🦈 conn, service: support MPTCP with explicit option

Unlike Go std's default, enabled, disabled states, our option explicitly enables/disables MPTCP.

Author: database64128

conn/conn.go

@@ -101,6 +101,17 @@ type ListenerSocketOptions struct {
 	// Available on Linux, macOS, FreeBSD, and Windows.
 	TCPFastOpen bool
 
+	// MultipathTCP enables multipath TCP on the listener.
+	//
+	// Unlike Go std, we make MPTCP strictly opt-in.
+	// That is, if this field is false, MPTCP will be explicitly disabled.
+	// This ensures that if Go std suddenly decides to enable MPTCP by default,
+	// existing configurations won't encounter issues due to missing features in the kernel MPTCP stack,
+	// such as TCP keepalive (as of Linux 6.5), and failed connect attempts won't always be retried once.
+	//
+	// Available on platforms supported by Go std's MPTCP implementation.
+	MultipathTCP bool
+
 	// ReceivePacketInfo enables the reception of packet information control messages on the listener.
 	//
 	// Available on Linux, macOS, and Windows.
@@ -114,12 +125,14 @@ type ListenerSocketOptions struct {
 
 // ListenConfig returns a [ListenConfig] with a control function that sets the socket options.
 func (lso ListenerSocketOptions) ListenConfig() ListenConfig {
-	return ListenConfig{
+	lc := ListenConfig{
 		ListenConfig: net.ListenConfig{
 			Control: lso.buildSetFns().controlFunc(),
 		},
 		DisableTFO: !lso.TCPFastOpen,
 	}
+	lc.SetMultipathTCP(lso.MultipathTCP)
+	return lc
 }
 
 var (
@@ -186,16 +199,29 @@ type DialerSocketOptions struct {
 	//
 	// Available on Linux, macOS, FreeBSD, and Windows.
 	TCPFastOpen bool
+
+	// MultipathTCP enables multipath TCP on the dialer.
+	//
+	// Unlike Go std, we make MPTCP strictly opt-in.
+	// That is, if this field is false, MPTCP will be explicitly disabled.
+	// This ensures that if Go std suddenly decides to enable MPTCP by default,
+	// existing configurations won't encounter issues due to missing features in the kernel MPTCP stack,
+	// such as TCP keepalive (as of Linux 6.5), and failed connect attempts won't always be retried once.
+	//
+	// Available on platforms supported by Go std's MPTCP implementation.
+	MultipathTCP bool
 }
 
 // Dialer returns a [Dialer] with a control function that sets the socket options.
 func (dso DialerSocketOptions) Dialer() Dialer {
-	return Dialer{
+	d := Dialer{
 		Dialer: net.Dialer{
 			ControlContext: dso.buildSetFns().controlContextFunc(),
 		},
 		DisableTFO: !dso.TCPFastOpen,
 	}
+	d.SetMultipathTCP(dso.MultipathTCP)
+	return d
 }
 
 var (

service/client.go

@@ -45,6 +45,17 @@ type ClientConfig struct {
 	EnableTCP bool `json:"enableTCP"`
 	DialerTFO bool `json:"dialerTFO"`
 
+	// MultipathTCP enables multipath TCP on the client.
+	//
+	// Unlike Go std, we make MPTCP strictly opt-in.
+	// That is, if this field is false, MPTCP will be explicitly disabled.
+	// This ensures that if Go std suddenly decides to enable MPTCP by default,
+	// existing configurations won't encounter issues due to missing features in the kernel MPTCP stack,
+	// such as TCP keepalive (as of Linux 6.5), and failed connect attempts won't always be retried once.
+	//
+	// Available on platforms supported by Go std's MPTCP implementation.
+	MultipathTCP bool `json:"multipathTCP"`
+
 	// AllowSegmentedFixedLengthHeader disables the requirement that
 	// the fixed-length header must be read in a single read call.
 	//
@@ -147,6 +158,7 @@ func (cc *ClientConfig) TCPClient() (zerocopy.TCPClient, error) {
 		Fwmark:       cc.DialerFwmark,
 		TrafficClass: cc.DialerTrafficClass,
 		TCPFastOpen:  cc.DialerTFO,
+		MultipathTCP: cc.MultipathTCP,
 	})
 
 	switch cc.Protocol {
@@ -193,6 +205,7 @@ func (cc *ClientConfig) UDPClient() (zerocopy.UDPClient, error) {
 			Fwmark:       cc.DialerFwmark,
 			TrafficClass: cc.DialerTrafficClass,
 			TCPFastOpen:  cc.DialerTFO,
+			MultipathTCP: cc.MultipathTCP,
 		})
 		return direct.NewSocks5UDPClient(cc.logger, cc.Name, cc.UDPAddress.String(), dialer, cc.MTU, listenConfig), nil
 	case "2022-blake3-aes-128-gcm", "2022-blake3-aes-256-gcm":

service/server.go

@@ -54,6 +54,17 @@ type TCPListenerConfig struct {
 	// Available on Linux, macOS, FreeBSD, and Windows.
 	FastOpen bool `json:"fastOpen"`
 
+	// Multipath enables multipath TCP on the listener.
+	//
+	// Unlike Go std, we make MPTCP strictly opt-in.
+	// That is, if this field is false, MPTCP will be explicitly disabled.
+	// This ensures that if Go std suddenly decides to enable MPTCP by default,
+	// existing configurations won't encounter issues due to missing features in the kernel MPTCP stack,
+	// such as TCP keepalive (as of Linux 6.5), and failed connect attempts won't always be retried once.
+	//
+	// Available on platforms supported by Go std's MPTCP implementation.
+	Multipath bool `json:"multipath"`
+
 	// DisableInitialPayloadWait disables the brief wait for initial payload.
 	// Setting it to true is useful when the listener only relays server-speaks-first protocols.
 	DisableInitialPayloadWait bool `json:"disableInitialPayloadWait"`
@@ -100,6 +111,7 @@ func (lnc *TCPListenerConfig) Configure(listenConfigCache conn.ListenConfigCache
 			ReusePort:    lnc.ReusePort,
 			Transparent:  transparent,
 			TCPFastOpen:  lnc.FastOpen,
+			MultipathTCP: lnc.Multipath,
 		}),
 		waitForInitialPayload:        !serverNativeInitialPayload && !lnc.DisableInitialPayloadWait,
 		initialPayloadWaitTimeout:    initialPayloadWaitTimeout,