Docker uses busybox to create a base image

The first line of a Docker image starts with an image such as FROM alpine, but how is the initial base image created? This article uses a busybox to create a base image. I believe that in this process, you will have a further understanding of some Docker-related concepts.

What is a base image?

Simply put, a base image is an image built by a Dockerfile that does not start with From or FROM scratch. For example, alpine, this very small Linux image is only about 4M

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

Its Dockerfile is very simple, with only three lines, which is a basic image.

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

In the next article we will create our own base image like alpine.

busybox

Summary

Busybox is called the Swiss Army Knife of Embedded Linux. This sentence was put forward when busybox introduced itself (The Swiss Army Knife of Embedded Linux). busybox integrates many small common functions under Unix into a small executable file. In short, you can find all the commonly used functions under Unix or Linux here, but for the goal of busybox: embedded Linux, size is a very important optimization factor and limitation for busybox. These functions may be castrated, but they are sufficient for general needs. Alpine adds its own package management tool apk and other functions based on busybox to create a small and popular image. Busybox is an open source project developed in C language based on GPL. The current stable version is 1.28.4

Host Machine

[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 ~]#

Download busybox

The latest binary version is 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 ~]#

Setting up 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 ~]#

Version confirmation

Type busybox to see the version and familiar Linux tools. After a careful look, you will understand that it is not an exaggeration for busybox to call itself the Swiss Army Knife. On the contrary, if the Swiss Army Knife dares to call itself the busybox in Linux, it may cause controversy.

[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 ~]

Create rootfs

This picture has been seen countless times when introducing the basics of mirroring. Rootfs is an important concept in Linux, and there is also a sentence like ADD rootfs.tar.xz in alpine. Next, we will learn how to generate a simple rootfs

Create a directory and enter

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

Create rootfs

Execute the following statement

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

Execution log

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

Result Confirmation

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

Copy busybox to the newly created ./bin directory

In this way, the link object of the above command exists

[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]#

Create rootfs.tar

Note the relative path here, and the subsequent relative paths will be expanded to /, thereby creating the rootfs of the new system. This is also one of the important operations of building Linux from scratch.

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

Brief description: busybox –list-modules lists all the modules of busybox, and then creates a small rootfs based on this.
[root@kong rootfs]# busybox –list-modules |wc -l
389
[root@kong rootfs]#

Prepare Dockerfile

Prepare a one-line Dockerfile

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

Create a base image. Since no specific content was found, no specific image was created. In addition, for the convenience of demonstration, this article directly creates a Dockerfile here, which is not a good idea. Please do not do this in practice. If there are 100G files in the current directory, it will be extremely slow and non-standard, and irrelevant items need to be cleared.

[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

There is From. Can this scratch be pulled? The current version has reserved it as a name.

[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]#

Add a line to this Dockerfile that has no practical effect to see what scratch is

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

After building, it was found that a 0-byte image file was generated, which is consistent with the original meaning of scratch.

[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]#

At this point, we understand that from scratch does not add anything extra. Next, we add the following two sentences like alpine

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

Our Dockerfile has almost the same three lines

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

This creates a 1M busybox-based image

[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]#

Run and use

Using docker run, it is found that there is nothing unusual in the container started by this image

[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
/ #

summary

This article introduces how to use busybox combined with the from scratch mechanism to create a basic docker image and related principles. This article uses busybox version 1.28.1 to create a usable basic image of 1.01M. From Swiss Army Knife to Swiss Nail Clipper, you can continue to optimize to several K. However, when it is below 10M, you should consider the subsequent scalability and functionality factors. But from another perspective, while integrating so many functions, the unstable factors of these functions are also integrated. Using the from scratch mechanism, it is very useful to create small context environments with few dependencies.

References

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

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