一、Dockerfile引用自建dns服务
一）指定DNS服务器
1、docker 启动容器dns会读取宿主机的配置
]# cat /etc/resolv.conf # Generated by NetworkManager search test.com nameserver 114.114.114.114 # docker images|grep centos centos 7 eeb6ee3f44bd 2 weeks ago 204MB
基于centos7打了一个镜像，dockerfile如下：
FROM centos:7 RUN echo -e "search test.com\nnameserver 8.8.8.8">/etc/resolv.conf && echo "`sed 'p' -n /etc/resolv.conf`" RUN echo "`sed 'p' -n /etc/resolv.conf`" RUN yum install -y wget && yum clean all
2、尝试使用RUN 直接修改resolv.conf文件失败，因为在容器启动时的操作，会被docker run指定的参数覆盖
这里指定了dns配置，然后再下面打印了一下，效果如下：
# docker build -t test:dns . Sending build context to Docker daemon 2.048kB Step 1/4 : FROM centos:7 ---> eeb6ee3f44bd Step 2/4 : RUN echo "search test.com\nnameserver 8.8.8.8">/etc/resolv.conf && echo "`sed 'p' -n /etc/resolv.conf`" ---> Running in f3ec71d93b5f search test.com nameserver 8.8.8.8 Removing intermediate container f3ec71d93b5f ---> c2a5a9f9985f Step 3/4 : RUN echo "`sed 'p' -n /etc/resolv.conf`" ---> Running in 08d22ef982d6 # Generated by NetworkManager search test.com nameserver 114.114.114.114 Removing intermediate container 08d22ef982d6 ---> 9c01241bad24 Step 4/4 : RUN yum install -y wget ---> Running in e831037cf89b Loaded plugins: fastestmirror, ovl Determining fastest mirrors * base: mirrors.tuna.tsinghua.edu.cn * extras: mirrors.bupt.edu.cn * updates: mirrors.tuna.tsinghua.edu.cn Resolving Dependencies --> Running transaction check ---> Package wget.x86_64 0:1.14-18.el7_6.1 will be installed --> Finished Dependency Resolution Dependencies Resolved ================================================================================ Package Arch Version Repository Size ================================================================================ Installing: wget x86_64 1.14-18.el7_6.1 base 547 k Transaction Summary ================================================================================ Install 1 Package Total download size: 547 k Installed size: 2.0 M Downloading packages: warning: /var/cache/yum/x86_64/7/base/packages/wget-1.14-18.el7_6.1.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID f4a80eb5: NOKEY Public key for wget-1.14-18.el7_6.1.x86_64.rpm is not installed Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7 Importing GPG key 0xF4A80EB5: Userid : "CentOS-7 Key (CentOS 7 Official Signing Key) <security@centos.org>" Fingerprint: 6341 ab27 53d7 8a78 a7c2 7bb1 24c6 a8a7 f4a8 0eb5 Package : centos-release-7-9.2009.0.el7.centos.x86_64 (@CentOS) From : /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7 Running transaction check Running transaction test Transaction test succeeded Running transaction Installing : wget-1.14-18.el7_6.1.x86_64 1/1 install-info: No such file or directory for /usr/share/info/wget.info.gz Verifying : wget-1.14-18.el7_6.1.x86_64 1/1 Installed: wget.x86_64 0:1.14-18.el7_6.1 Complete! Removing intermediate container e831037cf89b ---> 747d8f4768dd Successfully built 747d8f4768dd Successfully tagged test:dns # docker run --rm -it --name testdns test:dns /bin/sh -c "cat /etc/resolv.conf" # Generated by NetworkManager search test.com nameserver 114.114.114.114
从这可以看到，在第二步设置dns，第三步打印出的结果还是读取的宿主机的配置。
