From df9cadd9389b87632f53129123ab269d939118a1 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Thu, 19 Jul 2018 10:01:12 +0100
Subject: [PATCH] Cap MTU on Linux in TAP mode

---
 src/yggdrasil/tun_linux.go | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/yggdrasil/tun_linux.go b/src/yggdrasil/tun_linux.go
index 24c5aa9..14c2f0a 100644
--- a/src/yggdrasil/tun_linux.go
+++ b/src/yggdrasil/tun_linux.go
@@ -29,6 +29,14 @@ func (tun *tunDevice) setup(ifname string, iftapmode bool, addr string, mtu int)
 	}
 	tun.iface = iface
 	tun.mtu = getSupportedMTU(mtu)
+	// The following check is specific to Linux, as the TAP driver only supports
+	// an MTU of 65535-14 to make room for the ethernet headers. This makes sure
+	// that the MTU gets rounded down to 65521 instead of causing a panic.
+	if iftapmode {
+		if tun.mtu > 65535-tun_ETHER_HEADER_LENGTH {
+			tun.mtu = 65535-tun_ETHER_HEADER_LENGTH
+		}
+	}
 	return tun.setupAddress(addr)
 }