Survive the Zombie Apocalypse: recover lost data on Linux

26th Jan 2013 | 10:00

Survive the Zombie Apocalypse: recover lost data on Linux

Teach yourself the basics of Linux disk repair before the Zombies eat your brains

Recover lost data: get back your files

No matter how long you have been using computers, the chances are at some point you will have lost some valuable data. And, in the first of what will become an increasingly tenuous excuse to use some pictures of zombies, the chances of data loss rise when you're under attack from the undead.

If you are a relatively new user, chances are you will have botched up a regular disk cleanup task and accidentally deleted important files, or zapped memory cards without first transferring the images to a more permanent medium. Don't blame your inexperience - it happens to the best of us.

In fact, more experienced users make the most severe mistakes, such as messing up the partition table, wiping the MBR, or even worse formatting the wrong partition.

There is one simple solution - backups. Yet, despite the desktop distros making the process of taking regular automated backups completely painless, most of us only think of backups after deleting non-backed-up data.

That said, there are still lots of tools that'll help you out of a sticky situation. In this feature, we'll look at free software that'll carve data out of dead disks, repair your broken bootloader, restore deleted files and even entire partitions.

If you dual-boot into Windows, we'll show you how to reset forgotten Windows passwords, and keep the installation free of viruses and other nasties.

The success of these recovery tools depends on various factors, and is no match for a backup. Which is why we'll also talk about tools that'll back up your data and help you bounce back from data catastrophes.

Fsck things first

fdisk

Although filesystems have improved over the last decade, sometimes all it takes to mess up the hard disk is a misbehaving application that leaves you no option but to forcibly restart the computer. (this could also happen, for example, when you're fleeing from zombies and have to pull the plug out of your computer without shutting it down properly).

On restart, when your Linux distro detects an unclean shutdown it automatically launches the fsck filesystem check utility to verify the consistency of a filesystem. In many situations, that should do the trick. But sometimes, depending on factors such as the age of the disk, the filesystem, and the task that was interrupted, an automatic check wouldn't work.

In such a case, the distro would ask you to run the fsck tool manually. Although you can run fsck from the maintenance mode with your filesystem mounted as read-only, it's best to run fsck from a live CD without mounting the partition. You'll find fsck in almost every live distro, including the Redo Backup and Recovery distro.

To check a specific filesystem, say /dev/sda6, launch a terminal and enter sudo fsck /dev/sda6. fsck internally uses the respective filesystem checker command and will give you an error when it doesn't find a filesystem checker for the filesystem being checked.

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When called without any switches, the command will check the filesystem and ask you whether it should fix problems if it encounters any. Although you can use the -y switch to automate the process, it can potentially make some data irrecoverable depending on the errors in the filesystem.

One of the most common filesystem errors that fsck will stop at is a corrupt superblock. Since a filesystem cannot be used without a healthy superblock, there are several different backups of the superblock in different locations.

Enter sudo mke2fs -n /dev/sda6 to find out where the superblocks are kept (beware, missing out the -n may wipe your hard drive), which should be listed at the bottom of the output, such as:

Superblock backups stored on blocks: 32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208

Now use any of these addresses to replace the superblock of that partition, such as sudo e2fsck -b 32768 /dev/sda6, and you're good to go.

If the damaged filesystem is an NTFS partition and you can't boot into Windows, you can fix it using the ntfsfix utility. Ensure the partition isn't mounted and then run ntfsfix /dev/sda1, where /dev/sda1 is the Windows NTFS formatted partition. This utility will check and correct some basic NTFS faults, but more importantly it will schedule an NTFS consistency check for when you reboot into Windows.

chroot into a broken system

Sometimes, all the tools you need to repair a Linux distro are within the distro. But the one thing you don't have is access to the system. Maybe you have damaged Grub or accidentally removed important packages that won't let you boot into the distro. That's when you need chroot. It's a neat little tool that ships with every distro and gives you root access to an unbootable system.

Using chroot, you can boot from a live CD and then run programs as if you were actually booted into the broken system - a little like the way in which a genetically modified virus from outer space can take over a human brain.

