wgengine/magicsock: improve don't fragment bit set/get support

Add an enable/disable argument to setDontFragment() in preparation for dynamic
enable/disable of peer path MTU discovery. Add getDontFragment() to get the
status of the don't fragment bit from a socket.

Updates #311

Co-authored-by: James Tucker <james@tailscale.com>
Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/batching_conn.go

@@ -9,6 +9,7 @@ import (
 	"net/netip"
 	"sync"
 	"sync/atomic"
+	"syscall"
 	"time"
 
 	"golang.org/x/net/ipv6"
@@ -192,3 +193,11 @@ retry:
 	}
 	return err
 }
+
+func (c *batchingUDPConn) SyscallConn() (syscall.RawConn, error) {
+	sc, ok := c.pc.(syscall.Conn)
+	if !ok {
+		return nil, errUnsupportedConnType
+	}
+	return sc.SyscallConn()
+}

wgengine/magicsock/blockforever_conn.go

@@ -8,6 +8,7 @@ import (
 	"net"
 	"net/netip"
 	"sync"
+	"syscall"
 	"time"
 )
 
@@ -51,3 +52,4 @@ func (c *blockForeverConn) Close() error {
 func (c *blockForeverConn) SetDeadline(t time.Time) error         { return errors.New("unimplemented") }
 func (c *blockForeverConn) SetReadDeadline(t time.Time) error     { return errors.New("unimplemented") }
 func (c *blockForeverConn) SetWriteDeadline(t time.Time) error    { return errors.New("unimplemented") }
+func (c *blockForeverConn) SyscallConn() (syscall.RawConn, error) { return nil, errUnsupportedConnType }

wgengine/magicsock/magicsock.go

@@ -983,6 +983,8 @@ var errDropDerpPacket = errors.New("too many DERP packets queued; dropping")
 
 var errNoUDP = errors.New("no UDP available on platform")
 
+var errUnsupportedConnType = errors.New("unsupported connection type")
+
 var (
 	// This acts as a compile-time check for our usage of ipv6.Message in
 	// batchingUDPConn for both IPv6 and IPv4 operations.
@@ -2309,7 +2311,7 @@ func (c *Conn) bindSocket(ruc *RebindingUDPConn, network string, curPortFate cur
 		trySetSocketBuffer(pconn, c.logf)
 
 		if CanPMTUD() {
-			err = setDontFragment(pconn, network)
+			err = c.setDontFragment(network, true)
 			if err != nil {
 				c.logf("magicsock: set dontfragment failed for %v port %d: %v", network, port, err)
 				// TODO disable PMTUD in this case. We don't expect the setsockopt to fail on

wgengine/magicsock/peermtu_darwin.go

@@ -6,30 +6,46 @@
 package magicsock
 
 import (
-	"net"
 	"syscall"
 
 	"golang.org/x/sys/unix"
-	"tailscale.com/types/nettype"
 )
 
-func setDontFragment(pconn nettype.PacketConn, network string) (err error) {
+func getDontFragOpt(network string) int {
-	if c, ok := pconn.(*net.UDPConn); ok {
-		rc, err := c.SyscallConn()
-		if err == nil {
-			rc.Control(func(fd uintptr) {
 	if network == "udp4" {
-					err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, unix.IP_DONTFRAG, 1)
+		return unix.IP_DONTFRAG
 	}
-				if network == "udp6" {
+	return unix.IPV6_DONTFRAG
-					err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, unix.IPV6_DONTFRAG, 1)
+}
+
+func (c *Conn) setDontFragment(network string, enable bool) error {
+	optArg := 1
+	if enable == false {
+		optArg = 0
 	}
+	var err error
+	rcErr := c.connControl(network, func(fd uintptr) {
+		err = syscall.SetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network), optArg)
 	})
-		}
+
+	if rcErr != nil {
+		return rcErr
 	}
 	return err
 }
 
-func CanPMTUD() bool {
+func (c *Conn) getDontFragment(network string) (bool, error) {
-	return debugEnablePMTUD() // only if the envknob is for now.
+	var v int
+	var err error
+	rcErr := c.connControl(network, func(fd uintptr) {
+		v, err = syscall.GetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network))
+	})
+
+	if rcErr != nil {
+		return false, rcErr
+	}
+	if v == 1 {
+		return true, err
+	}
+	return false, err
 }

wgengine/magicsock/peermtu_linux.go

@@ -6,29 +6,44 @@
 package magicsock
 
 import (
-	"net"
 	"syscall"
-
-	"tailscale.com/types/nettype"
 )
 
-func setDontFragment(pconn nettype.PacketConn, network string) (err error) {
+func getDontFragOpt(network string) int {
-	if c, ok := pconn.(*net.UDPConn); ok {
-		rc, err := c.SyscallConn()
-		if err == nil {
-			rc.Control(func(fd uintptr) {
 	if network == "udp4" {
-					err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_MTU_DISCOVER, syscall.IP_PMTUDISC_DO)
+		return syscall.IP_MTU_DISCOVER
 	}
-				if network == "udp6" {
+	return syscall.IPV6_MTU_DISCOVER
-					err = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IPV6_MTU_DISCOVER, syscall.IP_PMTUDISC_DO)
+}
+
+func (c *Conn) setDontFragment(network string, enable bool) error {
+	optArg := syscall.IP_PMTUDISC_DO
+	if enable == false {
+		optArg = syscall.IP_PMTUDISC_DONT
 	}
+	var err error
+	rcErr := c.connControl(network, func(fd uintptr) {
+		err = syscall.SetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network), optArg)
 	})
-		}
+
+	if rcErr != nil {
+		return rcErr
 	}
 	return err
 }
 
