If no one can ping the system, the security is naturally increased. To this end, we can add the following line to the /etc/rc.d/rc.local file
The Linux kernel provides several SYN-related configurations with the command:
sysctl -a | grep syn
tcp_max_syn_backlog is the length of the SYN queue, tcp_syncookies is a switch, whether to enable the SYN cookie function, this function can prevent some SYN attacks. tcp_synack_retries and tcp_syn_retries define the number of SYN retries.