You have this old server, it does it's job correctly and you forgot about it. More important, you forget to update it's packages from time to time.
Well, you remember about it but meanwhile this OS has become obsolete and the apt sources are not valid anymore. What to do?
Create another server with new OS and migrate the application/data?
Upgrade the existing server?
Hold on, you do not want to upgrade for now? Just want to update the existing OS? OK, I have a solution for that too.
The solution that does not involves OS upgrade (yet)
This server is old, version 9 of Debian called Stretch and the apt update command fails because the packages for this release are no more in the main repository:
11:10:10 root@jupiter:~# cat /etc/debian_version
11:10:15 root@jupiter:~# cat /etc/apt/sources.list
deb http://ftp.us.debian.org/debian/ stretch main non-free contrib
deb-src http://ftp.us.debian.org/debian/ stretch main non-free contrib
deb http://security.debian.org/ stretch/updates main contrib non-free
deb-src http://security.debian.org/ stretch/updates main contrib non-free
# stretch-updates, previously known as 'volatile'
deb http://ftp.us.debian.org/debian/ stretch-updates main contrib non-free
deb-src http://ftp.us.debian.org/debian/ stretch-updates main contrib non-free
11:10:20 root@jupiter:~# apt update
Ign:1 http://security.debian.org stretch/updates InRelease
Ign:2 http://ftp.debian.org/debian stretch-backports InRelease
Err:3 http://security.debian.org stretch/updates Release
404 Not Found
Err:4 http://ftp.debian.org/debian stretch-backports Release
404 Not Found
Hit:5 https://download.docker.com/linux/debian stretch InRelease
Ign:6 http://ftp.us.debian.org/debian stretch InRelease
Ign:7 http://ftp.us.debian.org/debian stretch-updates InRelease
Err:8 http://ftp.us.debian.org/debian stretch Release
404 Not Found
Err:9 http://ftp.us.debian.org/debian stretch-updates Release
404 Not Found
Reading package lists... Done
E: The repository 'http://security.debian.org stretch/updates Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://ftp.debian.org/debian stretch-backports Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://ftp.us.debian.org/debian stretch Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://ftp.us.debian.org/debian stretch-updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
All I have to do now is to remove all those entries in /etc/apt/sources.list, consult the page mentioned above and put there the archived repository for stretch.
11:14:52 root@jupiter:~# cat /etc/apt/sources.list
deb http://archive.debian.org/debian/ stretch contrib main non-free
11:14:57 root@jupiter:~# apt update
Ign:1 http://archive.debian.org/debian stretch InRelease
Hit:2 http://archive.debian.org/debian stretch Release
Hit:3 https://download.docker.com/linux/debian stretch InRelease
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
