move set DF implementation to separate files & avoid the need for OOBCapablePacketConn

conn.go

@@ -29,6 +29,19 @@ type OOBCapablePacketConn interface {
 var _ OOBCapablePacketConn = &net.UDPConn{}
 
 func wrapConn(pc net.PacketConn) (connection, error) {
+	conn, ok := pc.(interface {
+		SyscallConn() (syscall.RawConn, error)
+	})
+	if ok {
+		rawConn, err := conn.SyscallConn()
+		if err != nil {
+			return nil, err
+		}
+		err = setDF(rawConn)
+		if err != nil {
+			return nil, err
+		}
+	}
 	c, ok := pc.(OOBCapablePacketConn)
 	if !ok {
 		utils.DefaultLogger.Infof("PacketConn is not a net.UDPConn. Disabling optimizations possible on UDP connections.")

err_size.go → conn_df.go

@@ -3,6 +3,13 @@
 
 package quic
 
+import "syscall"
+
+func setDF(rawConn syscall.RawConn) error {
+	// no-op on unsupported platforms
+	return nil
+}
+
 func isMsgSizeErr(err error) bool {
 	// to be implemented for more specific platforms
 	return false

conn_df_linux.go

@@ -0,0 +1,40 @@
+//go:build linux
+// +build linux
+
+package quic
+
+import (
+	"errors"
+	"syscall"
+
+	"github.com/lucas-clemente/quic-go/internal/utils"
+	"golang.org/x/sys/unix"
+)
+
+func setDF(rawConn syscall.RawConn) error {
+	// Enabling IP_MTU_DISCOVER will force the kernel to return "sendto: message too long"
+	// and the datagram will not be fragmented
+	var errDFIPv4, errDFIPv6 error
+	if err := rawConn.Control(func(fd uintptr) {
+		errDFIPv4 = unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_MTU_DISCOVER, unix.IP_PMTUDISC_DO)
+		errDFIPv6 = unix.SetsockoptInt(int(fd), unix.IPPROTO_IPV6, unix.IPV6_MTU_DISCOVER, unix.IPV6_PMTUDISC_DO)
+	}); err != nil {
+		return err
+	}
+	switch {
+	case errDFIPv4 == nil && errDFIPv6 == nil:
+		utils.DefaultLogger.Debugf("Setting DF for IPv4 and IPv6.")
+	case errDFIPv4 == nil && errDFIPv6 != nil:
+		utils.DefaultLogger.Debugf("Setting DF for IPv4.")
+	case errDFIPv4 != nil && errDFIPv6 == nil:
+		utils.DefaultLogger.Debugf("Setting DF for IPv6.")
+	case errDFIPv4 != nil && errDFIPv6 != nil:
+		return errors.New("setting DF failed for both IPv4 and IPv6")
+	}
+	return nil
+}
+
+func isMsgSizeErr(err error) bool {
+	// https://man7.org/linux/man-pages/man7/udp.7.html
+	return errors.Is(err, unix.EMSGSIZE)
+}

conn_df_windows.go

@@ -0,0 +1,46 @@
+//go:build windows
+// +build windows
+
+package quic
+
+import (
+	"errors"
+	"syscall"
+
+	"github.com/lucas-clemente/quic-go/internal/utils"
+	"golang.org/x/sys/windows"
+)
+
+const (
+	// same for both IPv4 and IPv6 on Windows
+	// https://microsoft.github.io/windows-docs-rs/doc/windows/Win32/Networking/WinSock/constant.IP_DONTFRAG.html
+	// https://microsoft.github.io/windows-docs-rs/doc/windows/Win32/Networking/WinSock/constant.IPV6_DONTFRAG.html
+	IP_DONTFRAGMENT = 14
+	IPV6_DONTFRAG   = 14
+)
+
+func setDF(rawConn syscall.RawConn) error {
+	var errDFIPv4, errDFIPv6 error
+	if err := rawConn.Control(func(fd uintptr) {
+		errDFIPv4 = windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, IP_DONTFRAGMENT, 1)
+		errDFIPv6 = windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IPV6, IPV6_DONTFRAG, 1)
+	}); err != nil {
+		return err
+	}
+	switch {
+	case errDFIPv4 == nil && errDFIPv6 == nil:
+		utils.DefaultLogger.Debugf("Setting DF for IPv4 and IPv6.")
+	case errDFIPv4 == nil && errDFIPv6 != nil:
+		utils.DefaultLogger.Debugf("Setting DF for IPv4.")
+	case errDFIPv4 != nil && errDFIPv6 == nil:
+		utils.DefaultLogger.Debugf("Setting DF for IPv6.")
+	case errDFIPv4 != nil && errDFIPv6 != nil:
+		return errors.New("setting DF failed for both IPv4 and IPv6")
+	}
+	return nil
+}
+
+func isMsgSizeErr(err error) bool {
+	// https://docs.microsoft.com/en-us/windows/win32/winsock/windows-sockets-error-codes-2
+	return errors.Is(err, windows.WSAEMSGSIZE)
+}

conn_oob.go

@@ -87,8 +87,6 @@ func newConn(c OOBCapablePacketConn) (*oobConn, error) {
 			errPIIPv4 = unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, ipv4RECVPKTINFO, 1)
 			errPIIPv6 = unix.SetsockoptInt(int(fd), unix.IPPROTO_IPV6, ipv6RECVPKTINFO, 1)
 		}
-
-		setOOBSockOpts(fd)
 	}); err != nil {
 		return nil, err
 	}

conn_windows.go

@@ -9,40 +9,10 @@ import (
 	"net"
 	"syscall"
 
-	"github.com/lucas-clemente/quic-go/internal/utils"
 	"golang.org/x/sys/windows"
 )
 
-const (
-	// same for both IPv4 and IPv6 on Windows
-	// https://microsoft.github.io/windows-docs-rs/doc/windows/Win32/Networking/WinSock/constant.IP_DONTFRAG.html
-	// https://microsoft.github.io/windows-docs-rs/doc/windows/Win32/Networking/WinSock/constant.IPV6_DONTFRAG.html
-	IP_DONTFRAGMENT = 14
-	IPV6_DONTFRAG   = 14
-)
-
 func newConn(c OOBCapablePacketConn) (connection, error) {
-	rawConn, err := c.SyscallConn()
-	if err != nil {
-		return nil, fmt.Errorf("couldn't get syscall.RawConn: %w", err)
-	}
-	var errDFIPv4, errDFIPv6 error
-	if err := rawConn.Control(func(fd uintptr) {
-		errDFIPv4 = windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, IP_DONTFRAGMENT, 1)
-		errDFIPv6 = windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IPV6, IPV6_DONTFRAG, 1)
-	}); err != nil {
-		return nil, err
-	}
-	switch {
-	case errDFIPv4 == nil && errDFIPv6 == nil:
-		utils.DefaultLogger.Debugf("Setting DF for IPv4 and IPv6.")
-	case errDFIPv4 == nil && errDFIPv6 != nil:
-		utils.DefaultLogger.Debugf("Setting DF for IPv4.")
-	case errDFIPv4 != nil && errDFIPv6 == nil:
-		utils.DefaultLogger.Debugf("Setting DF for IPv6.")
-	case errDFIPv4 != nil && errDFIPv6 != nil:
-		return nil, errors.New("setting Df failed for both IPv4 and IPv6")
-	}
 	return &basicConn{PacketConn: c}, nil
 }