6️⃣ conn: drop getsockopt(IPPROTO_IPV6, IPV6_V6ONLY)

database64128 · database64128 · commit 4e8689233fe9 · 2025-03-03T22:47:41.000+08:00
A user reports that TCP servers won't create MPTCP sockets when the option is enabled: #59 Turns out in addition to TCP keepalive socket options (multipath-tcp/mptcp_net-next#383), the MPTCP folks also forgot to implement getsockopt(2) for IPv6 socket options (multipath-tcp/mptcp_net-next#550). Our getsockopt(2) call fails with -EOPNOTSUPP and causes Go to silently fall back to standard TCP. A closer look at this reveals that the result of our getsockopt(2) call is only ever used on Windows to guide the setting of IP_MTU_DISCOVER and IP_PKTINFO socket options, and is unlikely to be useful elsewhere. All this seems like overkill. An extra bit of error handling should be enough to solve the original problem. This commit does exactly that, and adds tests to prevent regressions. This partially reverts commit 48be0ff.
diff --git a/conn/conn.go b/conn/conn.go
@@ -19,9 +19,6 @@ type SocketInfo struct {
 
 	// UDPGenericReceiveOffload indicates whether UDP GRO is enabled on the socket.
 	UDPGenericReceiveOffload bool
-
-	// IPv6Only reports whether the socket is IPv6-only.
-	IPv6Only bool
 }
 
 type setFunc = func(fd int, network string, info *SocketInfo) error
diff --git a/conn/conn_bufsize_posix.go b/conn/conn_bufsize_posix.go
@@ -2,10 +2,6 @@
 
 package conn
 
-func (fns setFuncSlice) appendGetIPv6Only() setFuncSlice {
-	return append(fns, getIPv6Only)
-}
-
 func (fns setFuncSlice) appendSetSendBufferSize(size int) setFuncSlice {
 	if size > 0 {
 		return append(fns, func(fd int, _ string, _ *SocketInfo) error {
diff --git a/conn/conn_bufsize_reuseport_tclass.go b/conn/conn_bufsize_reuseport_tclass.go
@@ -4,7 +4,6 @@ package conn
 
 func (lso ListenerSocketOptions) buildSetFns() setFuncSlice {
 	return setFuncSlice{}.
-		appendGetIPv6Only().
 		appendSetSendBufferSize(lso.SendBufferSize).
 		appendSetRecvBufferSize(lso.ReceiveBufferSize).
 		appendSetTrafficClassFunc(lso.TrafficClass).
diff --git a/conn/conn_darwin.go b/conn/conn_darwin.go
@@ -6,7 +6,7 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func setRecvPktinfo(fd int, network string, _ *SocketInfo) error {
+func setRecvPktinfo(fd int, network string) error {
 	switch network {
 	case "udp4":
 		if err := unix.SetsockoptInt(fd, unix.IPPROTO_IP, unix.IP_RECVPKTINFO, 1); err != nil {
@@ -24,7 +24,6 @@ func setRecvPktinfo(fd int, network string, _ *SocketInfo) error {
 
 func (lso ListenerSocketOptions) buildSetFns() setFuncSlice {
 	return setFuncSlice{}.
-		appendGetIPv6Only().
 		appendSetSendBufferSize(lso.SendBufferSize).
 		appendSetRecvBufferSize(lso.ReceiveBufferSize).
 		appendSetTrafficClassFunc(lso.TrafficClass).
diff --git a/conn/conn_darwinfreebsd.go b/conn/conn_darwinfreebsd.go
@@ -8,7 +8,7 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func setPMTUD(fd int, network string, _ *SocketInfo) error {
+func setPMTUD(fd int, network string) error {
 	switch network {
 	case "udp4":
 		if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_DONTFRAG, 1); err != nil {
diff --git a/conn/conn_darwinlinuxwindows.go b/conn/conn_darwinlinuxwindows.go
@@ -4,7 +4,9 @@ package conn
 
 func (fns setFuncSlice) appendSetRecvPktinfoFunc(recvPktinfo bool) setFuncSlice {
 	if recvPktinfo {
-		return append(fns, setRecvPktinfo)
+		return append(fns, func(fd int, network string, _ *SocketInfo) error {
+			return setRecvPktinfo(fd, network)
+		})
 	}
 	return fns
 }
diff --git a/conn/conn_freebsd.go b/conn/conn_freebsd.go
@@ -15,7 +15,6 @@ func setFwmark(fd, fwmark int) error {
 
 func (lso ListenerSocketOptions) buildSetFns() setFuncSlice {
 	return setFuncSlice{}.
-		appendGetIPv6Only().
 		appendSetSendBufferSize(lso.SendBufferSize).
 		appendSetRecvBufferSize(lso.ReceiveBufferSize).
 		appendSetFwmarkFunc(lso.Fwmark).
diff --git a/conn/conn_linux.go b/conn/conn_linux.go
@@ -98,7 +98,7 @@ func setTransparent(fd int, network string) error {
 	return nil
 }
 
