Add TCP_USER_TIMEOUT support

network/v2/sockopts.go

@@ -24,6 +24,7 @@ func setCommonSocketOptions(conn *net.TCPConn, keepAliveConfig net.KeepAliveConf
 	}
 
 	setCongestionControl(rawConn)
+	setTCPUserTimeout(rawConn, keepAliveConfig)
 
 	return nil
 }

network/v2/sockopts_linux.go

@@ -3,11 +3,50 @@
 package network
 
 import (
+	"net"
 	"syscall"
+	"time"
 
 	"golang.org/x/sys/unix"
 )
 
+// Go runtime defaults for KeepAliveConfig when fields are zero.
+const (
+	goDefaultKeepAliveIdle     = 15 * time.Second
+	goDefaultKeepAliveInterval = 15 * time.Second
+	goDefaultKeepAliveCount    = 9
+)
+
+// setTCPUserTimeout sets TCP_USER_TIMEOUT on a socket. If transmitted
+// data remains unacknowledged for this long, the kernel closes the
+// connection. As recommended by Cloudflare
+// (https://blog.cloudflare.com/when-tcp-sockets-refuse-to-die/),
+// the value is computed as: keepidle + keepintvl * keepcnt. This
+// ensures TCP_USER_TIMEOUT and keepalives agree on when to give up.
+// Best-effort: silently ignored if unsupported.
+func setTCPUserTimeout(conn syscall.RawConn, cfg net.KeepAliveConfig) {
+	idle := cfg.Idle
+	if idle == 0 {
+		idle = goDefaultKeepAliveIdle
+	}
+
+	interval := cfg.Interval
+	if interval == 0 {
+		interval = goDefaultKeepAliveInterval
+	}
+
+	count := cfg.Count
+	if count == 0 {
+		count = goDefaultKeepAliveCount
+	}
+
+	timeout := idle + interval*time.Duration(count)
+
+	conn.Control(func(fd uintptr) { //nolint: errcheck
+		unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_USER_TIMEOUT, int(timeout.Milliseconds())) //nolint: errcheck
+	})
+}
+
 func setCongestionControl(conn syscall.RawConn) {
 	conn.Control(func(fd uintptr) { //nolint: errcheck
 		// BBR provides better throughput over lossy and high-latency links compared

network/v2/sockopts_nolinux.go

@@ -2,6 +2,10 @@
 
 package network
 
-import "syscall"
+import (
+	"net"
+	"syscall"
+)
 
-func setCongestionControl(conn syscall.RawConn) {}
+func setCongestionControl(conn syscall.RawConn)                       {}
+func setTCPUserTimeout(conn syscall.RawConn, cfg net.KeepAliveConfig) {}