To use chroot, boot from any live distro, launch a terminal and become the root user, now assuming /dev/sda1 houses the unbootable distro:

# mkdir /broken
# mount /dev/sda1 /broken
# chroot /broken

From this point on, any command you run, or any file you edit is actually being executed on your broken system. So, you can reinstall Grub to the MBR after installing Windows by running update-grub, or remove unstable packages, or undo any other changes that could have broken the system.

Mirror mirror on the wall

PhotoRec

While fsck should fix most unmountable disks, sometimes the problems are more severe. Dying hard disks don't make for a pretty sight. Even more so if they've got some valuable data on them - such as the location of a top-secret research facility that you need to get to in order to find an antidote for the strain of zombie infection.

In such a case, the best thing you can do to recover your data is to minimise using the dying disk. So, instead of getting the disk to cough up your data, which will put it under more stress and push it over the hill, you should immediately make a mirror copy of the disk. You can then use the data recovery tools on this copy instead.

Under normal circumstances, we'd recommend using the venerable dd command to copy the disk. But dd is designed to quit as soon as it encounters any errors, and wouldn't be of much help to us on our malfunctioning disk.

This is where GNU ddrescue comes in. It works like dd and makes block-level copies of a disk. Unlike dd, however, GNU ddrescue skips over the bad block and copies the good blocks first.

You need a healthy drive to copy your rescued data to. It could be an external USB drive or an internal one. Also, make sure that while this disk is mounted, the troubled drive isn't. Now enter:

sudo ddrescue /dev/sda1 /media/backupdrive/sda1.image /media/backupdrive/logfile

This will back up the sda1 partition on the failing drive to the sda1.image file on the device mounted at /media/backupdrive. The logfile records all activity in a log file, which is useful for resuming from where it left off if you interrupt the copy process.

Once you have a good image of the disk, you can ask ddrescue to re-read portions of the disk it couldn't read earlier.

sudo ddrescue -r 3 -C /dev/sda1 /media/backupdrive/sda1.image /media/backupdrive/logfile

With the -r option, you are telling ddrescue the number of times it should try to read the data when it encounters an error. This will put the failing hard disk under stress, which is why you should only run it after you have one good image of the disk.

Thanks to the log file, ddrescue will only try to fill in the gaps and not attempt to read the good portions again. Also remember that, although ddrescue can back up an entire drive with multiple partitions into one image file, you'll save yourself a lot of trouble by rescuing individual partitions in different image files.

Once you have imaged all the partitions of the disk, you can attempt to recover data from them. Depending on how effective ddrescue has been, it can even recover complete filesystems. So once you have the image, you should first check it with fsck, such as:

sudo fsck /media/backupdrive/sda1.image

When it's done, mount the image as a loopback device:

sudo mount -o loop /media/backupdrive/sda1.image /media/sda1imge

Now have a look at /media/sda1image, and you should find all your data in there!

If that doesn't work, it's time to call in the experts. Foremost and Scalpel are file carving utilities that can extract files from disk images. Both utilities hunt for files using the headers and footers of the different file formats, but the latter is considered more efficient. You'll find both in the official repos of most distros.

Foremost is the simplest of the two to use:

sudo foremost -t all -i sda1.image -o rescuedfiles

The command will search for all file types Foremost can understand in the sda1.image file and save them under the rescued-files directory.

Before you can use Scalpel, you'll have to edit its configuration file /etc/scalpel/scalpel.conf and uncomment all the file types you want to recover. Save the file and run:

sudo scalpel sda1.image -o rescued-files

Recover deleted files

The tools we've seen up to now are used to recover files from damaged disks. But you can't always blame data loss on a hardware failure. A clumsy operator - for example, one who is in the early stages of zombie infection and losing control of fine motor skills - can be just as harmful. So now let's look at tools that can restore accidentally deleted files from otherwise functional disks.

Before we get started, you should understand how the filesystem handles files. When you delete a file, it isn't zapped out of existence. Instead, the filesystem marks it as deleted, and makes the space it occupies available to other files. This means that until another app uses that space, the original file is still there, and can be retrieved by file recovery tools. For this reason, it's important that you minimise, if not cease, interactions with the disk on which you have deleted files.

