Download files over the internet, especially if large, can lead to errors in the files themselves. In the case of ISO of Ubuntu could be the case that equipment malfunction in which we used.

For these cases there is a utility that allows checking the integrity of the downloaded file. So before you burn to CD then I think is a valid ISO image.

To do this we use the MD5 encryption utility. This utility generates a string of 32 hexadecimal characters, which is almost unique and serves as a signature file. This string will compare with the one we provided in the official Ubuntu downloads.

With the md5sum command will generate the string for the downloaded file. Open a terminal and run where we have the downloaded file, eg:

md5sum ubuntu-9.04-desktop-amd64.iso

We will return a string that will compare with that provided by Canonical. For each ISO of Ubuntu Jaunty will be:

3b5e9861910463374bb0d4ba9025bbb1 - ubuntu-9.04-alternate-amd64.iso
c564ae16dffb51a922aef74a07250473 - ubuntu-9.04-alternate-i386.iso
cace6ea9dde8dc158174e345aabe3fae - ubuntu-9.04-desktop-amd64.iso
66fa77789c7b8ff63130e5d5a272d67b - ubuntu-9.04-desktop-i386.iso
8f921e001aebc3e98e8e8e7d29ee1dd4 - ubuntu-9.04-netbook-remix-i386.img
78cf52114804f80576b0bfc8f5984339 - ubuntu-9.04-server-amd64.iso
20480057590ff8b80ad9094f40698030 - ubuntu-9.04-server-i386.iso

This list can be downloaded from http://noncdn.releases.ubuntu.com/releases/9.04/MD5SUMS

For previous editions can visit UbuntuHashes where have all the chains of the recently published ISO of Ubuntu.