-func setPMTUD(fd int, network string, _ *SocketInfo) error {
+func setPMTUD(fd int, network string) error {
 	// Set IP_MTU_DISCOVER for both v4 and v6.
 	if err := unix.SetsockoptInt(fd, unix.IPPROTO_IP, unix.IP_MTU_DISCOVER, unix.IP_PMTUDISC_DO); err != nil {
 		return fmt.Errorf("failed to set socket option IP_MTU_DISCOVER: %w", err)
@@ -117,7 +117,7 @@ func setPMTUD(fd int, network string, _ *SocketInfo) error {
 	return nil
 }
 
-func setRecvPktinfo(fd int, network string, _ *SocketInfo) error {
+func setRecvPktinfo(fd int, network string) error {
 	switch network {
 	case "udp4":
 		if err := unix.SetsockoptInt(fd, unix.IPPROTO_IP, unix.IP_PKTINFO, 1); err != nil {
@@ -190,7 +190,6 @@ func (fns setFuncSlice) appendSetRecvOrigDstAddrFunc(recvOrigDstAddr bool) setFu
 
 func (lso ListenerSocketOptions) buildSetFns() setFuncSlice {
 	return setFuncSlice{}.
-		appendGetIPv6Only().
 		appendSetSendBufferSize(lso.SendBufferSize).
 		appendSetRecvBufferSize(lso.ReceiveBufferSize).
 		appendSetFwmarkFunc(lso.Fwmark).
diff --git a/conn/conn_pmtud.go b/conn/conn_pmtud.go
@@ -4,7 +4,9 @@ package conn
 
 func (fns setFuncSlice) appendSetPMTUDFunc(pmtud bool) setFuncSlice {
 	if pmtud {
-		return append(fns, setPMTUD)
+		return append(fns, func(fd int, network string, _ *SocketInfo) error {
+			return setPMTUD(fd, network)
+		})
 	}
 	return fns
 }
diff --git a/conn/conn_test.go b/conn/conn_test.go
@@ -0,0 +1,37 @@
+package conn
+
+import "testing"
+
+func TestListenUDP(t *testing.T) {
+	for _, lcc := range []struct {
+		name string
+		lc   ListenConfig
+	}{
+		{"DefaultUDPServerListenConfig", DefaultUDPServerListenConfig},
+		{"DefaultUDPClientListenConfig", DefaultUDPClientListenConfig},
+	} {
+		t.Run(lcc.name, func(t *testing.T) {
+			for _, nac := range []struct {
+				name    string
+				network string
+				address string
+			}{
+				{"udp+zero", "udp", ""},
+				{"udp+loopback4", "udp4", "127.0.0.1:"},
+				{"udp+loopback6", "udp6", "[::1]:"},
+				{"udp4+zero", "udp4", ""},
+				{"udp4+loopback4", "udp4", "127.0.0.1:"},
+				{"udp6+zero", "udp6", ""},
+				{"udp6+loopback6", "udp6", "[::1]:"},
+			} {
+				t.Run(nac.name, func(t *testing.T) {
+					uc, _, err := lcc.lc.ListenUDP(t.Context(), nac.network, nac.address)
+					if err != nil {
+						t.Fatal(err)
+					}
+					_ = uc.Close()
+				})
+			}
+		})
+	}
+}
diff --git a/conn/conn_v6only.go b/conn/conn_v6only.go
diff --git a/conn/conn_windows.go b/conn/conn_windows.go
@@ -6,18 +6,6 @@ import (
 	"golang.org/x/sys/windows"
 )
 
-func getIPv6Only(fd int, network string, info *SocketInfo) error {
-	switch network {
-	case "tcp6", "udp6":
-		v6only, err := windows.GetsockoptInt(windows.Handle(fd), windows.IPPROTO_IPV6, windows.IPV6_V6ONLY)
-		if err != nil {
-			return fmt.Errorf("failed to get socket option IPV6_V6ONLY: %w", err)
-		}
-		info.IPv6Only = v6only != 0
-	}
-	return nil
-}
-
 func setSendBufferSize(fd, size int) error {
 	if err := windows.SetsockoptInt(windows.Handle(fd), windows.SOL_SOCKET, windows.SO_SNDBUF, size); err != nil {
 		return fmt.Errorf("failed to set socket option SO_SNDBUF: %w", err)
@@ -46,17 +34,17 @@ const (
 	IP_PMTUDISC_MAX
 )
 