Perhaps the most comprehensive open source file recovery tool is PhotoRec. It can sniff the most common image formats and can additionally pick out files in various formats, including ODF, PDF, 7ZIP, ZIP, TAR, RPM, DEB and even virtual disks. The tool works on all sorts of disks, including hard disks and removable media such as USB disks. In addition to reading unbootable disks, PhotoRec will recover files from partitions that have been formatted and reinstalled into. It ships along with the TestDisk tool that we'll look into later, and it's in most recovery distros.

Although PhotoRec is a command-line application, it breaks the process of recovering files into steps, much like a wizard. When you launch the tool, it will first ask you to select the disk and then point it to the partition that housed the lost file, and also specify its filesystem. It'll then ask you if you want it to search only the free unallocated space or the entire partition.

You'll also have to point it to a folder where it should store the recovered files. Depending on the size of the partition, PhotoRec can take quite a while to complete. By default, it looks for files of all the formats it supports, but you can limit the filetypes to recover using the File Opt option.

Once PhotoRec is done, you'll discover gazillions of weirdly-named files of all different formats under one directory. PhotoRec names them as it finds them, leaving the sorting to you.

Just like with files, it doesn't take much effort to corrupt a healthy disk. A wrong keypress (this is particularly likely if you find that a reanimated corpse has gnawed off part of your finger while you were distracted by getting PulseAudio to work) in fdisk or Gparted can wipe the MBR, or banish a partition to oblivion. And as with files, the situation is salvageable, more so if you stop using the disk straightaway.

Using TestDisk

TestDisk

TestDisk is the best tool to fix partition tables and put non-bootable disks back into service again. Using TestDisk is quite similar to PhotoRec.

When launched, it first asks you to create a log (which will come in handy for later analysis if the recovery fails), and then displays a list of all the disks attached to the computer. After you select the disk on which you've lost a partition, it'll ask you to select a partition table type, such as Intel, Mac, Sun and so on.

Next, you are shown the various TestDisk recovery options. Select the default Analyse option, which reads the partition structure and hunts for lost partitions. It then displays the current partition structure. Now select the Quick Search option to ask TestDisk to look for deleted partitions.

Depending on the age of your disk, TestDisk might display several partitions. To figure out which is the correct partition that you want to recover, look for the partition label listed at the end of each entry in square brackets. If that doesn't help you, press 'P' on a selected partition to see a list of files that TestDisk has found on that partition. Repeat this with all partitions until you find the right one.

When you've found your partition, it's best to copy over the data just in case TestDisk is unable to restore the partition. To do so, press 'P', and then with the 'a' key select all files. Then press 'C' to copy the files, which will ask you for the location to save the files. When it's done copying, press 'q' to return to the list of recovered partitions and press Enter to continue to the next step in restoring the partition.

TestDisk displays the partition structure again, this time with the missing partition accounted for. Select Write to save the partition table to the disk, and exit the program. If all goes well, when you reboot your partition will be restored.

Understand partitions

To successfully use TestDisk, you should first understand how disks are partitioned. A partition table contains four slots of 16 bytes each, which limits the number of primary partitions per hard disk to four. Typically, one of the four partitions is marked as extended, and contains a number of logical partitions.

You can have three primary partitions (sda1, sda2, sda3) and a fourth extended partition that contains several logical partitions (sda5, etc). When you're hunting for partitions, you'll find some that overlap others, or are beyond the boundaries of the partition table. Sometimes, TestDisk will complain because it sees a primary partition in between logical ones, which isn't possible.

When TestDisk can't place a partition, you'll not only have to find the correct partition, but also identify it as primary or logical. Just keep the following in mind. The first primary partition generally starts at cylinder 0, head 1, sector 1. If you have more primary partitions, they'll start at a non-zero cylinder (like 625), head 0, sector 1. On the other hand, logical partitions start at a non-zero cylinder, head 1, sector 1.

Dig deeper

Boot Repair

Although we hope TestDisk works for you as we illustrated in the previous section, there are times when it wouldn't. This is where the advanced options come into play.

