Welcome!

Linux Authors: Liz McMillan, Carmen Gonzalez, Rex Morrow, Datical, Elizabeth White, Pat Romanski

Related Topics: Linux

Linux: Article

Why Recovering a Deleted Ext3 File Is Difficult . . .

and why you should back up important files

We have all done it before. You accidentally type in the wrong argument to rm or select the wrong file for deletion. As you hit enter, you notice your mistake and your stomach drops. You reach for the backup of the system and realize that there isn't one.

There are many undelete tools for FAT and NTFS file systems, but there are few for Ext3, which is currently the default file system for most Linux distributions. This is because of the way that Ext3 files are deleted. Crucial information that stores where the file content is located is cleared during the deletion process.

In this article, we take a low-level look at why recovery is difficult and look at some approaches that are sometimes effective. We will use some open source tools for the recovery, but the techniques are not completely automated.

What Is a File?
Before we can see how to recover files, we need to look at how files are stored. Typically, file systems are located inside of a disk partition. The partition is usually organized into 512-byte sectors. When the partition is formatted as Ext3, consecutive sectors will be grouped into blocks, whose size can range from 1,024 to 4,096 bytes. The blocks are grouped together into block groups, whose size will be tens of thousands of blocks. Each file has data stored in three major locations: blocks, inodes, and directory entries. The file content is stored in blocks, which are allocated for the exclusive use of the file. A file is allocated as many blocks as it needs. Ideally, the file will be allocated consecutive blocks, but this is not always possible.

The metadata for the file is stored in an inode structure, which is located in an inode table at the beginning of a block group. There are a finite number of inodes and each is assigned to a block group. File metadata includes the temporal data such as the last modified, last accessed, last changed, and deleted times. Metadata also includes the file size, user ID, group ID, permissions, and block addresses where the file content is stored.

The addresses of the first 12 blocks are saved in the inode and additional addresses are stored externally in blocks, called indirect blocks. If the file requires many blocks and not all of the addresses can fit into one indirect block, a double indirect block is used whose address is given in the inode. The double indirect block contains addresses of single indirect blocks, which contain addresses of blocks with file content. There is also a triple indirect address in the inode that adds one more layer of pointers.

Last, the file's name is stored in a directory entry structure, which is located in a block allocated to the file's parent directory. An Ext3 directory is similar to a file and its blocks contain a list of directory entry structures, each containing the name of a file and the inode address where the file metadata is stored. When you use the ls -i command, you can see the inode address that corresponds to each file name. We can see the relationship between the directory entry, the inode, and the blocks in Figure 1.

When a new file is created, the operating system (OS) gets to choose which blocks and inode it will allocate for the file. Linux will try to allocate the blocks and inode in the same block group as its parent directory. This causes files in the same directory to be close together. Later we'll use this fact to restrict where we search for deleted data.

The Ext3 file system has a journal that records updates to the file system metadata before the update occurs. In case of a system crash, the OS reads the journal and will either reprocess or roll back the transactions in the journal so that recovery will be faster then examining each metadata structure, which is the old and slow way. Example metadata structures include the directory entries that store file names and inodes that store file metadata. The journal contains the full block that is being updated, not just the value being changed. When a new file is created, the journal should contain the updated version of the blocks containing the directory entry and the inode.

Deletion Process
Several things occur when an Ext3 file is deleted from Linux. Keep in mind that the OS gets to choose exactly what occurs when a file is deleted and this article assumes a general Linux system.

At a minimum, the OS must mark each of the blocks, the inode, and the directory entry as unallocated so that later files can use them. This minimal approach is what occurred several years ago with the Ext2 file system. In this case, the recovery process was relatively simple because the inode still contained the block addresses for the file content and tools such as debugfs and e2undel could easily re-create the file. This worked as long as the blocks had not been allocated to a new file and the original content was not overwritten.

With Ext3, there is an additional step that makes recovery much more difficult. When the blocks are unallocated, the file size and block addresses in the inode are cleared; therefore we can no longer determine where the file content was located. We can see the relationship between the directory entry, the inode, and the blocks of an unallocated file in Figure 2.

