sudo apt-get install dirmngr

https://www.gnupg.org/documentation/manuals/dirmngr/

sudo apt-key list

ubuntu’s key servers – you should verify these yourself…..

sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys E0B11894F66AEC98

sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys 7638D0442B90D010

sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys 8B48AD6246925553

Debians Key servers

apt-key adv –keyserver keyring.debian.org –recv-keys 0x1827364554637281

https://keyring.debian.org/

backports repo

sudo nano /etc/apt/sources.list

deb http://ftp.debian.org/debian stretch-backports main (contrib nonfree)

sudo apt-get update  

now install – need to tell it what to install….. and from where

sudo apt-get install -t stretch-backports cacti