Sometimes, TestDisk will detect deleted partitions but fail to identify their type. When it finds a partition, TestDisk displays the type of partition in the first column, where you'll find a * for a bootable partition, P for primary, L for logical and E for extended. You can use the left and right arrow keys on a highlighted partition to change its type.

There are a couple more partition options available at the start of the recovery procedure, right after you've selected the type of partition table on the disk. Immediately below the Analyse option is the Advanced tab, which lists some advanced filesystem tricks. With the Type option, you can change the format of the partition, which is useful when a partition you want to recover has been reformatted.

Then there will be times when TestDisk's Quick Search option won't be able to find all of your missing partitions. That's when you need the Deeper Search option, which scans each cylinder and also queries the backup's boot sectors and superblocks to find more partitions.

Don't be surprised if it discovers more partitions than you had on your disk to begin with. Most of them are just ghost images of partitions that once existed - not all will be recoverable, and many will occupy the same disk space.

In the list of partitions found, the ones highlighted in green are recoverable because they exist in the backup boot sector or superblocks. The issue is with partitions that occupy the same space. As with Quick Search, once TestDisk finds partitions you can use the 'P' key to list files on the selected partition. One or more of the duplicates won't display any files, and will complain that the filesystem is broken.

Mark the broken filesystems as Deleted (D) and continue until you've found all the deleted partitions.

Boot camp

CLamAV

There are several ways to lose the Grub bootloader. Perhaps your PC has succumbed to the deadliest zombie infection of all - a Windows installation!

Although installing Windows after Linux will surely wipe the MBR, sometimes an esoteric Linux distro can also take over the MBR and prevent you from booting other operating systems.

There are various methods to repair a damaged MBR, the simplest being the graphical Boot-Repair tool. You'll find it in most system rescue distros.

The tool is designed to automatically fix most common Grub issues with a single click. Furthermore, it'll also be of use to advanced users who want to tweak the finer aspects of Grub, such as passing additional kernel options, or changing the boot order or timeout, or the default OS to boot.

When you launch the tool, it'll check for and install updates over the internet if a connection is available. It'll then scan your disks and the partitions on them. When it's done, it'll display a simple graphical interface with a couple of buttons.

For most users, the Recommended Repair button will do the trick. Optionally, you can use the Advanced options pull-down and modify various options of the Grub installation. Then click on the Apply button. In either case, after it's done restoring Grub, the tool will display a URL to a diagnostic report of your computer. This will come in handy in case the tool hasn't worked and you need to ask for help on your distro's forum.

You can use the Create a BootInfo summary button to generate this report before repairing Grub. At the end of the report, it'll list the actions the tool will perform to repair Grub on your computer.

Recover lost data: Reset Linux passwords

Ophcrack

Unless you use the same password for all online and offline accounts (which we don't recommend), chances are that you may forget your obscure, intricately-crafted password. Or maybe you've arrived at the research facility, and the computers that store the procedure for synthesizing antidote, but the scientists aren't alive to tell you their passwords.

For crises like these, you need the Rescatux distro. The distro has the usual recovery tools to repair broken filesystems and bootloaders. But what sets this distro apart from others is its ability to change passwords on a Linux installation and regenerate a broken sudoers file.

When you boot the live distro, it'll automatically launch its custom rescue app, called Rescapp. This app has various buttons. When you click on the Password (+) button, the tool will ask you if you wish to change a password or regenerate the sudoers file. Both options will search for Linux installations on your computer, and then display the list of users on the distro you select.

If you are changing the password, the tool will prompt you to enter a new password for the selected user. If you are regenerating the sudoers file, the selected user will be added to /etc/sudoers. If the scientists were using old copies of Vista on their government-funded hardware (which is probably related to the dreadful outbreak in the first place), there are several tools that'll help you recover or reset the password on the Windows installation as well.

One of the easiest is Ophcrack. The best way to use Ophcrack is via its SliTaz-based live CD. The live CD is available in two flavours: one has tables that'll help you recover passwords from Windows XP and earlier distros, and the other does the same for Windows Vista and later releases.

