How to block output outgoing connections to private networks from your server using ufw or iptables (firewall on linux) using ssh console?

Here is how to using iptables:

iptables -A OUTPUT -d 10.0.0.0/8 -j DROP                                                                                                 
iptables -A OUTPUT -d 172.16.0.0/12 -j DROP                                                                                              
iptables -A OUTPUT -d 192.168.0.0/16 -j DROP                                                                                             
iptables -A OUTPUT -d 100.64.0.0/10 -j DROP
iptables -A OUTPUT -d 198.18.0.0/15 -j DROP
iptables -A OUTPUT -d 169.254.0.0/16 -j DROP

if you want to use ufw

ufw deny out from any to 10.0.0.0/8
ufw deny out from any to 172.16.0.0/12
ufw deny out from any to 192.168.0.0/16
ufw deny out from any to 100.64.0.0/10
ufw deny out from any to 198.18.0.0/15
ufw deny out from any to 169.254.0.0/16