ACCEPT)
target prot opt source destination 
DOCKER all -- 0.0.0.0/0 !127.0.0.0/8 ADDRTYPE match dst-type LOCAL

Chain POSTROUTING (policy ACCEPT)
target prot opt source destination 
MASQUERADE all -- 172.17.0.0/16 0.0.0.0/0 
MASQUERADE tcp -- 172.17.0.2 172.17.0.2 tcp dpt:80
MASQUERADE tcp -- 172.17.0.2 172.17.0.2 tcp dpt:22

Chain DOCKER (2 references)
target prot opt source destination 
DNAT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:8000 to:172.17.0.2:80
DNAT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:2200 to:172.17.0.2:22

Links:

  • https://docs.docker.com/
  • http://developerblog.redhat.com/2014/05/15/practical-introduction-to-docker-containers/

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