The tcp_keepalive_time is part of a three-parameter system:
tcp_keepalive_time – the initial idle time before the first probe is sent (default: 7.200 seconds)
tcp_keepalive_intvl – the interval (in seconds) between subsequent keepalive probes if the previous one doesn’t receive an acknowledgment (default: 75 seconds)
tcp_keepalive_probes – the number of failed probes after which the connection is considered broken and is closed (default: 9)
root@pve-ubuntu-01:~# sysctl net.ipv4.tcp_keepalive_time net.ipv4.tcp_keepalive_intvl net.ipv4.tcp_keepalive_probes
net.ipv4.tcp_keepalive_time = 7200
net.ipv4.tcp_keepalive_intvl = 75
net.ipv4.tcp_keepalive_probes = 9
root@pve-ubuntu-01:~#
Change temporarily for testing:
root@pve-ubuntu-01:~# sysctl -w net.ipv4.tcp_keepalive_time=900 # Sets the time to 15 minutes
root@pve-ubuntu-01:~# sysctl -a | grep keep
net.ipv4.tcp_keepalive_intvl = 75
net.ipv4.tcp_keepalive_probes = 9
net.ipv4.tcp_keepalive_time = 900
net.ipv6.conf.all.keep_addr_on_down = 0
net.ipv6.conf.default.keep_addr_on_down = 0
net.ipv6.conf.eth0.keep_addr_on_down = 0
net.ipv6.conf.lo.keep_addr_on_down = 0
Always set keepalive less than firewall timeout with intvl and probes tuned for your use case