# Checksumming CD or DVD media

Note on the MD5 command: On OS X, it’s md5. On RHEL it’s md5sum.

### Doing it properly

• For this demo, I’ll attempt to burn a CentOS 5.5 DVD
• The ISO name is centos-5.5.i386.iso
• It was burned to a DVD drive with device name /dev/sdc

Checksumming an ISO image is easy:

[user@localhost ~]# md5sum centos-5.5.i386.iso
75c92246479df172de41b14c9b966344


When you burn this to a DVD, you can verify that everything’s good by first getting the size of the ISO in bytes:

[user@localhost ~]#  ls -l centos-5.5.i386.iso
-rw-r--r-- 1 user user 4185118720 May 20 15:40 centos-5.5.i386.iso


Now divide that by 2048 (4185118720 ÷ 2048 = 2043515). You can now verify the DVD the ISO was burned to using:

[user@localhost ~]# dd if=/dev/sdc bs=2048 count=2043515 | md5sum
75c92246479df172de41b14c9b966344


This verifies a successful burn. Merely using dd if=/dev/sdc | md5sum will always give you an incorrect checksum!