Recovery Approaches
Now that we know the components involved with files and which ones are cleared during deletion, we can examine two approaches to file recovery (besides using a backup). The first approach uses the application type of the deleted file and the second approach uses data in the journal. Regardless of the approach, you should stop using the file system because you could create a file that overwrites the data you are trying to recover. You can power the system off and put the drive in another Linux computer as a slave drive or boot from a Linux CD.

The first step for both techniques is to determine the deleted file's inode address. This can be determined from debugfs or The Sleuth Kit (TSK). I'll give the debugfs method here. debugfs comes with most Linux distributions and is a file system debugger. To start debugfs, you'll need to know the device name for the partition that contains the deleted file. In my example, I have booted from a CD and the file is located on /dev/hda5:

# debugfs /dev/hda5
debugfs 1.37 (21-Mar-2005)
debugfs:

We can then use the cd command to change to the directory of the deleted file:

debugfs: cd /home/carrier/

The ls -d command will list the allocated and deleted files in the directory. Remember that the directory entry structure stores the name and the inode of the file and this listing will give us both values because neither is cleared during the deletion process. The deleted files have their inode address surrounded by "<" and ">":

debugfs: ls -d
415848 (12) . 376097 (12) .. 415864 (16) .bashrc
[...]
<415926> (28) oops.dat

More Stories By Brian Carrier

Brian Carrier has authored several leading computer forensic tools, including The Sleuth Kit (formerly The @stake Sleuth Kit) and the Autopsy Forensic Browser. He has authored several peer-reviewed conference and journal papers and has created publicly available testing images for forensic tools. Currently pursuing a Ph.D. in Computer Science and Digital Forensics at Purdue University, he is also a research assistant at the Center for Education and Research in Information Assurance and Security
(CERIAS) there. He formerly served as a research scientist at @stake and as the lead for the @stake Response Team and Digital Forensic Labs. Carrier has taught forensics, incident response, and file systems at SANS, FIRST, the @stake Academy, and SEARCH. He is the author of File System Forensic Analysis (Addison-Wesley, ISBN 0321268172).

Comments (6) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
theusr 07/09/09 09:29:00 AM EDT

The figure 2 maybe misleading: the links between the address blocks and the file content are still there (though the address blocks are unallocated), that what's make the recovery possible.

Mike Kay 01/15/08 03:57:07 PM EST

Excellent article. Followed it step by step and successfully recovered a .XLS spreadsheet that had been deleted from the /tmp folder on Ubuntu Gutsy. It also found an associated .jpg that I wasn't looking for!

Saved me hours of retyping. Thanks a lot.

Jahangir 10/22/07 05:26:36 PM EDT

This was really the best article i could find inspite of 3 hrs of googling.

But what if you are trying to recover a 6GB VM.
Since VMware files are not recognized by foremost, how can we get the magic number to get the header for the VM files ??

ruintower 04/23/06 09:07:29 PM EDT

Trackback Added: ext3 undelete;   I “mis-deleted” a big file several days ago. So I umount the the partition immediately and searched the recovery method because I knew (but forgot) some methods to recovery file in Linux. However, the result is disappointed. Alt...

marco 03/13/06 08:04:20 AM EST

U have saved my life.
I had lost all my application files under tomcat with de deploy command... no backup ..gulp
now I have a 128MB ascii file with my lost files, it's great.

U are a GURU,
thanx

marco 03/13/06 08:04:04 AM EST

U have saved my life.
I had lost all my application files under tomcat with de deploy command... no backup ..gulp
now I have a 128MB ascii file with my lost files, it's great.

U are a GURU,
thanx

@ThingsExpo Stories
SYS-CON Events announced today that Matrix.org has been named “Silver Sponsor” of Internet of @ThingsExpo, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Matrix is an ambitious new open standard for open, distributed, real-time communication over IP. It defines a new approach for interoperable Instant Messaging and VoIP based on pragmatic HTTP APIs and WebRTC, and provides open source reference implementations to showcase and bootstrap the new standard. Our focus is on simplicity, security, and supporting the fullest feature set.
WebRTC defines no default signaling protocol, causing fragmentation between WebRTC silos. SIP and XMPP provide possibilities, but come with considerable complexity and are not designed for use in a web environment. In his session at Internet of @ThingsExpo, Matthew Hodgson, technical co-founder of the Matrix.org, will discuss how Matrix is a new non-profit Open Source Project that defines both a new HTTP-based standard for VoIP & IM signaling and provides reference implementations.

