A file containing an operating system should not be treated the same way as a regular file. While you can download files using a web browser, this method is the least secure for this purpose. An operating system is provided as a disk image of a certain size, and when using a web browser, it can lose data along the way.
This might be a tiny amount, and the size of the image may even appear to be the same as the one online, but even a minimal difference can compromise the installation on your computer.
How to do it?
If you are using Windows
Preferably, use a program designed to manage downloads, such as Free Download Manager, a GPL-licensed program, or a torrent client, such as Transmission. Once you’ve finished downloading an ISO image of your future distribution: NEVER UNARCHIVE IT! It would be unusable…
Once your download is complete, verify the integrity of the download using a program that calculates the MD5 checksum of the ISO file and compares it with the one in the file with the .md5sum extension which comes with the ISO image.
For example, you can use Md5summer or WxChecksums, which are free software for Windows.
You can also use Install-winMd5Sum.exe or Checksum Calculator, both mentioned on the Ubuntu community wiki. You can then burn the ISO image with a program like InfraRecorder, another free software for Windows. Burn at a slow speed (e.g. 5x) and on a non-rewritable CD, the result of the burn is often more reliable this way.
If you are already using GNU/Linux
Open a console, type the md5sum command, followed by the -c option, and the name of the md5 file which must be placed in the same directory as the ISO image it accompanies. For example:
md5sum -c name-of-file.md5sum Once the command is validated, the console should return the following message:
name-of-your-file.iso: OKBy having both the iso image and the accompanying md5sum file, some burning software will check the MD5 sum for you before starting.
All Operating Systems
Additional information in 2024 : MD5 is a hash algorithm which has been less and less in use for this purpose, for security reasons. Now SHA-256 algorithms are most often used. The principle is the same as for MD5, you need a method for each OS which you use when checking a bootable ISO file. In Windows, a method similar to the one used in Linux is available, using the PowerShell console. And of course in a Linux terminal you will just type something such as:
sha256sum -c name-of-file.sha256 and once the command is validated, the console should return the following message:
name-of-your-file.iso: Success