docker中使用systemd

由于以下几个原因，docker的官方centos镜像中没有提供systemd服务：

• systemd requires the CAP_SYS_ADMIN capability.
  This means running docker with --privileged. Not good for a base image.
• systemd requires access to the cgroups filesystem.
• systemd has a number of unit files that don’t matter in a container, and they cause errors if they’re
  not removed

但在可控环境下，我们还是希望使用systemd来管理我们的服务，如何开启systemd呢？

首先创建一个Dockerfile用来创建systemd的base image：

FROM centos:latest

MAINTAINER feisky

RUN yum -y install systemd systemd-libs

#RUN yum -y update

RUN yum clean all; \

(cd /lib/systemd/system/sysinit.target.wants/; for i in *; \

do [ $i == systemd-tmpfiles-setup.service ] || rm -f $i; done); \

rm -f /lib/systemd/system/multi-user.target.wants/*;\

rm -f /etc/systemd/system/*.wants/*;\

rm -f /lib/systemd/system/local-fs.target.wants/*; \

rm -f /lib/systemd/system/sockets.target.wants/*udev*; \

rm -f /lib/systemd/system/sockets.target.wants/*initctl*; \

rm -f /lib/systemd/system/basic.target.wants/*;\

rm -f /lib/systemd/system/anaconda.target.wants/*;

VOLUME [ "/sys/fs/cgroup" ]

CMD ["/usr/sbin/init"]

执行 docker build --rm -t centos:systemd .来创建一个systemd base image:

[[email protected] ~]# docker build --rm -t centos:systemd .

Sending build context to Docker daemon 490 MB

Sending build context to Docker daemon

Step 0 : FROM centos:latest

---> b157b77b1a65

Step 1 : MAINTAINER feisky

---> Using cache

---> 4add1a12ff67

Step 2 : RUN yum -y install systemd systemd-libs

---> Using cache

---> 2bc5274f7dd2

Step 3 : RUN yum clean all; (cd /lib/systemd/system/sysinit.target.wants/; for i in *; do [ $i == systemd-tmpfiles-setup.service ] || rm -f $i; done); rm -f /lib/systemd/system/multi-user.target.wants/*;rm -f /etc/systemd/system/*.wants/*;rm -f /lib/systemd/system/local-fs.target.wants/*;
rm -f /lib/systemd/system/sockets.target.wants/*udev*; rm -f /lib/systemd/system/sockets.target.wants/*initctl*; rm -f /lib/systemd/system/basic.target.wants/*;rm -f /lib/systemd/system/anaconda.target.wants/*;

---> Running in d132481218d7

Loaded plugins: fastestmirror

Cleaning repos: base extras updates

Cleaning up everything

Cleaning up list of fastest mirrors

---> 7a9492819396

Removing intermediate container d132481218d7

Step 4 : VOLUME /sys/fs/cgroup

---> Running in d5a93c8ee296

---> 10b06676ea4d

Removing intermediate container d5a93c8ee296

Step 5 : CMD /usr/sbin/init

---> Running in 6ab276a1974b

---> b519e838367f

Removing intermediate container 6ab276a1974b

Successfully built b519e838367f

下面就可以基于该镜像来创建http服务的镜像了，当然这儿就需要另外一个Dockerfile：

FROM centos:systemd

MAINTAINER feisky

RUN yum -y install httpd; systemctl enable httpd.service

RUN yum -y install mariadb-server mariadb; systemctl enable mariadb.service

RUN yum clean all

EXPOSE 80

CMD ["/usr/sbin/init"]

执行docker build -t http_server .来创建http镜像。

镜像创建好后可以通过下面的命令来启动http docker（注意需要--privileged）：

docker run --privileged -d -v /sys/fs/cgroup:/sys/fs/cgroup:ro -h server.http.local -p 8081:80 -t http_server

通过下面的命令可以进入docker的shell（其中3df7d8c59f1e为容器ID）：

nsenter
--mount --uts --ipc --net --pid --target $(docker inspect --format "{{.State.Pid}}" "605bae76456d")

或者可以通过下面的命令来在容器内执行命令：

# docker exec 605bae76456d ps -ef

UID PID PPID C STIME TTY TIME CMD

root 1 0 0 07:33 ? 00:00:00 /usr/sbin/init

root 15 1 0 07:33 ? 00:00:00 /usr/lib/systemd/systemd-journald

root 16 1 0 07:33 ? 00:00:00 /usr/sbin/httpd
-DFOREGROUND

apache 18 16 0 07:33 ? 00:00:00 /usr/sbin/httpd
-DFOREGROUND

apache 19 16 0 07:33 ? 00:00:00 /usr/sbin/httpd
-DFOREGROUND

apache 20 16 0 07:33 ? 00:00:00 /usr/sbin/httpd
-DFOREGROUND

apache 21 16 0 07:33 ? 00:00:00 /usr/sbin/httpd
-DFOREGROUND

apache 22 16 0 07:33 ? 00:00:00 /usr/sbin/httpd
-DFOREGROUND

dbus 187 1 0 07:41 ? 00:00:00 /bin/dbus-daemon
--system --address=systemd: --nofork --nopidfile --systemd-activation

mysql 248 1 0 07:41 ? 00:00:00 /bin/sh /usr/bin/mysqld_safe
--basedir=/usr

mysql 404 248 1 07:41 ? 00:00:00 /usr/libexec/mysqld
--basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin --log-error=/var/log/mariadb/mariadb.log --pid-file=/var/run/mariadb/mariadb.pid --socket=/var/lib/mysql/mysql.sock

root 458 0 0 07:41 ? 00:00:00 ps -ef

访问服务器的8081端口发现Apache正常启动起来了。