Accept exchanging an MTU of 0 to signify that TUN/TAP is disabled, don't send traffic to a node in that case

src/yggdrasil/admin.go

@@ -280,6 +280,17 @@ func (a *admin) startTunWithMTU(ifname string, iftapmode bool, ifmtu int) error
 		if err != nil {
 			return err
 		}
+		// If we have open sessions then we need to notify them
+		// that our MTU has now changed
+		for _, sinfo := range a.core.sessions.sinfos {
+			if ifname == "none" {
+				sinfo.myMTU = 0
+			} else {
+				sinfo.myMTU = uint16(ifmtu)
+			}
+			a.core.sessions.sendPingPong(sinfo, false)
+		}
+		// Aaaaand... go!
 		go a.core.tun.read()
 	}
 	go a.core.tun.write()

src/yggdrasil/router.go

@@ -164,6 +164,31 @@ func (r *router) sendPacket(bs []byte) {
 		}
 		fallthrough // Also send the packet
 	default:
+		// Drop packets if the session MTU is 0 - this means that one or other
+		// side probably has their TUN adapter disabled
+		if sinfo.getMTU() == 0 {
+			// Get the size of the oversized payload, up to a max of 900 bytes
+			window := 900
+			if len(bs) < window {
+				window = len(bs)
+			}
+
+			// Create the Destination Unreachable response
+			ptb := &icmp.DstUnreach{
+				Data: bs[:window],
+			}
+
+			// Create the ICMPv6 response from it
+			icmpv6Buf, err := r.core.tun.icmpv6.create_icmpv6_tun(
+				bs[8:24], bs[24:40],
+				ipv6.ICMPTypeDestinationUnreachable, 1, ptb)
+			if err == nil {
+				r.recv <- icmpv6Buf
+			}
+
+			// Don't continue - drop the packet
+			return
+		}
 		// Generate an ICMPv6 Packet Too Big for packets larger than session MTU
 		if len(bs) > int(sinfo.getMTU()) {
 			// Get the size of the oversized payload, up to a max of 900 bytes

src/yggdrasil/session.go

@@ -65,7 +65,7 @@ func (s *sessionInfo) update(p *sessionPing) bool {
 		s.theirNonce = boxNonce{}
 		s.nonceMask = 0
 	}
-	if p.mtu >= 1280 {
+	if p.mtu >= 1280 || p.mtu == 0 {
 		s.theirMTU = p.mtu
 	}
 	s.coords = append([]byte{}, p.coords...)
@@ -313,6 +313,9 @@ func (n *boxNonce) minus(m *boxNonce) int64 {
 }
 
 func (sinfo *sessionInfo) getMTU() uint16 {
+	if sinfo.theirMTU == 0 || sinfo.myMTU == 0 {
+		return 0
+	}
 	if sinfo.theirMTU < sinfo.myMTU {
 		return sinfo.theirMTU
 	}