Click here to close now.

Welcome!

Linux Authors: Roger Strukhoff, AppDynamics Blog, Carmen Gonzalez, Bob Gourley, Hovhannes Avoyan

Related Topics: Linux

Linux: Article

Why Recovering a Deleted Ext3 File Is Difficult . . .

and why you should back up important files

The file we are trying to recover is /home/carrier/oops.dat and we can see it previously allocated to inode 415,926. The "(28)" shows us that the directory entry structure is 28 bytes long, but we don't care about that.

File Carving Recovery
The first recovery technique, called file carving, uses signatures from the deleted file. Many file types have standard values in the first bytes of the file header, and this recovery technique looks for the header value of the deleted file to determine where the file may have started. For example, JPEG files start with 0xffd8 and end with 0xffd9. To recover a deleted JPEG file, we would look at the first two bytes of each block and look for one with 0xffd8 in the first two bytes. When we find such a block, we look for a block that has 0xffd9 in it. The data in between are assumed to be the file. Unfortunately, not all file types have a standard footer signature, so determining where to end is difficult. An example of an open source tool that does file carving is foremost and there are several commercial options as well.

We can run a tool like foremost on the full file system, but we'll probably end up with way too many files, including allocated ones. We therefore want to run it on as little data as possible. The first way we can restrict the data size is to examine only the block group where the file was located. Remember that inodes and blocks for a file are allocated to the same block group, if there is room. In our case, we know which inode the file used and therefore we can examine only the blocks in the same group. The imap command in debugfs will tell us to which block group an inode belongs:

debugfs: imap <415926>
Inode 415926 is part of block group 25
    located at block 819426, offset 0x0a80

The output of the fsstat command in TSK would also tell us this:

# fsstat /dev/hda5
[...]
Group: 25:
   Inode Range: 408801 - 425152
   Block Range: 819200 - 851967

We next need to determine the blocks that are in the block group of the deleted file. We can see them in the previous fsstat output, but if we're using debugfs , we'll need to calculate the range. The stats command gives us the number of blocks in each group:

debugfs: stats
[...]
Blocks per group: 32768
[...]

Since we are looking at block group 25, then the block range is from 819,200 (25 * 32,768) to 851,967 (26 * 32,768 - 1). By focusing on only these blocks, we are looking at 128MB instead of the full file system. Although if we can't find the file in these blocks, we'll still need to search the full file system.

The next step to reduce the data we analyze is to extract the unallocated blocks from the file system because that is where our deleted file will be located. debugfs does not currently allow us to extract the unallocated space from only a specific block group, so we will need to use the dls tool from TSK.

# dls /dev/hda5 819200-851867 > /mnt/unalloc.dat

The above command will save the unallocated blocks in block group 25 to a file named /mnt/unalloc.dat. Make sure that this file is on a different file system because otherwise you may end up overwriting your deleted file.

Now we can run the foremost tool on the unallocated data. foremost can recover only file types for which it has been configured. If foremost doesn't have the header signature for the type of the deleted file, you'll need to examine some similar files and customize the configuration file. We can run it as follows:

# foremost -d -i /mnt/unalloc.dat -o /mnt/output/

The -d option will try to detect which blocks are indirect blocks and won't include them in the final output file. The /mnt/output/ directory will contain the files that could be recovered. If your file is not in there, you can expand your search to all unallocated blocks in the file system instead of only the blocks in the block group.

Journal-Based Recovery
The second method for trying to recover the files is to use the journal. We already saw that inode updates are first recorded in the journal, but the important concept here is that the entire block in which an inode is located is recorded in the journal. Therefore, when one inode is updated, the journal will contain copies of other inodes stored in the same block. The previous version of our deleted file's inode may exist in the journal because another file was updated before the deletion.

The easiest way to look for previous versions of the inode is by using the logdump -i command in debugfs:

debugfs: logdump -i <415926>
Inode 415926 is at group 25, block 819426, offset 2688
Journal starts at block 1, transaction 104588
  FS block 819426 logged at sequence 104940, journal block 2687
   (inode block for inode 415926):
   Inode: 415926 Type: regular Mode: 0664 Flags: 0x0
   User: 500 Group: 500 Size: 2048000
   [...]
   Blocks: (0+12): 843274 (IND): 843286
[...]

