file integrity fails LXFDVD155

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file integrity

Postby steve_usa_2011 » Thu Feb 09, 2012 6:04 pm

Hi Ben,

If you want to duplicate this problem using Brasero as mentioned in your post you need the real LXFDVD155 from the magazine, and a burned dvd from the 155.iso (from torrent). The filed integrity you mention looks exactly like the when I use a shortcut from the desktop, which happens after inserting the dvd, canceling the prompt which appears with an Autorun option, and then right-clicking the LXFDVD155 icon which shows on the desktop and picking Check disc.

Both have matching md5sums (md5sum /dev/dvd ---067c2fe8009ed614207096674ec9db07). If generating and iso from each dvd as nelz suggested (cp /dev/dvd fromdvd.iso from the burned dvd and cp /dev/dvd realdvd.iso from the real dvd) and comparing the size to the torrent 155.iso they all match in size --- 4379901952. If I run and md5sum against each of my 3 iso files ( 155.iso -- fromdvd.iso -- realdvd.iso) they are all the same --- -067c2fe8009ed614207096674ec9db07.

My question boils down to why does the real dvd pass the integrity check, but the burned dvd does not?


Thanks alot for your help, still having alot of fun!

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Re: file integrity

Postby towy71 » Thu Feb 09, 2012 6:24 pm

steve_usa_2011 wrote:Thanks alot for your help, still having alot of fun!steve_usa_2011
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Postby nelz » Thu Feb 09, 2012 9:51 pm

Well, it's nothing do do with it being a hybrid ISO, I can see that now having tested it. Since the checksums from the pressed and burned DVDs (and the original ISO image) are the same, the DVD you have burned is identical to the pressed one. Therefore it should pass any valid integrity test.

It seems that Brassero is doing something odd. It may be trying to use an embedded checksum that some discs use, but the LXFDVD does not contain.
"Insanity: doing the same thing over and over again and expecting different results." (Albert Einstein)
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file integrity fails

Postby steve_usa_2011 » Thu Feb 09, 2012 10:20 pm

Thank You nelz, I couldn't clarify my problem without the input from you and the others. That is what I was trying to say but couldn't nail it.

I never got Brasero 3.0.0 to install in Ubuntu 10.04 LTS, but I booted from the LXFDVD155 in a connected usb dvd drive and thus was able to run the file integrity check on my burned dvd using Brasero 3.2 (I think). Failed again! There seems to be no checksum on the burned version according to Brasero --- maybe this can lead to a solution.

Now that you have identified the basis for my misery, maybe someone can shed light on the actual differences between a burned dvd and the "pressed one" and why the "checksum" is only visible to Brasero on the pressed dvd --- and now--- where is it located specificallyon the dvd, and how can I see it?

still having fun, a trying to learn alot!
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Postby nelz » Fri Feb 10, 2012 12:47 am

There isn't a checksum on the DVD. There is a program to embed an md5 checksum in an ISO image, I have used it in the past, but I didn't use it on recent LXFDVDs as I wasn't sure how nicely it would play with the hybrid ISO[1]. It is used by some distro discs, they usually offer to check disc integrity before installation.

However, if this is the cause, Brassero is wrong. If there is no implanted md5sum, it should state so rather that reporting a failed integrity check.

[1] The implantisomd5 man page only states that it

[code]embeds an MD5 checksum in an unused section of and ISO9660 (.iso) image[/quote]

isohybrid also uses a normally unused area, the first 512 bytes. If implantisomd5 uses this area, it would break both USB booting and checksum checking.
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Postby Ben » Fri Feb 10, 2012 11:32 am

Hi Steve,

When you put in the DVD that came with the magazine and run a check, how long does it take? is it coming back almost straight away or does it take a few minutes?

It should take a little while to check a DVD against it's MD5 (as it will if you manually create an MD5 file in the way I mentioned in the previous post), and what I think is happenning is that the check is failing, but it's returning the wrong error code so the application thinks it's passed. If this happens, the check will return a result quickly (more-or-less straight away).

The error happens when the application checks the medium. The burned disk, on the other hand, gets passed this point (the application was designed to check burned disks) and fails the check at a later stage (when it looks for the MD5 file) which returns the correct error code, and so displays an error message.

I should point out that I'm not an expert on this application's code. However, the amount of time it's taking to check will tell you if it's actually doing an MD5 check or not. If it's taking a while to check the pressed DVD and returning a correct result, then ignore the above, the problem must be something else.

Cheers,

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file integrity fails

Postby steve_usa_2011 » Fri Feb 10, 2012 6:18 pm

Hi Ben,

The Disc Checking utility does not seem to be using an md5 since the "Use an MD5 file to check the disc" option box is not checked (if used, you have to provide path to the file). To answer about the length of time the error message takes to appear:

burned dvd approx. 1 second to generate the failed message.
real dvd from magazine almost instantly passes.


Please note that the original error was:

"The file integrity check could not be performed. No checksum file could be found on the disc."

This is a tough one for me, I just can't get my head around it.


Having alot of fun still
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Postby nelz » Fri Feb 10, 2012 10:06 pm

That error message confirms what I said in my previous post. There is no checksum to check because none was added to the ISO. Ben's theory about why the pressed DVD passes sounds about right, it should take much longer than a second to check the disc properly.

Since Brassero is trying to use an implanted checksum that was never implanted, any results it gives are meaningless and can be ignored. If the torrent downloaded without complaint, the ISO image has already passed a checksum test.
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file integrity

Postby steve_usa_2011 » Sat Feb 11, 2012 3:01 pm

Thanks for the help to all who contributed. I'll consider this one closed. Another topic for another day.

still having fun!
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