Iptables is best linux firewall available. Use this to secure your linux with iptables.
Make sure to save the iptables configuration.
vi /etc/sysconfig/iptables
Secure IPTables Configuration:
*filter :FORWARD DROP [0:0] :INPUT DROP [0:0] :OUTPUT DROP [0:0] ## Block IPs here -A INPUT -s xxx.xxx.xxx.xxx -j DROP -A INPUT -i eth0 -f -j DROP -A INPUT -i lo -j ACCEPT -A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT -A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP -A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG FIN,SYN,RST,PSH,ACK,URG -j DROP -A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG FIN,PSH,URG -j DROP -A INPUT -p tcp -m tcp --tcp-flags FIN,SYN FIN,SYN -j DROP -A INPUT -p tcp -m tcp --tcp-flags SYN,RST SYN,RST -j DROP -A INPUT -p tcp -m tcp --tcp-flags FIN,RST FIN,RST -j DROP -A INPUT -p tcp -m tcp --tcp-flags FIN,ACK FIN -j DROP -A INPUT -p tcp -m tcp --tcp-flags PSH,ACK PSH -j DROP -A INPUT -p tcp -m tcp --tcp-flags ACK,URG URG -j DROP ## Open all your needed ports here -A INPUT -p tcp -m state -m tcp --dport 25 --state NEW -j ACCEPT -A INPUT -p tcp -m state -m tcp --dport 80 --state NEW -j ACCEPT ## Allow your IPs here -A INPUT -p tcp -m state -m tcp -s xxx.xxx.xxx.xxx/255.255.255.0 --dport 10000 --state NEW -j ACCEPT -A INPUT -p tcp -m tcp -m state ! --tcp-flags FIN,SYN,RST,ACK SYN --state NEW -j DROP -A INPUT -f -j DROP -A OUTPUT -o lo -j ACCEPT -A OUTPUT -p tcp -m state --state NEW,ESTABLISHED -j ACCEPT -A OUTPUT -p udp -m state --state NEW,ESTABLISHED -j ACCEPT -A OUTPUT -p icmp -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT COMMIT *nat :PREROUTING ACCEPT [0:0] :POSTROUTING ACCEPT [0:0] :OUTPUT ACCEPT [0:0] COMMIT *mangle :PREROUTING ACCEPT [0:0] :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] :POSTROUTING ACCEPT [0:0] COMMIT