SUNNYVALE, Calif., Oct. 20, 2014 /PRNewswire/ -- Spansion Inc. (NYSE: CODE), a global leader in embedded systems, today added 96 new products to the Spansion® FM4 Family of flexible microcontrollers (MCUs). Based on the ARM® Cortex®-M4F core, the new MCUs boast a 200 MHz operating frequency and support a diverse set of on-chip peripherals for enhanced human machine interfaces (HMIs) and machine-to-machine (M2M) communications. The rich set of periphera...

SYS-CON Events announced today that Aria Systems, the recurring revenue expert, has been named "Bronze Sponsor" of SYS-CON's 15th International Cloud Expo®, which will take place on November 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Aria Systems helps leading businesses connect their customers with the products and services they love. Industry leaders like Pitney Bowes, Experian, AAA NCNU, VMware, HootSuite and many others choose Aria to power their recurring revenue business and deliver exceptional experiences to their customers.
The Internet of Things (IoT) is going to require a new way of thinking and of developing software for speed, security and innovation. This requires IT leaders to balance business as usual while anticipating for the next market and technology trends. Cloud provides the right IT asset portfolio to help today’s IT leaders manage the old and prepare for the new. Today the cloud conversation is evolving from private and public to hybrid. This session will provide use cases and insights to reinforce the value of the network in helping organizations to maximize their company’s cloud experience.
The Internet of Things (IoT) is making everything it touches smarter – smart devices, smart cars and smart cities. And lucky us, we’re just beginning to reap the benefits as we work toward a networked society. However, this technology-driven innovation is impacting more than just individuals. The IoT has an environmental impact as well, which brings us to the theme of this month’s #IoTuesday Twitter chat. The ability to remove inefficiencies through connected objects is driving change throughout every sector, including waste management. BigBelly Solar, located just outside of Boston, is trans...
SYS-CON Events announced today that Matrix.org has been named “Silver Sponsor” of Internet of @ThingsExpo, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Matrix is an ambitious new open standard for open, distributed, real-time communication over IP. It defines a new approach for interoperable Instant Messaging and VoIP based on pragmatic HTTP APIs and WebRTC, and provides open source reference implementations to showcase and bootstrap the new standard. Our focus is on simplicity, security, and supporting the fullest feature set.
Predicted by Gartner to add $1.9 trillion to the global economy by 2020, the Internet of Everything (IoE) is based on the idea that devices, systems and services will connect in simple, transparent ways, enabling seamless interactions among devices across brands and sectors. As this vision unfolds, it is clear that no single company can accomplish the level of interoperability required to support the horizontal aspects of the IoE. The AllSeen Alliance, announced in December 2013, was formed with the goal to advance IoE adoption and innovation in the connected home, healthcare, education, aut...
SYS-CON Events announced today that Red Hat, the world's leading provider of open source solutions, will exhibit at Internet of @ThingsExpo, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Red Hat is the world's leading provider of open source software solutions, using a community-powered approach to reliable and high-performing cloud, Linux, middleware, storage and virtualization technologies. Red Hat also offers award-winning support, training, and consulting services. As the connective hub in a global network of enterprises, partners, a...
The only place to be June 9-11 is Cloud Expo & @ThingsExpo 2015 East at the Javits Center in New York City. Join us there as delegates from all over the world come to listen to and engage with speakers & sponsors from the leading Cloud Computing, IoT & Big Data companies. Cloud Expo & @ThingsExpo are the leading events covering the booming market of Cloud Computing, IoT & Big Data for the enterprise. Speakers from all over the world will be hand-picked for their ability to explore the economic strategies that utility/cloud computing provides. Whether public, private, or in a hybrid form, clo...
Software AG helps organizations transform into Digital Enterprises, so they can differentiate from competitors and better engage customers, partners and employees. Using the Software AG Suite, companies can close the gap between business and IT to create digital systems of differentiation that drive front-line agility. We offer four on-ramps to the Digital Enterprise: alignment through collaborative process analysis; transformation through portfolio management; agility through process automation and integration; and visibility through intelligent business operations and big data.
