linuxexpert-practice-labs/labs/ubuntu-desktop/3-3-bootable-usb

Ubuntu Desktop 3.3: A bootable USB drive

Checking the signature on Ubuntu's checksum file, seeing a changed copy fail, and writing an ISO image to a drive with dd. A spare disk stands in for the USB drive.

Before you start

The lab adds an empty spare disk, /dev/loop40 (64 MiB), and an empty folder /root/usb to work in. The VM needs to reach releases.ubuntu.com. xorriso isn't installed.

Tasks

1. In /root/usb, download SHA256SUMS and SHA256SUMS.gpg for Ubuntu 24.04 (wget https://releases.ubuntu.com/24.04/SHA256SUMS https://releases.ubuntu.com/24.04/SHA256SUMS.gpg) and check the signature.
2. Copy SHA256SUMS to /root/usb/SHA256SUMS.copy, change one character of the copy, and check its signature again.
3. Make a small ISO with the volume label TEST (install xorriso, then xorriso -as mkisofs -V TEST -o test.iso SOMEDIR) and write it to the spare disk /dev/loop40 with dd.

Hints

• gpgv --keyring /usr/share/keyrings/ubuntu-archive-keyring.gpg SHA256SUMS.gpg SHA256SUMS should print Good signature.
• Change a single letter or digit, for example with sed -i '1s/^./0/' or in an editor; gpgv should now print BAD signature.
• dd if=test.iso of=/dev/loop40 bs=4M conv=fsync, then blkid /dev/loop40 shows LABEL="TEST" and TYPE="iso9660".