file: /proc/sys/net/ipv4/tcp_max_syn_backlog
variable: net.ipv4.tcp_max_syn_backlog
Official reference

Maximal number of remembered connection requests, which have not received an acknowledgment from connecting client. The minimal value is 128 for low memory machines, and it will increase in proportion to the memory of machine. If server suffers from overload, try increasing this number.