iptables

Example for creating a whitelist of clients for SSH

iptables -N ssh_whitelist 
iptables -A INPUT -p tcp --syn --dport ssh -j ssh_whitelist 
iptables -A ssh_whitelist -j DROP 
iptables -I ssh_whitelist -j ACCEPT -s a.b.c.d
iptables -I ssh_whitelist -j ACCEPT -s a.b.c.d/mask 
iptables -I ssh_whitelist -j ACCEPT -s FQDN