ufw

root@majoit:~# ufw disable
Firewall stopped and disabled on system startup
root@majoit:~# ufw enable
Command may disrupt existing ssh connections. Proceed with operation (y|n)? y
ERROR: problem running ufw-init
iptables-restore: line 69 failed
iptables-restore: line 30 failed
sysctl: permission denied on key 'net.ipv4.tcp_sack'

Problem running '/etc/ufw/before.rules'
Problem running '/etc/ufw/after.rules'

root@majoit:~# sysctl -a | grep net.ipv4.tcp_sack
sysctl: separators should not be repeated: /.fake
sysctl: separators should not be repeated: /.fake
net.ipv4.tcp_sack = 1
root@majoit:~#