Mikrotik Router SYN Flood Protection

Use terminal

/ip firewall filter 
add chain=forward protocol=tcp tcp-flags=syn connection-state=new action=jump jump-target=SYN-Protect comment="SYN Flood protect" disabled=yes
add chain=SYN-Protect protocol=tcp tcp-flags=syn limit=400,5 connection-state=new action=accept comment="" disabled=no
add chain=SYN-Protect protocol=tcp tcp-flags=syn connection-state=new action=drop comment="" disabled=no

Add Connection Tracking

For v6.x:

/ip settings set tcp-syncookies=yes

For older RouterOS versions:

/ip firewall connection tracking set tcp-syncookie=yes