/etc/apt/sources.list
deb http://archive.ubuntu.com/ubuntu bionic universe multiverse deb-src http://archive.ubuntu.com/ubuntu bionic universe multiverse deb http://us.archive.ubuntu.com/ubuntu/ bionic universe deb-src http://us.archive.ubuntu.com/ubuntu/ bionic universe deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe deb http://us.archive.ubuntu.com/ubuntu/ bionic multiverse deb-src http://us.archive.ubuntu.com/ubuntu/ bionic multiverse deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates multiverse deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates multiverse deb http://security.ubuntu.com/ubuntu bionic-security universe deb-src http://security.ubuntu.com/ubuntu bionic-security universe deb http://security.ubuntu.com/ubuntu bionic-security multiverse deb-src http://security.ubuntu.com/ubuntu bionic-security multiverse
References
https://help.ubuntu.com/lts/serverguide/configuration.html.en