Add dontfrag control

common/control/frag_linux.go

@@ -0,0 +1,31 @@
+package control
+
+import (
+	"os"
+	"syscall"
+
+	N "github.com/sagernet/sing/common/network"
+
+	"golang.org/x/sys/unix"
+)
+
+func DisableUDPFragment() Func {
+	return func(network, address string, conn syscall.RawConn) error {
+		switch N.NetworkName(network) {
+		case N.NetworkUDP:
+		default:
+			return nil
+		}
+		return Raw(conn, func(fd uintptr) error {
+			if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_MTU_DISCOVER, unix.IP_PMTUDISC_DO); err != nil {
+				return os.NewSyscallError("SETSOCKOPT IP_MTU_DISCOVER IP_PMTUDISC_DO", err)
+			}
+			if network == "udp6" {
+				if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_IPV6, unix.IPV6_MTU_DISCOVER, unix.IP_PMTUDISC_DO); err != nil {
+					return os.NewSyscallError("SETSOCKOPT IPV6_MTU_DISCOVER IP_PMTUDISC_DO", err)
+				}
+			}
+			return nil
+		})
+	}
+}