hotfix(pmtud/icmp): set IPv6 dont fragment options just in case

internal/pmtud/icmp/df_linux.go

@@ -4,7 +4,11 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func setDontFragment(fd uintptr) (err error) {
-	return unix.SetsockoptInt(int(fd), unix.IPPROTO_IP,
-		unix.IP_MTU_DISCOVER, unix.IP_PMTUDISC_PROBE)
+func setDontFragment(fd uintptr, ipv4 bool) (err error) {
+	if ipv4 {
+		return unix.SetsockoptInt(int(fd), unix.IPPROTO_IP,
+			unix.IP_MTU_DISCOVER, unix.IP_PMTUDISC_PROBE)
+	}
+	return unix.SetsockoptInt(int(fd), unix.IPPROTO_IPV6,
+		unix.IPV6_MTU_DISCOVER, unix.IPV6_PMTUDISC_PROBE)
 }

internal/pmtud/icmp/df_unspecified.go

@@ -5,6 +5,6 @@ package icmp
 // setDontFragment for platforms other than Linux and Windows
 // is not implemented, so we just return assuming the don't
 // fragment flag is set on IP packets.
-func setDontFragment(fd uintptr) (err error) {
+func setDontFragment(fd uintptr, ipv4 bool) (err error) {
 	return nil
 }

internal/pmtud/icmp/df_windows.go

@@ -4,8 +4,11 @@ import (
 	"golang.org/x/sys/windows"
 )
 
-func setDontFragment(fd uintptr) (err error) {
-	// https://docs.microsoft.com/en-us/troubleshoot/windows/win32/header-library-requirement-socket-ipproto-ip
-	// #define IP_DONTFRAGMENT        14     /* don't fragment IP datagrams */
-	return windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, 14, 1)
+func setDontFragment(fd uintptr, ipv4 bool) (err error) {
+	if ipv4 {
+		// https://docs.microsoft.com/en-us/troubleshoot/windows/win32/header-library-requirement-socket-ipproto-ip
+		// #define IP_DONTFRAGMENT        14     /* don't fragment IP datagrams */
+		return windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, 14, 1)
+	}
+	return windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IPV6, 14, 1)
 }

internal/pmtud/icmp/ipv4.go

@@ -25,7 +25,8 @@ func listenICMPv4(ctx context.Context) (conn net.PacketConn, err error) {
 	listenConfig.Control = func(_, _ string, rawConn syscall.RawConn) error {
 		var setDFErr error
 		err := rawConn.Control(func(fd uintptr) {
-			setDFErr = setDontFragment(fd) // runs when calling ListenPacket
+			const ipv4 = true
+			setDFErr = setDontFragment(fd, ipv4) // runs when calling ListenPacket
 		})
 		if err == nil {
 			err = setDFErr

internal/pmtud/icmp/ipv6.go

@@ -6,6 +6,7 @@ import (
 	"net"
 	"net/netip"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/qdm12/gluetun/internal/pmtud/constants"
@@ -19,6 +20,17 @@ const (
 
 func listenICMPv6(ctx context.Context) (conn net.PacketConn, err error) {
 	var listenConfig net.ListenConfig
+	listenConfig.Control = func(_, _ string, rawConn syscall.RawConn) error {
+		var setDFErr error
+		err := rawConn.Control(func(fd uintptr) {
+			const ipv4 = false
+			setDFErr = setDontFragment(fd, ipv4) // runs when calling ListenPacket
+		})
+		if err == nil {
+			err = setDFErr
+		}
+		return err
+	}
 	const listenAddress = ""
 	packetConn, err := listenConfig.ListenPacket(ctx, "ip6:ipv6-icmp", listenAddress)
 	if err != nil {