$ cat /proc/sys/net/ipv4/tcp_keepalive_time
1800

$ cat /etc/sysctl.conf
...
#### Net IPv4 TCP ####
# Default: 0
net.ipv4.tcp_abort_on_overflow = 0
# Default: bic
net.ipv4.tcp_congestion_control = cubic
# Decrease fin_timeout default 60
net.ipv4.tcp_fin_timeout = 12
# Decrease keepalive time default = 7200 (2 hour)
net.ipv4.tcp_keepalive_time = 1800
# Default: 1024
net.ipv4.tcp_max_syn_backlog = 16000