Docker利用busybox创建基础镜像(base image)

Docker镜像的首行从FROM alpine之类的镜像开始，但是最初的基础镜像是如何创建的，本文使用一个busybox创建一个基础镜像，相信在此过程中会对docker一些相关的概念有进一步的理解。

什么是基础镜像（base image）

简单来说，基础镜像就是没有From或者FROM scratch开头的Dockerfile所构建出来的镜像。比如alpine，这个很小的linux镜像目前只有4M左右

[root@kong ~]# docker images |grep alpine
docker.io/alpine        latest       3fd9065eaf02    4 months ago    4.15 MB
[root@kong ~]#

它的Dockerfile很简单，只有三行，这就是一个基础镜像,

FROM scratch
ADD rootfs.tar.xz /
CMD ["/bin/sh"]

在接下来的文章中我们将会像alpine那样来创建一个自己的基础镜像。

busybox

概要说明

busybox被称为嵌入式linux的瑞士军刀，这句话是在busybox自己介绍自己的时候提出的(The Swiss Army Knife of Embedded Linux)。busybox整合了很多小的unix下的通用功能到一个小的可执行文件之中，简单来说在unix或者linux下常用的那些功能在这里你都能找到，但是为了busybox的目标：嵌入式的linux，大小对于busybox来说是非常重要的优化要素和限制，这些功能有可能会有所阉割，但是对于一般需求来说已经足够。而alpine就是在busybox基础上增加了自己的包管理工具apk等功能创建了风靡一时的小巧镜像。busybox是用C语言开发的基于GPL的开源项目，目前的稳定版本为1.28.4

宿主机器

[root@kong ~]# uname -a
Linux kong 3.10.0-693.el7.x86_64 #1 SMP Tue Aug 22 21:09:27 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
[root@kong ~]# cat /etc/redhat-release 
CentOS Linux release 7.4.1708 (Core) 
[root@kong ~]#

下载busybox

简单可以直接使用的二进制最新的版本为1.28.1

[root@kong ~]# wget https://busybox.net/downloads/binaries/1.28.1-defconfig-multiarch/busybox-x86_64
--2018-05-25 04:51:20-- https://busybox.net/downloads/binaries/1.28.1-defconfig-multiarch/busybox-x86_64
Resolving busybox.net (busybox.net)... 140.211.167.122
Connecting to busybox.net (busybox.net)|140.211.167.122|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1001112 (978K)
Saving to: 'busybox-x86_64'
100%[==============================================================================================>] 1,001,112  19.3KB/s  in 30s  
2018-05-25 04:51:57 (32.4 KB/s) - 'busybox-x86_64' saved [1001112/1001112]
[root@kong ~]#

设定busybox

[root@kong ~]# cp busybox-x86_64 /usr/local/bin/busybox
[root@kong ~]# chmod +x /usr/local/bin/busybox
[root@kong ~]# which busybox
/usr/local/bin/busybox
[root@kong ~]#

版本确认

输入busybox可以看出版本以及熟悉的linux下的工具，仔细看一遍就会理解busybox号称自己是瑞士军刀一点都不夸张，反过来说，瑞士军刀如果敢号称linux里的busybox可能会引起非议。

