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ojek avatar Ubuntu

Customizing LiveCD 22.04 - unable to boot - Squashfs GPG signature / keyring issues

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ojek

I have spent last few weeks trying to customize Ubuntu LiveCD. Main guide which I follow is https://help.ubuntu.com/community/LiveCDCustomization This produces a modified image, but upon booting a newly created image I receive an error, after passing the GRUB screen, Unable to find a medium containing a live file system. Attempt interactive netboot from a URL?. I have followed the linked guide extremely closely, to ensure everything is step by step like described in the guide, but there are two caveats which are likely a cause of my failure:

  1. I am using a newer image than described in the guide (so maybe some steps have changed),
  2. (most likely reason of why it fails) In the guide, there is a section Assembling the file system which instructs to remove old squashfs and it's GPG, and recreate them. I completely do not understand the GPG part, thus I am very likely failing that. At the same time throughout my investigation it looks like Ubuntu did change something in recent releases, and now uses something called "GPG Keyring" which probably also affects mksquashFS. To make things worse the guide basically skimps over this part which does not help at all. I can't find any real information about this process in context of LiveCD customization

As such, I would highly appreciate any help here, my main two questions are:

  1. Is generating GPG signature a must when recreating squashFS for custom liveCD? If yes - How do I do it / where can I read about it? If no,
  2. How can I debug error during boot of my custom LiveCD (quoted above)? Is there any verbose mode for booting up? I tried using different flags within GRUB, but nothing really produces any meaningful output that would guide me as to what is the root cause of the error.
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guiverc avatar
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guiverc
You haven't said what release you're trying to use, but releases from 20.10 & newer differ from each other in minor ways as Ubuntu works now to keep all architectures booting the same way for the same release. Your *media check & offer to download* implies you're on a 20.10 or later release but you don't say which. If you don't regenerate GPG then expect the message you got (*as it'll look for an ISO that matches the key; if it doesn't find one it'll offer to download one*). Did you look up any of the GPG doco? eg. https://help.ubuntu.com/community/GnuPrivacyGuardHowto etc
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ojek avatar
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ojek
Thanks for reply @guiverc! I have stated in the title that I am trying out the newest release, 22.04 - I have read your linked guide and whilst very helpful, it's also worrying for me. There is a section on uploading my generated key to a keyserver - this is likely the step I am missing. I generate my own key, sign the squashFS with it, but that's it, on boot ubuntu likely doesn't recognize my key. Two questions - does that mean that I must be connected to internet during bootup? Is there any way of signing squashfs and **not** upload anything to external servers?
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guiverc avatar
cn flag
guiverc
Details in the title should also exist in your question, as not all readers of the site use large screen PCs and the much larger title is not always readable when reading your actual question. I have no experience with creating my own ISOs, but would be inclined to trust the editor of the wiki/help pages, the Ubuntu ISOs will install offline as the key is found on the ISO itself; if the checksum mismatches stored key, then internet is needed to download an ISO from cdimage.ubuntu.com to replace the invalid ISO that is detected (that it won't allow boot from for security reasons)
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