why is the iso image corrupt?

[regsim]

hello,
i'm trying to prepare an ubuntu installation disk for version 8.10-alternate.
* i downloaded the iso image from an official download site
* ran the md5sum check, which gave a hash string identical to the official one for the ubuntu version i chose, confirming that the iso image downloaded was not corrupted.
* i then proceeded to burn the disk using nero express, i chose the image option, and a slow speed x 8 on a brand new non-rewritable disk.
* when i checked the file burnt onto the disk (using the command "md5sum -c md5sum.txt"), the result was 3 files not readable, and one that did not match.
* i repeated the burning and the checking again, but obtained the same result.
can the problem possibly be in the downloaded file, despite the fact that the hash codes matched?
any ideas on what may be the cause of this problem?
thanks!

[_madman_]

First off, Nero is a MS Windows program (the linux version is almost a joke), that shouldn't matter though since an iso image is an iso image (not system dependent). Did you just try booting the CD/DVD? Optical drives can be very flakey; for instance my Knoppix 5.1 CD some times spits out error messages when booting. According to the help documents, this is a result of a badly burned disc - not so. The disc boots fine 29 out of 30 times. I think it's just a problem with the drive staying in sync and not timing out or the like.

You checked the file you downloaded - it's fine. You burned it and got no errors during the burn - it's fine. You put it in your drive and reboot, it fires up your new GNU/Linux installation with no strange error(s) - it's fine.

Otherwise blame it on some big software corporation that is trying to dominate the industry, but slowly loosing the battle with each new release of their "OS"

...oh yah, and the new disc is no longer the original .iso file, it's all the files extracted with the file system in place (so some of the data is now physical so to speak) so it probably shouldn't match!

[regsim]

thank you for replying,
you are right, my current operating system is windows. the reason i'm doing all this is because i'm trying to move over to linux.
as far as the booting is concerned, i never got that far - when the checking of the burnt discs gave me failures twice, i decided to post the question. i did not want to risk using a faulty installer disk, especially since this is my very first linux installation.
so you think i could try using one of these ubuntu installation disks i created safely for my first installation?

[Roxoff]

You've got a few options available to you. Try one or more of these:

* Is there a local Linux Users Group close to where you are? They may be able to provide you with a tested and working disk
* You can order a disk through Ubuntu, I believe. They'll send it you for free, the only drawback is it may take a little time to turn up.
* You could try a live CD version of the OS, it may be a smaller disk and burn a lot easier.
* Does Ubuntu have a network install version? The CD images for these are _really_ small, and hence are less prone to problems.
* Do you have any close-by friends who could try and burn a disk for you?
* You could purchase a disk from CheapBytes (USA) or LinuxEmporium (UK). I'm sure there are similar suppliers around the world too.

[_madman_]

Go for it !

The whole point of the md5sums is to make sure that each byte in the file you retrieve (the .iso image) is the same as each byte that the developer (that you trust, such as Ubuntu) intended. This is really for the somewhat paranoid, who think that someone may try to slip a trojan version of the file into their download stream, or possibly a corrupted image on some mirror site. You could compare the actual total byte sizes, but a talented hacker (or untalented wanna-be using software written by a talented hacker) could alter the bits inside the file with some sort of hex editor, leaving the file size the same as the original. This is what checking md5sums guards against.

Since you checked the .iso file after download and it passed, you know that the file you now have on your local system is legitimate. No further checking is necessary. Besides, when was the last time you ran an md5sum on a MS Windows file?

[regsim]

thanks a lot!
i discovered in the end that - despite the "failed" md5sum check, the disk installed ubuntu without any problems.

[elija]

Surely the MD5 of the files on the CD will be different to the MD5 of the .iso as they are not the .iso, they are the contents of the .iso

Or have I missed something?

[regsim]

indeed - the cd no longer contained one (iso) file, but a whole host of files. and in fact, the test i ran using the command
"md5sum -c md5sum.txt" (see link to my instructions below), yields an output which is a long long list of paths/filenames, each with a corresponding outcome: OK or FAILED.

my problem was, that this test kept giving me FAILED on three of the files on the installation disk. thanks to _madman_ i finally mustered the guts to try installing ubuntu with one of the disks i had prepared, ignoring the FAILED tests, and succeeded!

thanks again to _madman_, and to all of you who replied!

the instructions i followed:
How to check an Ubuntu .iso file b-initials