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If you edited your /etc/apt/sources.list this is the default. Some sources can also be under /etc/apt/sources.list.d/
It is advised you read the whole page so you understand what is happening. You dont need the mirror section for example.
Sources.list example:
deb http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware deb-src http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware deb http://deb.debian.org/debian-security/ bookworm-security main contrib non-free non-free-firmware deb-src http://deb.debian.org/debian-security/ bookworm-security main contrib non-free non-free-firmware deb http://deb.debian.org/debian bookworm-updates main contrib non-free non-free-firmware deb-src http://deb.debian.org/debian bookworm-updates main contrib non-free non-free-firmware
If you want to also download from another mirror at the exact same time so you get concurrent downloads, you can add a mirror (security mirrors usually dont work for testing) but on a stable release it would be like this:
#Mirror deb http://ftp.us.debian.org/debian bookworm main contrib non-free non-free-firmware deb http://ftp.us.debian.org/debian-security/ bookworm-security main contrib non-free non-free-firmware deb http://ftp.us.debian.org/debian bookworm-updates main contrib non-free non-free-firmware
A complete file would be similar to this :
# This system was installed using small removable media # (e.g. netinst, live or single CD). The matching "deb cdrom" # entries were disabled at the end of the installation process. # For information about how to configure apt package sources, # see the sources.list(5) manual. # deb http://deb.debian.org/debian/ bookworm-backports main contrib non-free deb http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware deb-src http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware deb http://deb.debian.org/debian-security/ bookworm-security main contrib non-free non-free-firmware deb-src http://deb.debian.org/debian-security/ bookworm-security main contrib non-free non-free-firmware deb http://deb.debian.org/debian bookworm-updates main contrib non-free non-free-firmware deb-src http://deb.debian.org/debian bookworm-updates main contrib non-free non-free-firmware #Mirror deb http://ftp.us.debian.org/debian bookworm main contrib non-free non-free-firmware deb http://ftp.us.debian.org/debian-security/ bookworm-security main contrib non-free non-free-firmware deb http://ftp.us.debian.org/debian bookworm-updates main contrib non-free non-free-firmware
The mirror section is not required and not advised to add unless you know what you are doing. Just putting it out there if you want to test and learn more (which is why we are all here).
While running testing since mirrors dont exist for testing security you would get an error like this:
Err:16 http://ftp.us.debian.org/debian-security bookworm-security Release 404 Not Found [IP: 208.80.154.139 80] E: The repository 'http://ftp.us.debian.org/debian-security bookworm-security Release' does not have a Release file.
This would work when it became stable due to how Debian is packaged. So you cant get concurrent downloads on security fixes if you run testing.
