From da7e7e0a3e6f93a4555737c8b3cb19ce60dfa54f Mon Sep 17 00:00:00 2001
From: Matthew Ellison <matt@arroyonetworks.com>
Date: Fri, 12 May 2017 14:07:24 -0400
Subject: [PATCH] darwin: Detects IP Ver. for NULL Header on Write

Adds the ability for both IPv4 and IPv6 to be written to the TUN device.
---
 syscalls_darwin.go | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/syscalls_darwin.go b/syscalls_darwin.go
index df2b285..3bdbc50 100644
--- a/syscalls_darwin.go
+++ b/syscalls_darwin.go
@@ -159,7 +159,16 @@ func (t *tunReadCloser) Write(from []byte) (int, error) {
 	}
 	t.wBuf = t.wBuf[:len(from)+4]
 
-	t.wBuf[3] = 2 // Family: IP (2)
+	// Determine the IP Family for the NULL L2 Header
+	ipVer := from[0] >> 4
+	if ipVer == 4 {
+		t.wBuf[3] = syscall.AF_INET
+	} else if ipVer == 6 {
+		t.wBuf[3] = syscall.AF_INET6
+	} else {
+		return 0, errors.New("Unable to determine IP version from packet.")
+	}
+
 	copy(t.wBuf[4:], from)
 
 	n, err := t.f.Write(t.wBuf)