Hi! Is it possible to read the MBR of an image and to store it inside a file?

(If you want to know what I really want to do: Creating a report on all contents of an vm image. [1] [2] Create the image on two different machines, compare them and see, that there are no important differences besides temporary files.) Cheers, adrelanos [1] https://github.com/Whonix/Whonix/issues/113 [2] https://github.com/Whonix/Whonix/blob/master/release/analyze_image

what about just using dd? dd if=disk.img of=mbr bs=512b count=1

> This will work for any format of disk. Of course for a raw format > disk this is just a slower way of reading the first 512 bytes from the > raw file.

Thank you all for your suggestions! Richard W.M. Jones: > I keep meaning to write a comprehensive "virt-diff" tool. I needed it > myself just yesterday. Most interesting. I guess there are two reasons for creating such a tool: just compare the images (show the diff) and/or check for malicious additions in the other image. Did you consider implementing the former or both? Do you think it's realistic to compare vm images with the goal of eventually finding deliberately hard to detect (malicious) changes? At the moment I am not trying to write a virt-diff like tool, but something simpler. A tool to create a report of all of a vm image's contents. (Checksums for all files, filesystem, for MBR and Volume Boot Record.) When publishing VM images, it might be useful to publish such a report together with the image, so others who re-build from source can be certain, they ended up with a very similar image. When having created two such reports, one could easily get a virt-diff like tool. > although that *only* > compares files, not the other data outside the filesystem What other data can there be outside the filesystem? I can think of: - MBR - Volume Boot Record Anything else? If these have been compared, the compared image should be as safe to use as the original one? (I could imagine that there can be extra data outside filesystem, maybe in regions outside the partition table, but those data shouldn't get executed after starting the image in a VM.) Cheers, adrelanos _______________________________________________ Libguestfs mailing list Libguestfs(a)redhat.com https://www.redhat.com/mailman/listinfo/libguestfs

Hi all, Piping in here as someone who has worked on file system and Registry differencing for a few years now. Taking diffs of a storage system is not a straightforward task. Hopefully, this message saves you some re-implementation heartache. In the forensics world, there is a tool called Fiwalk, which enumerates the contents of a file system and its metadata (with some basic data summaries, including libmagic and checksums). The tool "idifference" compares file system states and enumerates differences, using the Digital Forensics XML output from Fiwalk. A research publication on the forensic differencing process and idifference is here: http://dfrws.org/2012/proceedings/DFRWS2012-6.pdf Fiwalk is a component of The SleuthKit, here: https://github.com/sleuthkit/sleuthkit If you wish to use Fiwalk on your images, you should convert any of your disk images to a raw image or Expert Witness Format. Actually, I don't suppose qemu-img has a FUSE-like wrapper that exposes the underlying image as a raw file? DFXML has an entry on the Forensics Wiki: http://www.forensicswiki.org/wiki/Category:Digital_Forensics_XML As for your external-to-filesystem data question: I think you got the essential non-file-system data. I can imagine data fragments from past/shrunken file systems, or hidden-data regions that fall outside what's recorded in the partition table. My imagination runs dry there, though. --Alex On Fri, Nov 22, 2013 at 12:56 PM, adrelanos <adrelanos(a)riseup.net&gt; wrote: > Thank you all for your suggestions! > > Richard W.M. Jones: >> I keep meaning to write a comprehensive "virt-diff" tool. I needed it >> myself just yesterday. > > Most interesting. I guess there are two reasons for creating such a > tool: just compare the images (show the diff) and/or check for malicious > additions in the other image. > > Did you consider implementing the former or both? > > Do you think it's realistic to compare vm images with the goal of > eventually finding deliberately hard to detect (malicious) changes? > > At the moment I am not trying to write a virt-diff like tool, but > something simpler. A tool to create a report of all of a vm image's > contents. (Checksums for all files, filesystem, for MBR and Volume Boot > Record.) When publishing VM images, it might be useful to publish such a > report together with the image, so others who re-build from source can > be certain, they ended up with a very similar image. When having created > two such reports, one could easily get a virt-diff like tool. > >> although that *only* >> compares files, not the other data outside the filesystem > > What other data can there be outside the filesystem? > > I can think of: > > - MBR > - Volume Boot Record > > Anything else? > > If these have been compared, the compared image should be as safe to use > as the original one? > > (I could imagine that there can be extra data outside filesystem, maybe > in regions outside the partition table, but those data shouldn't get > executed after starting the image in a VM.) > > Cheers, > adrelanos > > _______________________________________________ > Libguestfs mailing list > Libguestfs(a)redhat.com > https://www.redhat.com/mailman/listinfo/libguestfs >

On Fri, Nov 22, 2013 at 05:56:00PM +0000, adrelanos wrote: > Thank you all for your suggestions! > > Richard W.M. Jones: > > I keep meaning to write a comprehensive "virt-diff" tool. I needed it > > myself just yesterday. > > Most interesting. I guess there are two reasons for creating such a > tool: just compare the images (show the diff) and/or check for malicious > additions in the other image. > > Did you consider implementing the former or both? For all the reasons that Alex goes into, it would just be for checking NON-malicious differences. The use case is to reverse engineer what files change in a guest when you perform an action (eg. install a Windows driver or run some Linux administrative command). [...] > At the moment I am not trying to write a virt-diff like tool, but > something simpler. A tool to create a report of all of a vm image's > contents. (Checksums for all files, filesystem, for MBR and Volume Boot > Record.) When publishing VM images, it might be useful to publish such a > report together with the image, so others who re-build from source can > be certain, they ended up with a very similar image. When having created > two such reports, one could easily get a virt-diff like tool. I think Matt Booth was doing something like this for Windows systems, with the aim of being able to recreate a Windows VM from a (smaller) description. Don't know what state that was/is in.

[...] > What other data can there be outside the filesystem? > > I can think of: > > - MBR > - Volume Boot Record > > Anything else? Potentially all unused space inside and between partitions / filesystems / logical volumes. The boot loader is sometimes stored in the space between the MBR and the first partition. Other peculiar things lie in other spaces.

However if you don't care about guests that are malicious / hiding data, then you can ignore everything except for the MBR and any non-zero data between the MBR and the first partition. Note for GPT you have to take into account two partition tables as well. > If these have been compared, the compared image should be as safe to use > as the original one? > > (I could imagine that there can be extra data outside filesystem, maybe > in regions outside the partition table, but those data shouldn't get > executed after starting the image in a VM.)

For Linux guests, you could boot from an external kernel & initrd ...