这里分析了一下原因：由于以Dockerfile的形式构建镜像，其中每一个指令都是一层，它的过程是基于基础镜像运行一个容器，然后按指令执行，第一个指令完毕后，commit为一个新的镜像层，docker再运行一个基于新镜像的容器，执行下一步指令，直到结束。这样的好处是，假如共有四个指令，第三个失败了，那么可以基于第二个指令生成的镜像继续操作，不需要再从头操作一遍，节省资源。
那么，由此可见，我在第二步设置完，docker提交一个镜像，docker接下来运行这个镜像的时候，会读取宿主机配置，所以我的设置也就不生效了。使用docker commit构建镜像也是一样的效果。
如果想让它生效，需要在启动命令里指定。
3、使用ENTRYPOINT在容器启动时的操作，而且不会被docker run指定的参数覆盖。
]# cat Dockerfile FROM centos:7 ENTRYPOINT echo -e "search test.com\nnameserver 8.8.8.8">/etc/resolv.conf && echo "`sed 'p' -n /etc/resolv.conf`" && tail -f /dev/null RUN echo "`sed 'p' -n /etc/resolv.conf`" RUN yum install -y wget && yum clean all 使用entrypoint指令，这个指令的作用是在容器启动时的操作，而且不会被docker run指定的参数覆盖。（当然也可以覆盖，在启动的时候docker run --entrypoint= 可以这样指定临时的启动命令来覆盖dockerfile里的entrypoint） # docker build -t test:dns . Sending build context to Docker daemon 2.048kB Step 1/4 : FROM centos:7 ---> eeb6ee3f44bd Step 2/4 : ENTRYPOINT echo -e "search test.com\nnameserver 8.8.8.8">/etc/resolv.conf && echo "`sed 'p' -n /etc/resolv.conf`" && tail -f /dev/null ---> Running in 361ec0e4cf3b Removing intermediate container 361ec0e4cf3b ---> e062358b2b4d Step 3/4 : RUN echo "`sed 'p' -n /etc/resolv.conf`" ---> Running in 1272cc8f69cb # Generated by NetworkManager search test.com nameserver 114.114.114.114 Removing intermediate container 1272cc8f69cb ---> 08b7026e183e Step 4/4 : RUN yum install -y wget ---> Running in 5346d673e0b7 Loaded plugins: fastestmirror, ovl Determining fastest mirrors * base: mirrors.tuna.tsinghua.edu.cn * extras: mirrors.bfsu.edu.cn * updates: mirrors.tuna.tsinghua.edu.cn Resolving Dependencies --> Running transaction check ---> Package wget.x86_64 0:1.14-18.el7_6.1 will be installed --> Finished Dependency Resolution Dependencies Resolved ================================================================================ Package Arch Version Repository Size ================================================================================ Installing: wget x86_64 1.14-18.el7_6.1 base 547 k Transaction Summary ================================================================================ Install 1 Package Total download size: 547 k Installed size: 2.0 M Downloading packages: warning: /var/cache/yum/x86_64/7/base/packages/wget-1.14-18.el7_6.1.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID f4a80eb5: NOKEY Public key for wget-1.14-18.el7_6.1.x86_64.rpm is not installed Retrieving key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7 Importing GPG key 0xF4A80EB5: Userid : "CentOS-7 Key (CentOS 7 Official Signing Key) <security@centos.org>" Fingerprint: 6341 ab27 53d7 8a78 a7c2 7bb1 24c6 a8a7 f4a8 0eb5 Package : centos-release-7-9.2009.0.el7.centos.x86_64 (@CentOS) From : /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7 Running transaction check Running transaction test Transaction test succeeded Running transaction Installing : wget-1.14-18.el7_6.1.x86_64 1/1 install-info: No such file or directory for /usr/share/info/wget.info.gz Verifying : wget-1.14-18.el7_6.1.x86_64 1/1 Installed: wget.x86_64 0:1.14-18.el7_6.1 Complete! Removing intermediate container 5346d673e0b7 ---> e8c04c88aed2 Successfully built e8c04c88aed2 Successfully tagged test:dns # docker run --rm -it --name testdns test:dns /bin/sh -c "cat /etc/resolv.conf" search test.com nameserver 8.8.8.8
可以看到，使用entrypoint之后，这个指定的配置便生效了。