Modify linux system TCP connections
centOS 6.x
(1) we /etc/sysctl.conf
(2) add the parameter
net.nf_conntrack_max = 655360
（3）sysctl -p
centOS 7.x
(1) we /etc/sysctl.conf
(2) add the parameter
net.ipv4.ip_conntrack_max = 65536
（3）sysctl -p
CONNTRACK_MAX optimal settings
CONNTRACK_MAX = RAMSIZE (in bytes) / 16384 / (x / 32)
Where x is the number of bit pointer (e.g., 32 or 64bit)
For example: a 32-bit PC with 512M of memory can be processed in default 1024 * 1024 * 512/16384 / (32/32) = 32768 netfilter concurrent connections.