-func setPMTUD(fd int, network string, info *SocketInfo) error {
+func setPMTUD(fd int, network string) error {
 	switch network {
 	case "tcp4", "udp4":
 		if err := windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_DO); err != nil {
 			return fmt.Errorf("failed to set socket option IP_MTU_DISCOVER: %w", err)
 		}
 	case "tcp6", "udp6":
-		if !info.IPv6Only {
-			if err := windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_DO); err != nil {
-				return fmt.Errorf("failed to set socket option IP_MTU_DISCOVER: %w", err)
-			}
+		// For dual-stack IPv6 sockets, both IP_MTU_DISCOVER and IPV6_MTU_DISCOVER need to be set.
+		// However, if IPV6_V6ONLY is set to true, setting IP_MTU_DISCOVER will fail with WSAEINVAL.
+		if err := windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_DO); err != nil && err != windows.WSAEINVAL {
+			return fmt.Errorf("failed to set socket option IP_MTU_DISCOVER: %w", err)
 		}
 		if err := windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IPV6, IPV6_MTU_DISCOVER, IP_PMTUDISC_DO); err != nil {
 			return fmt.Errorf("failed to set socket option IPV6_MTU_DISCOVER: %w", err)
@@ -85,17 +73,16 @@ func setUDPGenericReceiveOffload(fd int, info *SocketInfo) {
 	}
 }
 
-func setRecvPktinfo(fd int, network string, info *SocketInfo) error {
+func setRecvPktinfo(fd int, network string) error {
 	switch network {
 	case "udp4":
 		if err := windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, windows.IP_PKTINFO, 1); err != nil {
 			return fmt.Errorf("failed to set socket option IP_PKTINFO: %w", err)
 		}
 	case "udp6":
-		if !info.IPv6Only {
-			if err := windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, windows.IP_PKTINFO, 1); err != nil {
-				return fmt.Errorf("failed to set socket option IP_PKTINFO: %w", err)
-			}
+		// This behaves just like IP_MTU_DISCOVER. See comments above for more details.
+		if err := windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, windows.IP_PKTINFO, 1); err != nil && err != windows.WSAEINVAL {
+			return fmt.Errorf("failed to set socket option IP_PKTINFO: %w", err)
 		}
 		if err := windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IPV6, windows.IPV6_PKTINFO, 1); err != nil {
 			return fmt.Errorf("failed to set socket option IPV6_PKTINFO: %w", err)
@@ -109,7 +96,6 @@ func setRecvPktinfo(fd int, network string, info *SocketInfo) error {
 
 func (lso ListenerSocketOptions) buildSetFns() setFuncSlice {
 	return setFuncSlice{}.
-		appendGetIPv6Only().
 		appendSetSendBufferSize(lso.SendBufferSize).
 		appendSetRecvBufferSize(lso.ReceiveBufferSize).
 		appendSetPMTUDFunc(lso.PathMTUDiscovery).

Original file line number	Diff line number	Diff line change
`@@ -19,9 +19,6 @@ type SocketInfo struct {`
`19`	`19`
`20`	`20`	`// UDPGenericReceiveOffload indicates whether UDP GRO is enabled on the socket.`
`21`	`21`	`UDPGenericReceiveOffload bool`
`22`		`-`
`23`		`- // IPv6Only reports whether the socket is IPv6-only.`
`24`		`- IPv6Only bool`
`25`	`22`	`}`
`26`	`23`
`27`	`24`	`type setFunc = func(fd int, network string, info *SocketInfo) error`
Original file line number	Diff line number	Diff line change
`@@ -8,7 +8,7 @@ import (`
`8`	`8`	`"golang.org/x/sys/unix"`
`9`	`9`	`)`
`10`	`10`
`11`		`-func setPMTUD(fd int, network string, _ *SocketInfo) error {`
	`11`	`+func setPMTUD(fd int, network string) error {`
`12`	`12`	`switch network {`
`13`	`13`	`case "udp4":`
`14`	`14`	`if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_DONTFRAG, 1); err != nil {`
Original file line number	Diff line number	Diff line change
`@@ -4,7 +4,9 @@ package conn`
`4`	`4`
`5`	`5`	`func (fns setFuncSlice) appendSetRecvPktinfoFunc(recvPktinfo bool) setFuncSlice {`
`6`	`6`	`if recvPktinfo {`
`7`		`- return append(fns, setRecvPktinfo)`
	`7`	`+ return append(fns, func(fd int, network string, _ *SocketInfo) error {`
	`8`	`+ return setRecvPktinfo(fd, network)`
	`9`	`+ })`
`8`	`10`	`}`
`9`	`11`	`return fns`
`10`	`12`	`}`
Original file line number	Diff line number	Diff line change
`@@ -4,7 +4,9 @@ package conn`
`4`	`4`
`5`	`5`	`func (fns setFuncSlice) appendSetPMTUDFunc(pmtud bool) setFuncSlice {`
`6`	`6`	`if pmtud {`
`7`		`- return append(fns, setPMTUD)`
	`7`	`+ return append(fns, func(fd int, network string, _ *SocketInfo) error {`
	`8`	`+ return setPMTUD(fd, network)`
	`9`	`+ })`
`8`	`10`	`}`
`9`	`11`	`return fns`
`10`	`12`	`}`