Debian 9 Stretch

https://www.debian.org/releases/stretch/index.en.html

Debian LTS July 6, 2020 to June 30, 2022

Archive:

deb http://archive.debian.org/debian/ stretch contrib main non-free

Backports:

deb http://ftp.debian.org/debian stretch-backports main

Security:

deb http://security.debian.org/debian-security stretch/updates main
deb-src http://security.debian.org/debian-security stretch/updates main