[H-GEN] dd or shred for secure deletes.

Anthony Irwin irwa82 at yahoo.com
Mon Nov 28 23:29:43 EST 2005


Hi,
 
 I was under the impression that dd bs=1024 if=/dev/zero of=foo_file would securely delete a file by adding /0 to every byte of the file and the dd bs=1024 if=/dev/zero of=/dev/hda would securely delete a hard drive.
 
 I have however seen a web site that said that you should use a tool like shred because the method i mentioned above is not a secure way to delete files. Reading the man page of shred also said:
 
 ----------------------------
        CAUTION:  Note  that  shred relies on a very important assumption: that
        the filesystem overwrites data in place.  This is the  traditional  way
        to  do  things,  but many modern filesystem designs do not satisfy this
        assumption.  The following are examples of filesystems on  which  shred
        is not effective:
 
        * log-structured or journaled filesystems, such as those supplied with
 
               AIX and Solaris (and JFS, ReiserFS, XFS, Ext3, etc.)
 
        *  filesystems  that  write  redundant  data  and carry on even if some
        writes
 
               fail, such as RAID-based filesystems
 
        * filesystems that make snapshots,  such  as  Network  Appliance's  NFS
        server
 
        * filesystems that cache in temporary locations, such as NFS
 
               version 3 clients
 
        * compressed filesystems
 
        In  addition, file system backups and remote mirrors may contain copies
        of the file that cannot be removed, and that will allow a shredded file
        to be recovered later.
 
 ------------------------------------
 
 So I was wondering if anyone knows the difference between the dd and shred methods of deleting files and hard drives and what the best way is.
 
 Kind Regards,
 Anthony Irwin
 

		
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