linux常用基本命令之二十二implantisomd5和checkisomd5

以前每做出一个ISO都会同时生成一个md5值文件，防止复制过程中数据损坏；现使用implantisomd5和checkisomd5两个工具，可以将md5值嵌入ISO中并直接效验，方便不少。

用法：

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implantisomd5 xxx.iso
checkisomd5 xxx.iso

$ implantisomd5 test2.iso

Inserting md5sum into iso image...

md5 = 1f7419e5fc8157e3c2a79a8023816454

Inserting fragment md5sums into iso image...

fragmd5 = ded8b32776d8ea11f436df6f6b4171

frags = 20

Setting supported flag to 0

$ checkisomd5 test2.iso

Press [Esc] to abort check.

The media check is complete, the result is: PASS.

It is OK to use this media.

注意：只能植入一次，若植入两次及两次以上就会报错。

$ implantisomd5 test2.iso

ERROR: Application data has been used - not implanting md5sum!

实验：

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$ mkdir iso
$ echo 'Hello World!' > iso/test
$ mkisofs -J -T -o test.iso iso/
$ cp test.iso test2.iso
$ implantisomd5 test2.iso
$ hexdump -C test.iso > test-dump
$ hexdump -C test2.iso > test2-dump
$ diff test-dump test2-dump

结果如下：
33,34c33,46
< 00008370 20 01 00 20 20 20 20 20 20 20 20 20 20 20 20 20 | .. |
< 00008380 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |
---
> 00008370 20 01 00 49 53 4f 20 4d 44 35 53 55 4d 20 3d 20 | ..ISO MD5SUM = |
> 00008380 31 66 37 34 31 39 65 35 66 63 38 31 35 37 65 33 |1f7419e5fc8157e3|
> 00008390 63 32 61 37 39 61 38 30 32 33 38 31 36 34 35 34 |c2a79a8023816454|
> 000083a0 3b 53 4b 49 50 53 45 43 54 4f 52 53 20 3d 20 31 |;SKIPSECTORS = 1|
> 000083b0 35 3b 52 48 4c 49 53 4f 53 54 41 54 55 53 3d 30 |5;RHLISOSTATUS=0|
> 000083c0 3b 46 52 41 47 4d 45 4e 54 20 53 55 4d 53 20 3d |;FRAGMENT SUMS =|
> 000083d0 20 64 65 64 38 62 33 32 37 37 36 64 38 65 61 31 | ded8b32776d8ea1|
> 000083e0 31 66 34 33 36 64 66 36 66 36 62 34 31 37 31 3b |1f436df6f6b4171;|
> 000083f0 46 52 41 47 4d 45 4e 54 20 43 4f 55 4e 54 20 3d |FRAGMENT COUNT =|
> 00008400 20 32 30 3b 54 48 49 53 20 49 53 20 4e 4f 54 20 | 20;THIS IS NOT |
> 00008410 54 48 45 20 53 41 4d 45 20 41 53 20 52 55 4e 4e |THE SAME AS RUNN|
> 00008420 49 4e 47 20 4d 44 35 53 55 4d 20 4f 4e 20 54 48 |ING MD5SUM ON TH|
> 00008430 49 53 20 49 53 4f 21 21 20 20 20 20 20 20 20 20 |IS ISO!! |
> 00008440 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | |

注意最后一句话："THIS IS NOT THE SAME AS RUNNING MD5SUM ON THIS ISO!!"

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$ md5sum test.iso
79771b464de80779e78b126ba0e918a7 test.iso

可见并不是简单将原ISO的md5值插入而已。

代码分析：

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$ rpm -qf `which implantisomd5`
isomd5sum-1.0.7-1.fc15.i686

$ yumdownloader --source isomd5sum-1.0.7-1.fc15
isomd5sum-1.0.7-1.fc15.src.rpm | 24 kB 00:00

$ rpm -ivh isomd5sum-1.0.7-1.fc15.src.rpm
1:isomd5sum warning: user mockbuild does not exist - using root
########################################### [100%]

$ rpmbuild -bp ~/rpmbuild/SPECS/isomd5sum.spec
Executing(%prep): /bin/sh -e /var/tmp/rpm-tmp.mwSxql
+ umask 022
+ cd /home/eric/rpmbuild/BUILD
+ LANG=C
+ export LANG
+ unset DISPLAY
+ cd /home/eric/rpmbuild/BUILD
+ rm -rf isomd5sum-1.0.7
+ /usr/bin/bzip2 -dc /home/eric/rpmbuild/SOURCES/isomd5sum-1.0.7.tar.bz2
+ /bin/tar -xf -
+ STATUS=0
+ '[' 0 -ne 0 ']'
+ cd isomd5sum-1.0.7
+ /bin/chmod -Rf a+rX,u+w,g-w,o-w .
+ exit 0

$ cd ~/rpmbuild/BUILD/isomd5sum-1.0.7/

# 在libimplantisomd5.c文件中找到核心代码，说明读取<16扇区 ~ END>; 根据ISO9660规范，扇区0~15未定义，全0；
/* read up to 15 sectors from end, due to problems reading last few */
/* sectors on burned CDs */
while (total < isosize - SKIPSECTORS*2048) {

# DUMP中的内容和注释一致
SKIPSECTORS = 15

# 测试是否读取<16扇区 ~ END>内容再计算MD5
2048*16=32768=0x8000
$ tail -c +32769 test.iso > test-sum.iso
$ md5sum test-sum.iso
91cccb33d25a976bd88911b71ce630a4 test-sum.iso

# 结果与预期不符，仔细查看while代码，跟进MD5_Final()函数，发现原来是用自己写的算法计算SUM。

# 插入位置为Primary Volume Descriptor的 APPDATA区域（偏移883=0x373; 0x8000+0x373=0x8373）