When you boot either live CD, it'll automatically launch the graphical Ophcrack tool. It will discover and list all the user accounts on your computer and attempt to recover their passwords. Unless the password is fairly complicated, has lots of characters, or you're on a dated machine, the tool shouldn't take long to crack the passwords.

When it's done, the passwords are listed in the NT Pwd column. While this should work for most users, if it doesn't you can increase your chances of cracking the passwords by downloading and installing additional tables from Ophcrack's website.

Depending on which live CD you've downloaded, you'll either have the XP Free Small or the Vista Free table. Besides these, only the 703MB XP Free Fast table is available for free. The others can be downloaded for a fee, and can be used to crack passwords that aren't based on dictionary words, include special characters, German characters or numbers, and are of various lengths.

Resetting the password

If Ophcrack isn't able to crack your passwords using the free tables, and you don't want to cough up for the additional paid tables, you can use the Offline NT Password and Registry Editor to reset the password.

Before you proceed, however, be aware that resetting a password has some disadvantages compared to recovering a password. If you have asked Windows to encrypt your files with your password, resetting your user account will not let you decrypt those files. This is why you should first try to recover your password.

The Offline NT Password and Registry Editor is available as a 4MB Live ISO image. When you boot from it, select the partition that houses the Windows installation whose password you need to reset. Next, the tool asks you the location of the password registry.

After reading the password registry, the tool prints a list of users, and gives you the option to set a new password, wipe the password, enable/disable a user, or escalate their privileges to those of an admin. Just make sure you write the changes to the registry before exiting the tool.

Dedicated recovery tools

RedoBackup

While TestDisk and Photorec will sniff out all sorts of files, the tools can sometimes be an overkill. If all you need to do is recover JPEGs or MOV files from a formatted memory card - perhaps to recover evidence of early infection in daytime TV presenters - you can use the recoverjpeg tool. You can install the tool from your favourite distro's repos.

To use the tool to recover images from a memory card at /dev/sdc, launch a terminal and enter:

sudo recoverjpeg /dev/sdc

The tool will store all the recovered images in the directory you have invoked it from.

Another useful tool is ntfsundelete, which is designed to recover files from an NTFS filesystem. Again, you'll find it in your distros repos. Assuming /dev/sda5 is your NTFS formatted partition, in a terminal enter:

sudo ntfsundelete /dev/sda5

This will display a list of files it has found. You can also use the -t switch to look for files modified in a specific time period. The command sudo ntfsundelete /dev/ sda5 -t 2d will look for files that were altered in the past two days.

The tool also has very flexible recovery options. This command will recover all PDF files and keep them in a recovered/ directory under your home directory:

sudo ntfsundelete /dev/sda5 -u -m *.pdf -d /recovered

Backup is a virtue

DejaDup

Despite all the excellent open source data recovery tools, there's only one sure shot way of recovering all your data - a backup. Although it isn't particularly time consuming, backing up data requires careful thought and preparation.

For starters, where do you store your data? Keeping it on another partition of the same disk isn't advisable - what if the whole disk fails? A copy on another disk is one solution. The kind of data also influences the choice of storage medium. Hard disks offer the best price-to-space ratio, and are also a convenient and readily-available option.

While flash drives offer portability, optical media is easily distributable, and online storage is globally accessible.

What to back up?

Some of us are more organised than others. So while the best strategy would be to keep all your data on a dedicated disk or partition away from all the other distro files, most of us will have data strewn all over the place.

Most desktop distros have dedicated directories to store your documents, downloads, pictures and videos. They also take the pain to ensure the bundled apps use these directories as the default storage location.

If you have been using these directories, you can back them up instead of the whole /home directory. Also, most apps create their own data repositories to store files. Many prompt you for the location, while some create them on their own. Check under their Preferences to search these out.

Point-and-click backup

There's no dearth of backup tools to choose from, but the best tool for a desktop user is Déjà Dup. It's a graphical front-end to duplicity that's easy to set up and has all the features you'd expect from a backup tool.

On some distros, such as Ubuntu, Déjà Dup ships pre-installed, while it's available in the official repos of most others. The tool has a very minimal interface so as to not overwhelm new users. But you'll need to configure the app before putting it into action.

