Welcome!

Linux Containers Authors: Yeshim Deniz, Liz McMillan, Elizabeth White, Pat Romanski, Jyoti Bansal

Related Topics: Linux Containers

Linux Containers: 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 delaPlex will exhibit at SYS-CON's @ThingsExpo, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. delaPlex pioneered Software Development as a Service (SDaaS), which provides scalable resources to build, test, and deploy software. It’s a fast and more reliable way to develop a new product or expand your in-house team.
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, whic...
In his keynote at @ThingsExpo, Chris Matthieu, Director of IoT Engineering at Citrix and co-founder and CTO of Octoblu, focused on building an IoT platform and company. He provided a behind-the-scenes look at Octoblu’s platform, business, and pivots along the way (including the Citrix acquisition of Octoblu).
With major technology companies and startups seriously embracing Cloud strategies, now is the perfect time to attend @CloudExpo | @ThingsExpo, June 6-8, 2017, at the Javits Center in New York City, NY and October 31 - November 2, 2017, Santa Clara Convention Center, CA. Learn what is going on, contribute to the discussions, and ensure that your enterprise is on the right path to Digital Transformation.
SYS-CON Events announced today that DivvyCloud will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. DivvyCloud software enables organizations to achieve their cloud computing goals by simplifying and automating security, compliance and cost optimization of public and private cloud infrastructure. Using DivvyCloud, customers can leverage programmatic Bots to identify and remediate common cloud problems in rea...
SYS-CON Events announced today that Tintri, Inc, a leading provider of enterprise cloud infrastructure, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Tintri offers an enterprise cloud platform built with public cloud-like web services and RESTful APIs. Organizations use Tintri all-flash storage with scale-out and automation as a foundation for their own clouds – to build agile development environments...
SYS-CON Events announced today that Carbonite will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Carbonite protects your entire IT footprint with the right level of protection for each workload, ensuring lower costs and dependable solutions with DoubleTake and Evault.
SYS-CON Events announced today that Technologic Systems Inc., an embedded systems solutions company, will exhibit at SYS-CON's @ThingsExpo, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Technologic Systems is an embedded systems company with headquarters in Fountain Hills, Arizona. They have been in business for 32 years, helping more than 8,000 OEM customers and building over a hundred COTS products that have never been discontinued. Technologic Systems’ pr...
SYS-CON Events announced today that Tappest will exhibit MooseFS at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. MooseFS is a breakthrough concept in the storage industry. It allows you to secure stored data with either duplication or erasure coding using any server. The newest – 4.0 version of the software enables users to maintain the redundancy level with even 50% less hard drive space required. The software func...
SYS-CON Events announced today that EARP will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. "We are a software house, so we perfectly understand challenges that other software houses face in their projects. We can augment a team, that will work with the same standards and processes as our partners' internal teams. Our teams will deliver the same quality within the required time and budget just as our partn...
SYS-CON Events announced today that Cloudistics, an on-premises cloud computing company, has been named “Bronze Sponsor” of SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Cloudistics delivers a complete public cloud experience with composable on-premises infrastructures to medium and large enterprises. Its software-defined technology natively converges network, storage, compute, virtualization, and management into a ...
New competitors, disruptive technologies, and growing expectations are pushing every business to both adopt and deliver new digital services. This ‘Digital Transformation’ demands rapid delivery and continuous iteration of new competitive services via multiple channels, which in turn demands new service delivery techniques – including DevOps. In this power panel at @DevOpsSummit 20th Cloud Expo, moderated by DevOps Conference Co-Chair Andi Mann, panelists will examine how DevOps helps to meet th...
DevOps at Cloud Expo – being held October 31 - November 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA – announces that its Call for Papers is open. Born out of proven success in agile development, cloud computing, and process automation, DevOps is a macro trend you cannot afford to miss. From showcase success stories from early adopters and web-scale businesses, DevOps is expanding to organizations of all sizes, including the world's largest enterprises – and delivering real r...
Most technology leaders, contemporary and from the hardware era, are reshaping their businesses to do software in the hope of capturing value in IoT. Although IoT is relatively new in the market, it has already gone through many promotional terms such as IoE, IoX, SDX, Edge/Fog, Mist Compute, etc. Ultimately, irrespective of the name, it is about deriving value from independent software assets participating in an ecosystem as one comprehensive solution.
SYS-CON Events announced today that A&I Solutions has been named “Bronze Sponsor” of SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Founded in 1999, A&I Solutions is a leading information technology (IT) software and services provider focusing on best-in-class enterprise solutions. By partnering with industry leaders in technology, A&I assures customers high performance levels across all IT environments including: mai...
Every successful software product evolves from an idea to an enterprise system. Notably, the same way is passed by the product owner's company. In his session at 20th Cloud Expo, Oleg Lola, CEO of MobiDev, will provide a generalized overview of the evolution of a software product, the product owner, the needs that arise at various stages of this process, and the value brought by a software development partner to the product owner as a response to these needs.
SYS-CON Events announced today that Systena America will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Systena Group has been in business for various software development and verification in Japan, US, ASEAN, and China by utilizing the knowledge we gained from all types of device development for various industries including smartphones (Android/iOS), wireless communication, security technology and IoT serv...
Five years ago development was seen as a dead-end career, now it’s anything but – with an explosion in mobile and IoT initiatives increasing the demand for skilled engineers. But apart from having a ready supply of great coders, what constitutes true ‘DevOps Royalty’? It’ll be the ability to craft resilient architectures, supportability, security everywhere across the software lifecycle. In his keynote at @DevOpsSummit at 20th Cloud Expo, Jeffrey Scheaffer, GM and SVP, Continuous Delivery Busine...
SYS-CON Events announced today that Outscale will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Outscale's technology makes an automated and adaptable Cloud available to businesses, supporting them in the most complex IT projects while controlling their operational aspects. You boost your IT infrastructure's reactivity, with request responses that only take a few seconds.
SYS-CON Events announced today that Progress, a global leader in application development, has been named “Bronze Sponsor” of SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Enterprises today are rapidly adopting the cloud, while continuing to retain business-critical/sensitive data inside the firewall. This is creating two separate data silos – one inside the firewall and the other outside the firewall. Cloud ISVs ofte...