From 22a703b669f399613c33b9d58bc5333d46d05061 Mon Sep 17 00:00:00 2001
From: Marten Seemann <martenseemann@gmail.com>
Date: Tue, 11 May 2021 12:33:40 -0700
Subject: [PATCH] fix DONT_FRAGMENT error when using a IPv6 connection on
 Windows

The syscall setting the IP_DONTFRAGMENT socket option will fail if the
underlying connection is a IPv6 connection. Since Go doesn't allow us to
distinguish between IPv6 and dual-stack connections, we can only ignore the
error here.
---
 conn_windows.go      | 18 +++++++-----------
 conn_windows_test.go | 14 ++++++++++++--
 2 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/conn_windows.go b/conn_windows.go
index d1f0240a4..2540d8516 100644
--- a/conn_windows.go
+++ b/conn_windows.go
@@ -13,24 +13,20 @@ import (
 
 const IP_DONTFRAGMENT = 14
 
-func newConn(c net.PacketConn) (connection, error) {
-	conn, ok := c.(interface {
-		SyscallConn() (syscall.RawConn, error)
-	})
-	if !ok {
-		return nil, errors.New("doesn't have a SyscallConn")
-	}
-	rawConn, err := conn.SyscallConn()
+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 serr error
 	if err := rawConn.Control(func(fd uintptr) {
-		serr = windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, IP_DONTFRAGMENT, 1)
+		// This should succeed if the connection is a IPv4 or a dual-stack connection.
+		// It will fail for IPv6 connections.
+		// TODO: properly handle error.
+		_ = windows.SetsockoptInt(windows.Handle(fd), windows.IPPROTO_IP, IP_DONTFRAGMENT, 1)
 	}); err != nil {
 		return nil, err
 	}
-	return &basicConn{PacketConn: c}, serr
+	return &basicConn{PacketConn: c}, nil
 }
 
 func inspectReadBuffer(c net.PacketConn) (int, error) {
diff --git a/conn_windows_test.go b/conn_windows_test.go
index 8e6ef9c29..de6758366 100644
--- a/conn_windows_test.go
+++ b/conn_windows_test.go
@@ -10,13 +10,23 @@ import (
 )
 
 var _ = Describe("Windows Conn Test", func() {
-	It("try newConn", func() {
+	It("works on IPv4", func() {
 		addr, err := net.ResolveUDPAddr("udp4", "localhost:0")
 		Expect(err).ToNot(HaveOccurred())
 		udpConn, err := net.ListenUDP("udp4", addr)
 		Expect(err).ToNot(HaveOccurred())
 		conn, err := newConn(udpConn)
 		Expect(err).ToNot(HaveOccurred())
-		_ = conn.Close()
+		Expect(conn.Close()).To(Succeed())
+	})
+
+	It("works on IPv6", func() {
+		addr, err := net.ResolveUDPAddr("udp6", "[::1]:0")
+		Expect(err).ToNot(HaveOccurred())
+		udpConn, err := net.ListenUDP("udp6", addr)
+		Expect(err).ToNot(HaveOccurred())
+		conn, err := newConn(udpConn)
+		Expect(err).ToNot(HaveOccurred())
+		Expect(conn.Close()).To(Succeed())
 	})
 })