-func CanPMTUD() bool {
+func (c *Conn) getDontFragment(network string) (bool, error) {
-	return debugEnablePMTUD() // only if the envknob is enabled, for now.
+	var v int
+	var err error
+	rcErr := c.connControl(network, func(fd uintptr) {
+		v, err = syscall.GetsockoptInt(int(fd), getIPProto(network), getDontFragOpt(network))
+	})
+
+	if rcErr != nil {
+		return false, rcErr
+	}
+	if v == syscall.IP_PMTUDISC_DO {
+		return true, err
+	}
+	return false, err
 }

wgengine/magicsock/peermtu_stubs.go

@@ -7,15 +7,27 @@ package magicsock
 
 import (
 	"errors"
-
-	"tailscale.com/types/nettype"
 )
 
-// setDontFragment sets the dontfragment sockopt on pconn on the platforms that support it,
+// setDontFragment sets the don't fragment sockopt on the underlying connection
-// for both IPv4 and IPv6.
+// specified by network, which must be "udp4" or "udp6". See
-// (C.f. https://datatracker.ietf.org/doc/html/rfc3542#section-11.2 for IPv6 fragmentation)
+// https://datatracker.ietf.org/doc/html/rfc3542#section-11.2 for details on
-func setDontFragment(pconn nettype.PacketConn, network string) (err error) {
+// IPv6 fragmentation.
-	return errors.New("setting don't fragment bit not supported on this OS")
+//
+// Return values:
+// - an error if peer MTU is not supported on this OS
+// - errNoActiveUDP if the underlying connection is not UDP
+// - otherwise, the result of setting the don't fragment bit
+func (c *Conn) setDontFragment(network string, enable bool) error {
+	return errors.New("peer path MTU discovery not supported on this OS")
+}
+
+// getDontFragment gets the don't fragment setting on the underlying connection
+// specified by network, which must be "udp4" or "udp6". Returns true if the
+// underlying connection is UDP and the don't fragment bit is set, otherwise
+// false.
+func (c *Conn) getDontFragment(network string) (bool, error) {
+	return false, nil
 }
 
 // CanPMTUD returns whether this platform supports performing peet path MTU discovery.

wgengine/magicsock/peermtu_unix.go

@@ -0,0 +1,46 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build (darwin && !ios) || (linux && !android)
+
+package magicsock
+
+import (
+	"syscall"
+)
+
+// getIPProto returns the value of the get/setsockopt proto argument necessary
+// to set an IP sockopt that corresponds with the string network, which must be
+// "udp4" or "udp6".
+func getIPProto(network string) int {
+	if network == "udp4" {
+		return syscall.IPPROTO_IP
+	}
+	return syscall.IPPROTO_IPV6
+}
+
+// connControl allows the caller to run a system call on the socket underlying
+// Conn specified by the string network, which must be "udp4" or "udp6". If the
+// pconn type implements the syscall method, this function returns the value of
+// of the system call fn called with the fd of the socket as its arg (or the
+// error from rc.Control() if that fails). Otherwise it returns the error
+// errUnsupportedConnType.
+func (c *Conn) connControl(network string, fn func(fd uintptr)) error {
+	pconn := c.pconn4.pconn
+	if network == "udp6" {
+		pconn = c.pconn6.pconn
+	}
+	sc, ok := pconn.(syscall.Conn)
+	if !ok {
+		return errUnsupportedConnType
+	}
+	rc, err := sc.SyscallConn()
+	if err != nil {
+		return err
+	}
+	return rc.Control(fn)
+}
+
+func CanPMTUD() bool {
+	return debugEnablePMTUD()
+}

wgengine/magicsock/rebinding_conn.go

@@ -9,6 +9,7 @@ import (
 	"net/netip"
 	"sync"
 	"sync/atomic"
+	"syscall"
 
 	"golang.org/x/net/ipv6"
 	"tailscale.com/net/netaddr"
@@ -166,3 +167,13 @@ func (c *RebindingUDPConn) writeToUDPAddrPortWithInitPconn(pconn nettype.PacketC
 func (c *RebindingUDPConn) WriteToUDPAddrPort(b []byte, addr netip.AddrPort) (int, error) {
 	return c.writeToUDPAddrPortWithInitPconn(*c.pconnAtomic.Load(), b, addr)
 }
+
+func (c *RebindingUDPConn) SyscallConn() (syscall.RawConn, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	sc, ok := c.pconn.(syscall.Conn)
+	if !ok {
+		return nil, errUnsupportedConnType
+	}
+	return sc.SyscallConn()
+}