In this case, we found a previous copy of the inode and the file content blocks are listed on the last line. The last line shows that the first block of the file is 843,274 and the next 12 blocks in the file system are the next 12 blocks in the file. The file is large and requires an indirect block, which is located in block 843,286. So far, all blocks are consecutive and there was no fragmentation. Block 843,286 contains the rest of the block addresses, so we should try to look at a previous version to learn where the rest of the file is located. We can see if there is a copy in the journal using logdump -b:

debugfs: logdump -b 843286 -c

Unfortunately, we don't find a copy of the block that contains the original list of block pointers so, if we want to recover the file, we need to assume that the remaining file content is stored in block 843,287 and onward. A more advanced approach would also consider which blocks are currently allocated and skip over those. The data can be extracted with tools such as dd or the Linux Disk Editor. The journal can also be searched using the jls and jcat tools from TSK.

Conclusion
File recovery with Ext3 is not a trivial matter, which reinforces the concept of making backups of important files. If the file was not fragmented, then searching for its header signature can be useful, but the tool needs to know to ignore the indirect blocks and where to stop copying (not all files have a standard footer signature). Restricting the search to the local block group can help save time. The journal could be useful if files near the deleted file were recently updated and a previous version of the inode existed, but this is not always guaranteed and the file's indirect block may not exist.

