🫀 SO_KEEPALIVE — How your server detects dead connections before the client knowsA client connects to your server. Then their laptop lid closes. WiFi drops. Router reboots.The TCP connection is dead — but your server has no idea. It just sits there. Holding a socket. Waiting forever. This is called a half-open connection — one of TCP’s most silent failure modes. The fix — one line: setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &flag, sizeof(flag));The kernel now sends small probe packets on idle connections. No response after a few tries? Connection gets cleaned up automatically.️ Three knobs you control:→ tcp_keepalive_time — idle time before first probe (default: 2h )→ tcp_keepalive_intvl — time between probes (default: 75s)→ tcp_keepalive_probes — failures before giving up (default: 9)The defaults are hilariously conservative. For a real server you want minutes, not hours. Without it you risk:→ File descriptor leaks→ Thread pool exhaustion→ Memory piling up for connections that died hours ago Who needs it most:→ WebSockets & long-lived connections→ Servers behind NAT — routers silently drop idle mappings→ Any server where clients disappear without sending FIN Your server shouldn’t mourn connections that are already gone.#Linux #Networking #SystemsProgramming #ServerDevelopment
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