Click here to close now.

Welcome!

Linux Authors: Mike Kavis, Elizabeth White, Liz McMillan, Esmeralda Swartz, Jason Bloomberg

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 robomq.io 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. robomq.io is an interoperable and composable platform that connects any device to any application. It helps systems integrators and the solution providers build new and innovative products and service for industries requiring monitoring or intelligence from devices and sensors.
The 17th International Cloud Expo has announced that its Call for Papers is open. 17th International Cloud Expo, to be held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, APM, APIs, Microservices, Security, Big Data, Internet of Things, DevOps and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportunity. Submit your speaking proposal today!
SYS-CON Events announced today that Litmus Automation 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. Litmus Automation’s vision is to provide a solution for companies that are in a rush to embrace the disruptive Internet of Things technology and leverage it for real business challenges. Litmus Automation simplifies the complexity of connected devices applications with Loop, a secure and scalable cloud platform.
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 ...
As Marc Andreessen says software is eating the world. Everything is rapidly moving toward being software-defined – from our phones and cars through our washing machines to the datacenter. However, there are larger challenges when implementing software defined on a larger scale - when building software defined infrastructure. In his session at 16th Cloud Expo, Boyan Ivanov, CEO of StorPool, will provide some practical insights on what, how and why when implementing "software-defined" in the datacenter.
SYS-CON Events announced today that Solgenia 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, and the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Solgenia is the global market leader in Cloud Collaboration and Cloud Infrastructure software solutions. Designed to “Bridge the Gap” between Personal and Professional Social, Mobile and Cloud user experiences, our solutions help large and medium-sized organizations dr...
Internet of Things (IoT) will be a hybrid ecosystem of diverse devices and sensors collaborating with operational and enterprise systems to create the next big application. In their session at @ThingsExpo, Bramh Gupta, founder and CEO of robomq.io, and Fred Yatzeck, principal architect leading product development at robomq.io, will discuss how choosing the right middleware and integration strategy from the get-go will enable IoT solution developers to adapt and grow with the industry, while at the same time reduce Time to Market (TTM) by using plug and play capabilities offered by a robust I...
The world's leading Cloud event, Cloud Expo has launched Microservices Journal on the SYS-CON.com portal, featuring over 19,000 original articles, news stories, features, and blog entries. DevOps Journal is focused on this critical enterprise IT topic in the world of cloud computing. Microservices Journal 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. Follow new article posts on Twitter at @MicroservicesE
After making a doctor’s appointment via your mobile device, you receive a calendar invite. The day of your appointment, you get a reminder with the doctor’s location and contact information. As you enter the doctor’s exam room, the medical team is equipped with the latest tablet containing your medical history – he or she makes real time updates to your medical file. At the end of your visit, you receive an electronic prescription to your preferred pharmacy and can schedule your next appointment.
Wearable technology was dominant at this year’s International Consumer Electronics Show (CES) , and MWC was no exception to this trend. New versions of favorites, such as the Samsung Gear (three new products were released: the Gear 2, the Gear 2 Neo and the Gear Fit), shared the limelight with new wearables like Pebble Time Steel (the new premium version of the company’s previously released smartwatch) and the LG Watch Urbane. The most dramatic difference at MWC was an emphasis on presenting wearables as fashion accessories and moving away from the original clunky technology associated with t...
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...
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...
The WebRTC Summit 2015 New York, to be held June 9-11, 2015, at the Javits Center in New York, NY, announces that its Call for Papers is open. Topics include all aspects of improving IT delivery by eliminating waste through automated business models leveraging cloud technologies. WebRTC Summit is co-located with 16th International Cloud Expo, @ThingsExpo, Big Data Expo, and DevOps Summit.
SOA Software has changed its name to Akana. With roots in Web Services and SOA Governance, Akana has established itself as a leader in API Management and is expanding into cloud integration as an alternative to the traditional heavyweight enterprise service bus (ESB). The company recently announced that it achieved more than 90% year-over-year growth. As Akana, the company now addresses the evolution and diversification of SOA, unifying security, management, and DevOps across SOA, APIs, microservices, and more.
The list of ‘new paradigm’ technologies that now surrounds us appears to be at an all time high. From cloud computing and Big Data analytics to Bring Your Own Device (BYOD) and the Internet of Things (IoT), today we have to deal with what the industry likes to call ‘paradigm shifts’ at every level of IT. This is disruption; of course, we understand that – change is almost always disruptive.
SYS-CON Events announced today that SafeLogic has been named “Bag Sponsor” of SYS-CON's 16th International Cloud Expo® New York, which will take place June 9-11, 2015, at the Javits Center in New York City, NY. SafeLogic provides security products for applications in mobile and server/appliance environments. SafeLogic’s flagship product CryptoComply is a FIPS 140-2 validated cryptographic engine designed to secure data on servers, workstations, appliances, mobile devices, and in the Cloud.
GENBAND has announced that SageNet is leveraging the Nuvia platform to deliver Unified Communications as a Service (UCaaS) to its large base of retail and enterprise customers. Nuvia’s cloud-based solution provides SageNet’s customers with a full suite of business communications and collaboration tools. Two large national SageNet retail customers have recently signed up to deploy the Nuvia platform and the company will continue to sell the service to new and existing customers. Nuvia’s capabilities include HD voice, video, multimedia messaging, mobility, conferencing, Web collaboration, deskt...
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 Cisco, the worldwide leader in IT that transforms how people connect, communicate and collaborate, has been named “Gold Sponsor” of 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. Cisco makes amazing things happen by connecting the unconnected. Cisco has shaped the future of the Internet by becoming the worldwide leader in transforming how people connect, communicate and collaborate. Cisco and our partners are building the platform for the Internet of Everything by connecting the...
Temasys has announced senior management additions to its team. Joining are David Holloway as Vice President of Commercial and Nadine Yap as Vice President of Product. Over the past 12 months Temasys has doubled in size as it adds new customers and expands the development of its Skylink platform. Skylink leads the charge to move WebRTC, traditionally seen as a desktop, browser based technology, to become a ubiquitous web communications technology on web and mobile, as well as Internet of Things compatible devices.