References and Bibliography

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 Intelligent Systems Services will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Established in 1994, Intelligent Systems Services Inc. is located near Washington, DC, with representatives and partners nationwide. ISS’s well-established track record is based on the continuous pursuit of excellence in designing, implementing and supporting nationwide clients’ mission-critical systems. ISS has completed many successful projects in Healthcare, Commercial, Manufacturing, ...
Sonus Networks introduced the Sonus WebRTC Services Solution, a virtualized Web Real-Time Communications (WebRTC) offer, purpose-built for the Cloud. The WebRTC Services Solution provides signaling from WebRTC-to-WebRTC applications and interworking from WebRTC-to-Session Initiation Protocol (SIP), delivering advanced real-time communications capabilities on mobile applications and on websites, which are accessible via a browser.
SYS-CON Events announced today that B2Cloud, a provider of enterprise resource planning software, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. B2cloud develops the software you need. They have the ideal tools to help you work with your clients. B2Cloud’s main solutions include AGIS – ERP, CLOHC, AGIS – Invoice, and IZUM
SYS-CON Events announced today that Tufin, the market-leading provider of Security Policy Orchestration Solutions, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. As the market leader of Security Policy Orchestration, Tufin automates and accelerates network configuration changes while maintaining security and compliance. Tufin's award-winning Orchestration Suite™ gives IT organizations the power and agility to enforce security policy across complex, multi-vendor enterprise networks. With more than 1...
SYS-CON Events announced today that Cloudian, Inc., the leading provider of hybrid cloud storage solutions, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Cloudian, Inc., is a Foster City, California - based software company specializing in cloud storage software. The main product is Cloudian, an Amazon S3-compliant cloud object storage platform, the bedrock of cloud computing systems, that enables cloud service providers and enterprises to build reliable, affordable and scalable cloud storage solu...
“With easy-to-use SDKs for Atmel’s platforms, IoT developers can now reap the benefits of realtime communication, and bypass the security pitfalls and configuration complexities that put IoT deployments at risk,” said Todd Greene, founder & CEO of PubNub. PubNub will team with Atmel at CES 2015 to launch full SDK support for Atmel’s MCU, MPU, and Wireless SoC platforms. Atmel developers now have access to PubNub’s secure Publish/Subscribe messaging with guaranteed ¼ second latencies across PubNub’s 14 global points-of-presence. PubNub delivers secure communication through firewalls, proxy ser...
SYS-CON Events announced today that Gridstore™, the leader in hyper-converged infrastructure purpose-built to optimize Microsoft workloads, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Gridstore™ is the leader in hyper-converged infrastructure purpose-built for Microsoft workloads and designed to accelerate applications in virtualized environments. Gridstore’s hyper-converged infrastructure is the industry’s first all flash version of HyperConverged Appliances that include both compute and storag...
SYS-CON Events announced today that IDenticard will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. IDenticard™ is the security division of Brady Corp (NYSE: BRC), a $1.5 billion manufacturer of identification products. We have small-company values with the strength and stability of a major corporation. IDenticard offers local sales, support and service to our customers across the United States and Canada. Our partner network encompasses some 300 of the world's leading systems integrators and security s...
Containers and microservices have become topics of intense interest throughout the cloud developer and enterprise IT communities. Accordingly, attendees at the upcoming 16th Cloud Expo at the Javits Center in New York June 9-11 will find fresh new content in a new track called PaaS | Containers & Microservices Containers are not being considered for the first time by the cloud community, but a current era of re-consideration has pushed them to the top of the cloud agenda. With the launch of Docker's initial release in March of 2013, interest was revved up several notches. Then late last...
So I guess we’ve officially entered a new era of lean and mean. I say this with the announcement of Ubuntu Snappy Core, “designed for lightweight cloud container hosts running Docker and for smart devices,” according to Canonical. “Snappy Ubuntu Core is the smallest Ubuntu available, designed for security and efficiency in devices or on the cloud.” This first version of Snappy Ubuntu Core features secure app containment and Docker 1.6 (1.5 in main release), is available on public clouds, and for ARM and x86 devices on several IoT boards. It’s a Trend! This announcement comes just as...
SYS-CON Events announced today the IoT Bootcamp – Jumpstart Your IoT Strategy, being held June 9–10, 2015, in conjunction with 16th Cloud Expo and Internet of @ThingsExpo at the Javits Center in New York City. This is your chance to jumpstart your IoT strategy. Combined with real-world scenarios and use cases, the IoT Bootcamp is not just based on presentations but includes hands-on demos and walkthroughs. We will introduce you to a variety of Do-It-Yourself IoT platforms including Arduino, Raspberry Pi, BeagleBone, Spark and Intel Edison. You will also get an overview of cloud technologies s...
Health care systems across the globe are under enormous strain, as facilities reach capacity and costs continue to rise. M2M and the Internet of Things have the potential to transform the industry through connected health solutions that can make care more efficient while reducing costs. In fact, Vodafone's annual M2M Barometer Report forecasts M2M applications rising to 57 percent in health care and life sciences by 2016. Lively is one of Vodafone's health care partners, whose solutions enable older adults to live independent lives while staying connected to loved ones. M2M will continue to gr...
While not quite mainstream yet, WebRTC is starting to gain ground with Carriers, Enterprises and Independent Software Vendors (ISV’s) alike. WebRTC makes it easy for developers to add audio and video communications into their applications by using Web browsers as their platform. But like any market, every customer engagement has unique requirements, as well as constraints. And of course, one size does not fit all. In her session at WebRTC Summit, Dr. Natasha Tamaskar, Vice President, Head of Cloud and Mobile Strategy at GENBAND, will explore what is needed to take a real time communications ...
The best mobile applications are augmented by dedicated servers, the Internet and Cloud services. Mobile developers should focus on one thing: writing the next socially disruptive viral app. Thanks to the cloud, they can focus on the overall solution, not the underlying plumbing. From iOS to Android and Windows, developers can leverage cloud services to create a common cross-platform backend to persist user settings, app data, broadcast notifications, run jobs, etc. This session provides a high level technical overview of many cloud services available to mobile app developers, includi...
“In the past year we've seen a lot of stabilization of WebRTC. You can now use it in production with a far greater degree of certainty. A lot of the real developments in the past year have been in things like the data channel, which will enable a whole new type of application," explained Peter Dunkley, Technical Director at Acision, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that Vicom Computer Services, Inc., a provider of technology and service solutions, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. They are located at booth #427. Vicom Computer Services, Inc. is a progressive leader in the technology industry for over 30 years. Headquartered in the NY Metropolitan area. Vicom provides products and services based on today’s requirements around Unified Networks, Cloud Computing strategies, Virtualization around Software defined Data Ce...
Dave will share his insights on how Internet of Things for Enterprises are transforming and making more productive and efficient operations and maintenance (O&M) procedures in the cleantech industry and beyond. Speaker Bio: Dave Landa is chief operating officer of Cybozu Corp (kintone US). Based in the San Francisco Bay Area, Dave has been on the forefront of the Cloud revolution driving strategic business development on the executive teams of multiple leading Software as a Services (SaaS) application providers dating back to 2004. Cybozu's kintone.com is a leading global BYOA (Build Your O...
SYS-CON Media announced today that @WebRTCSummit Blog, the largest WebRTC resource in the world, has been launched. @WebRTCSummit Blog offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. @WebRTCSummit Blog can be bookmarked ▸ Here @WebRTCSummit conference site can be bookmarked ▸ Here
SYS-CON Events announced today that Ciqada will exhibit at SYS-CON's @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Ciqada™ makes it easy to connect your products to the Internet. By integrating key components - hardware, servers, dashboards, and mobile apps - into an easy-to-use, configurable system, your products can quickly and securely join the internet of things. With remote monitoring, control, and alert messaging capability, you will meet your customers' needs of tomorrow - today! Ciqada. Let your products take flight. For more inform...
SYS-CON Events announced today that GENBAND, a leading developer of real time communications software solutions, has been named “Silver Sponsor” of SYS-CON's WebRTC Summit, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. The GENBAND team will be on hand to demonstrate their newest product, Kandy. Kandy is a communications Platform-as-a-Service (PaaS) that enables companies to seamlessly integrate more human communications into their Web and mobile applications - creating more engaging experiences for their customers and boosting collaboration and productiv...