Posts Tagged ‘file management’

Secure delete files irreverseble on Debian and Fedora GNU / Linux

Thursday, February 21st, 2013

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I just read an article in Linux-Magazine on Advanced File Management named – "Beyond the Basics". Most of what the article says is pretty trivial and known by any Linux enthusiast average user and administrator. There was one command mentioned shred which is probably not so well known among Free Software users shred allows the user to "secure delete files" / from the hard disk irreversible.

The tool is part of coreutils package and available across mostly all Linux distributions including Debian / Ubuntu debian derivatives and the RedHat based distros CentOS, Fedora, RHEL etc.

Just for info for those who don't know how to check, to which package a command belongs with rpm and dpkg, here is how;

[hipo@centos ~]$ rpm -qf /usr/bin/shred

hipo@debian:~$ dpkg -S /usr/bin/shred
coreutils: /usr/bin/shred

Here is how to delete a sample file ovewritting 3 times (-n2 – means 3 because in comuters we know we countr from 0 – 0 1 2 3), the z option fills up with zeros after overwritting the file ( just like seen on paste), -v option shows verbose what shred is doing and -u option truncates removes file after overwritting

noah:/var/tmp# shred -n2 -zvu crash20121113021508.txt
shred: crash20121113021508.txt: pass 1/3 (random)…
shred: crash20121113021508.txt: pass 2/3 (random)…
shred: crash20121113021508.txt: pass 3/3 (000000)…
shred: crash20121113021508.txt: removing
shred: crash20121113021508.txt: renamed to 00000000000000000000000
shred: 00000000000000000000000: renamed to 0000000000000000000000
shred: 0000000000000000000000: renamed to 000000000000000000000
shred: 000000000000000000000: renamed to 00000000000000000000
shred: 00000000000000000000: renamed to 0000000000000000000
shred: 0000000000000000000: renamed to 000000000000000000
shred: 000000000000000000: renamed to 00000000000000000
shred: 00000000000000000: renamed to 0000000000000000
shred: 0000000000000000: renamed to 000000000000000
shred: 000000000000000: renamed to 00000000000000
shred: 00000000000000: renamed to 0000000000000
shred: 0000000000000: renamed to 000000000000
shred: 000000000000: renamed to 00000000000
shred: 00000000000: renamed to 0000000000
shred: 0000000000: renamed to 000000000
shred: 000000000: renamed to 00000000
shred: 00000000: renamed to 0000000
shred: 0000000: renamed to 000000
shred: 000000: renamed to 00000
shred: 00000: renamed to 0000
shred: 0000: renamed to 000
shred: 000: renamed to 00
shred: 00: renamed to 0
shred: crash20121113021508.txt: removed

One common use of shred is by sysadmins who has to prepare old server containing lets say client data (SQL) – mail boxes or just file data and then sell it to third parties making sure data will be un-restorable for the new owner. Also shred is used a lot by crackers who set up "time bombs" activated on user activity or inactivity to destroy evidences in case of crackers PC is being captured by police. Though shred cannot guarantee 100% that deleted data can't be recoved within a special data recovery lab in most of cases it is enough to assure data with it will be almost impossible to recover.