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

Initial time period in second to disable Fastopen on active TCP sockets when a TFO firewall blackhole issue happens. This time period will grow exponentially when more blackhole issues get detected right after Fastopen is re-enabled and will reset to initial value when the blackhole issue goes away. 0 to disable the blackhole detection. By default, it is set to 1hr.