The Transparent Cloud-computing Consortium (abbreviation: T-Cloud Consortium) will conduct research activities into changes in the computing model as a result of collaboration between "device" and "cloud" and the creation of new value and markets through organic data processing High speed and high quality networks, and dramatic improvements in computer processing capabilities, have greatly changed the nature of applications and made the storing and processing of data on the network commonplace.
Be Among the First 100 to Attend & Receive a Smart Beacon. The Physical Web is an open web project within the Chrome team at Google. Scott Jenson leads a team that is working to leverage the scalability and openness of the web to talk to smart devices. The Physical Web uses bluetooth low energy beacons to broadcast an URL wirelessly using an open protocol. Nearby devices can find all URLs in the room, rank them and let the user pick one from a list. Each device is, in effect, a gateway to a web page. This unlocks entirely new use cases so devices can offer tiny bits of information or simple i...
The Internet of Things (IoT) is going to require a new way of thinking and of developing software for speed, security and innovation. This requires IT leaders to balance business as usual while anticipating for the next market and technology trends. Cloud provides the right IT asset portfolio to help today’s IT leaders manage the old and prepare for the new. Today the cloud conversation is evolving from private and public to hybrid. This session will provide use cases and insights to reinforce the value of the network in helping organizations to maximize their company’s cloud experience.
Things are being built upon cloud foundations to transform organizations. This CEO Power Panel at 15th Cloud Expo, moderated by Roger Strukhoff, Cloud Expo and @ThingsExpo conference chair, will address the big issues involving these technologies and, more important, the results they will achieve. How important are public, private, and hybrid cloud to the enterprise? How does one define Big Data? And how is the IoT tying all this together?
TechCrunch reported that "Berlin-based relayr, maker of the WunderBar, an Internet of Things (IoT) hardware dev kit which resembles a chunky chocolate bar, has closed a $2.3 million seed round, from unnamed U.S. and Switzerland-based investors. The startup had previously raised a €250,000 friend and family round, and had been on track to close a €500,000 seed earlier this year — but received a higher funding offer from a different set of investors, which is the $2.3M round it’s reporting."
The Industrial Internet revolution is now underway, enabled by connected machines and billions of devices that communicate and collaborate. The massive amounts of Big Data requiring real-time analysis is flooding legacy IT systems and giving way to cloud environments that can handle the unpredictable workloads. Yet many barriers remain until we can fully realize the opportunities and benefits from the convergence of machines and devices with Big Data and the cloud, including interoperability, data security and privacy.
All major researchers estimate there will be tens of billions devices - computers, smartphones, tablets, and sensors - connected to the Internet by 2020. This number will continue to grow at a rapid pace for the next several decades. Over the summer Gartner released its much anticipated annual Hype Cycle report and the big news is that Internet of Things has now replaced Big Data as the most hyped technology. Indeed, we're hearing more and more about this fascinating new technological paradigm. Every other IT news item seems to be about IoT and its implications on the future of digital busines...
Cultural, regulatory, environmental, political and economic (CREPE) conditions over the past decade are creating cross-industry solution spaces that require processes and technologies from both the Internet of Things (IoT), and Data Management and Analytics (DMA). These solution spaces are evolving into Sensor Analytics Ecosystems (SAE) that represent significant new opportunities for organizations of all types. Public Utilities throughout the world, providing electricity, natural gas and water, are pursuing SmartGrid initiatives that represent one of the more mature examples of SAE. We have s...
The Internet of Things needs an entirely new security model, or does it? Can we save some old and tested controls for the latest emerging and different technology environments? In his session at Internet of @ThingsExpo, Davi Ottenheimer, EMC Senior Director of Trust, will review hands-on lessons with IoT devices and reveal privacy options and a new risk balance you might not expect.