[root@kong ~]# busybox
BusyBox v1.28.1 (2018-02-15 14:34:02 CET) multi-call binary.
BusyBox is copyrighted by many authors between 1998-2015.
Licensed under GPLv2. See source distribution for detailed
copyright notices.
Usage: busybox [function [arguments]...]
  or: busybox --list[-full]
  or: busybox --install [-s] [DIR]
  or: function [arguments]...
  BusyBox is a multi-call binary that combines many common Unix
  utilities into a single executable. Most people will create a
  link to busybox for each function they wish to use and BusyBox
  will act like whatever it was invoked as.
Currently defined functions:
  [, [[, acpid, add-shell, addgroup, adduser, adjtimex, arch, arp, arping, ash, awk, base64, basename, beep, blkdiscard, blkid,
  blockdev, bootchartd, brctl, bunzip2, bzcat, bzip2, cal, cat, chat, chattr, chgrp, chmod, chown, chpasswd, chpst, chroot, chrt,
  chvt, cksum, clear, cmp, comm, conspy, cp, cpio, crond, crontab, cryptpw, cttyhack, cut, date, dc, dd, deallocvt, delgroup,
  deluser, depmod, devmem, df, dhcprelay, diff, dirname, dmesg, dnsd, dnsdomainname, dos2unix, dpkg, dpkg-deb, du, dumpkmap,
  dumpleases, echo, ed, egrep, eject, env, envdir, envuidgid, ether-wake, expand, expr, factor, fakeidentd, fallocate, false,
  fatattr, fbset, fbsplash, fdflush, fdformat, fdisk, fgconsole, fgrep, find, findfs, flock, fold, free, freeramdisk, fsck,
  fsck.minix, fsfreeze, fstrim, fsync, ftpd, ftpget, ftpput, fuser, getopt, getty, grep, groups, gunzip, gzip, halt, hd, hdparm,
  head, hexdump, hexedit, hostid, hostname, httpd, hush, hwclock, i2cdetect, i2cdump, i2cget, i2cset, id, ifconfig, ifdown,
  ifenslave, ifplugd, ifup, inetd, init, insmod, install, ionice, iostat, ip, ipaddr, ipcalc, ipcrm, ipcs, iplink, ipneigh,
  iproute, iprule, iptunnel, kbd_mode, kill, killall, killall5, klogd, last, less, link, linux32, linux64, linuxrc, ln, loadfont,
  loadkmap, logger, login, logname, logread, losetup, lpd, lpq, lpr, ls, lsattr, lsmod, lsof, lspci, lsscsi, lsusb, lzcat, lzma,
  lzop, makedevs, makemime, man, md5sum, mdev, mesg, microcom, mkdir, mkdosfs, mke2fs, mkfifo, mkfs.ext2, mkfs.minix, mkfs.vfat,
  mknod, mkpasswd, mkswap, mktemp, modinfo, modprobe, more, mount, mountpoint, mpstat, mt, mv, nameif, nanddump, nandwrite,
  nbd-client, nc, netstat, nice, nl, nmeter, nohup, nproc, nsenter, nslookup, ntpd, nuke, od, openvt, partprobe, passwd, paste,
  patch, pgrep, pidof, ping, ping6, pipe_progress, pivot_root, pkill, pmap, popmaildir, poweroff, powertop, printenv, printf, ps,
  pscan, pstree, pwd, pwdx, raidautorun, rdate, rdev, readahead, readlink, readprofile, realpath, reboot, reformime,
  remove-shell, renice, reset, resize, resume, rev, rm, rmdir, rmmod, route, rpm, rpm2cpio, rtcwake, run-init, run-parts,
  runlevel, runsv, runsvdir, rx, script, scriptreplay, sed, sendmail, seq, setarch, setconsole, setfattr, setfont, setkeycodes,
  setlogcons, setpriv, setserial, setsid, setuidgid, sh, sha1sum, sha256sum, sha3sum, sha512sum, showkey, shred, shuf, slattach,
  sleep, smemcap, softlimit, sort, split, ssl_client, start-stop-daemon, stat, strings, stty, su, sulogin, sum, sv, svc, svlogd,
  swapoff, swapon, switch_root, sync, sysctl, syslogd, tac, tail, tar, taskset, tcpsvd, tee, telnet, telnetd, test, tftp, tftpd,
  time, timeout, top, touch, tr, traceroute, traceroute6, true, truncate, tty, ttysize, tunctl, ubiattach, ubidetach, ubimkvol,
  ubirename, ubirmvol, ubirsvol, ubiupdatevol, udhcpc, udhcpd, udpsvd, uevent, umount, uname, unexpand, uniq, unix2dos, unlink,
  unlzma, unshare, unxz, unzip, uptime, users, usleep, uudecode, uuencode, vconfig, vi, vlock, volname, w, wall, watch, watchdog,
  wc, wget, which, who, whoami, whois, xargs, xxd, xz, xzcat, yes, zcat, zcip
[root@kong ~]

创建rootfs

这张图在镜像基础介绍的时候已经被无数次看到了，rootfs是linux中重要的概念，而alpine中也有ADD rootfs.tar.xz这样一句，接下来我们将了解一下如何生成一个简单的rootfs

创建目录并进入

[root@kong ~]# mkdir rootfs
[root@kong ~]# cd rootfs/
[root@kong rootfs]#

创建rootfs

执行如下语句

for module in `busybox --list-modules`
do
 mkdir -p `dirname "$module"`
 ln -sf /bin/busybox "$module"
done

执行日志

[root@kong rootfs]# for module in `busybox --list-modules`
> do
>  mkdir -p `dirname "$module"`
>  ln -sf /bin/busybox "$module"
> done
[root@kong rootfs]#

结果确认

[root@kong rootfs]# ls
bin linuxrc sbin usr
[root@kong rootfs]# find . -type d
.
./usr
./usr/bin
./usr/sbin
./sbin
./bin
[root@kong rootfs]#

将busybox拷贝至新创建的./bin目录下

这样，上述命令的链接对象就存在了

[root@kong rootfs]# cp /usr/local/bin/busybox bin/
[root@kong rootfs]# ls -l bin/busybox
-rwxr-xr-x. 1 root root 1001112 May 25 05:27 bin/busybox
[root@kong rootfs]#