Start by pointing it to the location where you want to house your backups. This can be a local hard disk, a remote location accessible via SSH or FTP, or a web-based storage service, such as Amazon S3 or Ubuntu One.

Next, you need to specify the list of directories you want to include in and exclude from the backup. By separating the two, Déjà Dup enables you to include a larger directory in your backup, such as a user's home directory, while giving you the flexibility to leave out certain parts, such as the .cache/ directory.

Lastly, you need to select a backup schedule to automate the process. By default, the tool will keep older backups forever, if you have the space on the backup location, but you can also specify a different time period depending on the criticality of the data.

When you're done, click the Backup icon to start the process. The tool will prompt you to enter a password to encrypt the backups. This initial backup may take some time, but subsequent ones are much faster because they are incremental and only back up data that has changed.

When restoring backups, Déjà Dup enables you to restore data to its original location or under a specific directory. You'll first be prompted for the password to decrypt the backups, and then shown a time-stamped list of backups to restore from.

Bare metal backup

CLoneZilla

While Déjà Dup is best for making targeted backups, sometimes you need to clone the entire disk. For situations such as these, where you need to replace your old disk with a new one, you need a tool that's capable of making a bare-metal backup of the disk.

The most popular tool for such a task is CloneZilla. But for all the tool's flexibility, it can be quite intimidating for desktop users. If all you need is an app that'll help you swap the old disk for a new one, on the same computer, then Redo Backup and Recovery is the easiest tool to use for the job.

The tool is available as a live CD. When you boot the live CD, you only get two buttons - to Backup the disk or Restore the backup on the new disk. The backup process will begin by first scanning your computer for drives. You'll then be asked to select the one you want to back up. The tool will then show you the list of partitions on the disc. Make sure that all of the partitions are selected.

You are then asked to select a destination to save the cloned image of the disk. This can be another hard drive on the computer, or a USB drive attached to the computer, or a shared folder on another computer. On this destination disk, you'll have to create a folder and then select a name which will be appended to the various files created by the tool. That's all there's to it.

Depending on the size of the disk and the location of the destination, the tool can take several hours to complete. When it's done, remove the old drive and plug in the new one. Now boot from the live CD again, and this time click the Restore button. Now navigate to the .backup file you wish to restore from, either on a local disk or over the network.

The most crucial step in the restoration process is selecting the destination drive to write the image over. Pay attention when making a selection here, because the tool will erase existing content on the drive, including the MBR.

When it's done, the new drive will exactly mirror the old one. If the previous drive had multiple operating systems or distros, and had a working bootloader, so will the new one. If you have moved the image to a bigger disk, you'll have to use Gparted to reclaim the additional, unpartitioned space at the end of the disk.

Make crash-proof discs

DVD disaster

Optical discs are commonly the preferred media for keeping backups. However, even when stored carefully, they'll eventually go bad. Making multiple copies of backup disks seems like an inefficient use of resources.

A better option is to use DVDisaster. The tool creates an error correction code (ECC) file from a healthy disk, which can be used later to recover data when the media is damaged.

DVDisaster works on ISO images. To image your disk, insert the disc into the drive and launch DVDisaster. Now click on the Image File Selection icon, type in a name for the ISO image and select a directory for it to be stored in. Lastly, click the Read button.

Now it's time to create an ECC file. DVDisaster supports two types: RS01 and RS02. We'd advise you to stick to the default RS01 method, and store the ECC file on a separate medium. Click the Create button to create and save the ECC file.

Using the default settings, the ECC file is about 15% the size of the ISO file. For better protection, head to Preferences > Error Correction and select the High option. This balloons the ECC file to about 35% the size of the image, but gives you a better chance of restoring badly damaged media.

Now, from time to time, scan the backup discs with DVDisaster using the Scan button after inserting the disc. If the scan reveals bad sectors in the media, it's time to recover the data.

For that, first create an ISO image of the damaged media using the same procedure as before. Then find the ECC file that you created earlier for the damaged media and point to it using the button for ECC file selection. With the image and ECC file in place, click on the Fix button, which reads and repairs the damaged image.

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