From 3443d3ffd38cd7e611513b644ea15b144e28a079 Mon Sep 17 00:00:00 2001
From: JackDoan <me@jackdoan.com>
Date: Fri, 14 Nov 2025 11:38:34 -0600
Subject: [PATCH] correctly set ethertype, at least for single-IP networks

---
 overlay/tun_linux.go |  8 ++++++--
 packet/outpacket.go  | 11 ++++++++---
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/overlay/tun_linux.go b/overlay/tun_linux.go
index 86e36e9..4108aad 100644
--- a/overlay/tun_linux.go
+++ b/overlay/tun_linux.go
@@ -43,7 +43,8 @@ type tun struct {
 	useSystemRoutes           bool
 	useSystemRoutesBufferSize int
 
-	l *logrus.Logger
+	isV6 bool
+	l    *logrus.Logger
 }
 
 func (t *tun) Networks() []netip.Prefix {
@@ -163,6 +164,9 @@ func newTunGeneric(c *config.C, l *logrus.Logger, file *os.File, vpnNetworks []n
 		useSystemRoutesBufferSize: c.GetInt("tun.use_system_route_table_buffer_size", 0),
 		l:                         l,
 	}
+	if len(vpnNetworks) != 0 {
+		t.isV6 = vpnNetworks[0].Addr().Is6() //todo what about multi-IP?
+	}
 
 	err := t.reload(c, true)
 	if err != nil {
@@ -738,7 +742,7 @@ func (t *tun) AllocSeg(pkt *packet.OutPacket, q int) (int, error) {
 	if err != nil {
 		return 0, err
 	}
-	x := pkt.UseSegment(idx, buf)
+	x := pkt.UseSegment(idx, buf, t.isV6)
 	return x, nil
 }
 
diff --git a/packet/outpacket.go b/packet/outpacket.go
index 42dd5dd..3ae13bb 100644
--- a/packet/outpacket.go
+++ b/packet/outpacket.go
@@ -39,7 +39,7 @@ func (pkt *OutPacket) Reset() {
 	pkt.wasSegmented = false
 }
 
-func (pkt *OutPacket) UseSegment(segID uint16, seg []byte) int {
+func (pkt *OutPacket) UseSegment(segID uint16, seg []byte, isV6 bool) int {
 	pkt.Valid = true
 	pkt.SegmentIDs = append(pkt.SegmentIDs, segID)
 	pkt.Segments = append(pkt.Segments, seg) //todo do we need this?
@@ -56,8 +56,13 @@ func (pkt *OutPacket) UseSegment(segID uint16, seg []byte) int {
 
 	hdr := seg[0 : virtio.NetHdrSize+14]
 	_ = vhdr.Encode(hdr)
-	hdr[virtio.NetHdrSize+14-2] = 0x86
-	hdr[virtio.NetHdrSize+14-1] = 0xdd //todo ipv6 ethertype
+	if isV6 {
+		hdr[virtio.NetHdrSize+14-2] = 0x86
+		hdr[virtio.NetHdrSize+14-1] = 0xdd
+	} else {
+		hdr[virtio.NetHdrSize+14-2] = 0x08
+		hdr[virtio.NetHdrSize+14-1] = 0x00
+	}
 
 	pkt.SegmentHeaders = append(pkt.SegmentHeaders, hdr)
 	pkt.SegmentPayloads = append(pkt.SegmentPayloads, seg[virtio.NetHdrSize+14:])