创建rootfs.tar

此处注意相对路径，而后续次相对路径会展开至/下，从而创建新的系统的rootfs，这也是从零搭建linux(linux from scratch)的重要操作之一。

[root@kong rootfs]# tar cpf rootfs.tar .
tar: ./rootfs.tar: file is the archive; not dumped
[root@kong rootfs]#

简单说明：busybox Clist-modules列出了busybox的所有模块，然后以此为基础，创建了一个小型的rootfs
[root@kong rootfs]# busybox Clist-modules |wc -l
389
[root@kong rootfs]#

准备Dockerfile

准备一个一行的Dockerfile

[root@kong rootfs]# vi Dockerfile
[root@kong rootfs]# cat Dockerfile 
From scratch
[root@kong rootfs]#

创建base镜像，由于没有发现具体的内容，所以未创建出具体镜像。另外，本文为了演示方便，直接在此处创建Dockerfile，这并不是一个好主意，实际的时候请不要这样做，如果当前目录下有100G的文件，就会无比缓慢，而且也不规范，无关物品需要清场。

[root@kong rootfs]# docker build -t busyboxbase:latest .
Sending build context to Docker daemon 2.415 MB
Step 1/1 : FROM scratch
 ---> 
No image was generated. Is your Dockerfile empty?
[root@kong rootfs]# docker images |grep busyboxbase
[root@kong rootfs]#

from scracth

有From，这个scratch可以pull么，目前的版本已经将其作为一个保留名称

[root@kong rootfs]# docker search scratch |grep 'an explicitly empty'
docker.io  docker.io/scratch              an explicitly empty image, especially for ...  407    [OK]    
[root@kong rootfs]# docker pull scratch
Using default tag: latest
Error response from daemon: 'scratch' is a reserved name
[root@kong rootfs]#

将此Dockerfile添加一行没有实际作用的，看看scratch到底是什么

[root@kong rootfs]# vi Dockerfile 
[root@kong rootfs]# cat Dockerfile 
From scratch
MAINTAINER LiuMiao <[email protected]>
[root@kong rootfs]#

进行构建，发现产生了一个0字节的镜像文件，也与scratch的原意相通

[root@kong rootfs]# docker build -t busyboxbase:latest .
Sending build context to Docker daemon 2.415 MB
Step 1/2 : FROM scratch
 ---> 
Step 2/2 : MAINTAINER LiuMiao <[email protected]>
 ---> Running in b118fd7c73a7
 ---> 2074dc76c09e
Removing intermediate container b118fd7c73a7
Successfully built 2074dc76c09e
[root@kong rootfs]# docker images |grep busyboxbase
busyboxbase           latest       2074dc76c09e    14 seconds ago   0 B
[root@kong rootfs]#

至此，我们理解了from scratch确实不会有额外的添加，接下来我们像alpine那样添加如下两句

ADD rootfs.tar /
CMD ["/bin/sh"]

我们的Dockerfile也是几乎一样的三行

[root@kong rootfs]# cat Dockerfile 
From scratch
ADD rootfs.tar /
CMD ["/bin/sh"]
[root@kong rootfs]#

这样就创建了一个1M的busybox为基础的镜像

[root@kong rootfs]# docker build -t busyboxbase:latest .
Sending build context to Docker daemon 2.415 MB
Step 1/3 : FROM scratch
 ---> 
Step 2/3 : ADD rootfs.tar /
 ---> 0fbb0c8c7579
Removing intermediate container 8311e96f456c
Step 3/3 : CMD /bin/sh
 ---> Running in efb85c4526bf
 ---> 02270c80a4e4
Removing intermediate container efb85c4526bf
Successfully built 02270c80a4e4
[root@kong rootfs]# docker images |grep busyboxbase
busyboxbase           latest       02270c80a4e4    9 seconds ago    1.01 MB
[root@kong rootfs]#

运行并使用

使用docker run发现此镜像所启动的容器并无异常之处

[root@kong rootfs]# docker run --rm -it busyboxbase sh
/ # hostname
b7f9e9646746
/ # uname -a
Linux b7f9e9646746 3.10.0-693.el7.x86_64 #1 SMP Tue Aug 22 21:09:27 UTC 2017 x86_64 GNU/Linux
/ #

小结

这篇文章介绍了如何使用busybox结合from scratch机制创建docker的基础镜像以及相关原理。本文利用busybox 1.28.1版本创建了一个1.01M的可用的基础镜像，从瑞士军刀到瑞士指甲刀，你可以继续优化到若干K都是可行的，但是到了10M以下其实应该更多考虑的是后续的扩展性和功能性的因素了。但是从另外的角度考虑，集成进来了那么多功能的同时，也将这些功能的不安定因素也集成进来了。利用from scratch机制，创建小并且依赖性少的上下文环境是非常有用的。

参考文献

https://docs.docker.com/develop/develop-images/baseimages/