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

Number of times SYNACKs for a passive TCP connection attempt will be retransmitted. Should not be higher than 255. Default value is 5, which corresponds to 31seconds till the last retransmission with the current initial RTO of 1second. With this the final timeout for a passive TCP connection will happen after 63seconds.