From e543bff1bae7f5eb03690e29800b962f8129141d Mon Sep 17 00:00:00 2001
From: David Wragg <david@wragg.org>
Date: Thu, 26 Feb 2015 13:10:51 +0000
Subject: [PATCH] Don't propagate errors from RelayBroadcast

If forwarding of a broadcast frame resulted in a FrameTooBigError, we
would generate an ICMP "fragmentation needed" packet that contravened
RFC1122 on two counts: First, we should not produce a ICMP error
message in response to a broadcast/multicast packet.  Secondly, we
should not produce an IP packet with a broadcast/multicast source IP
address.  (It also gave an ethernet frame with a broadcast source MAC,
which probably contravenes some IEEE spec.)

So when RelayBroadcast gets a FrameTooBigError, it should just drop
the packet.

Futhermore, if we fail to relay a broadcast on one connection, that's
not a good reason to give up entirely - we can proceed to attempt to
relay on the other next hop connections.  Errors other than
FrameTooBigError that propagated upwards were simply logged, so that
logging is moved down into RelayBroadcast.
---
 router/local_peer.go | 13 ++++++++-----
 router/router.go     |  6 ++++--
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/router/local_peer.go b/router/local_peer.go
index 1f2cf810..ff5b4e50 100644
--- a/router/local_peer.go
+++ b/router/local_peer.go
@@ -32,8 +32,8 @@ func (peer *LocalPeer) Forward(dstPeer *Peer, df bool, frame []byte, dec *Ethern
 	return peer.Relay(peer.Peer, dstPeer, df, frame, dec)
 }
 
-func (peer *LocalPeer) Broadcast(df bool, frame []byte, dec *EthernetDecoder) error {
-	return peer.RelayBroadcast(peer.Peer, df, frame, dec)
+func (peer *LocalPeer) Broadcast(df bool, frame []byte, dec *EthernetDecoder) {
+	peer.RelayBroadcast(peer.Peer, df, frame, dec)
 }
 
 func (peer *LocalPeer) Relay(srcPeer, dstPeer *Peer, df bool, frame []byte, dec *EthernetDecoder) error {
@@ -57,7 +57,7 @@ func (peer *LocalPeer) Relay(srcPeer, dstPeer *Peer, df bool, frame []byte, dec
 		dec)
 }
 
-func (peer *LocalPeer) RelayBroadcast(srcPeer *Peer, df bool, frame []byte, dec *EthernetDecoder) error {
+func (peer *LocalPeer) RelayBroadcast(srcPeer *Peer, df bool, frame []byte, dec *EthernetDecoder) {
 	for _, conn := range peer.NextBroadcastHops(srcPeer) {
 		err := conn.Forward(df, &ForwardedFrame{
 			srcPeer: srcPeer,
@@ -65,10 +65,13 @@ func (peer *LocalPeer) RelayBroadcast(srcPeer *Peer, df bool, frame []byte, dec
 			frame:   frame},
 			dec)
 		if err != nil {
-			return err
+			if ftbe, ok := err.(FrameTooBigError); ok {
+				log.Printf("dropping too big DF broadcast frame (%v -> %v): PMTU= %v\n", dec.ip.DstIP, dec.ip.SrcIP, ftbe.EPMTU)
+			} else {
+				log.Println(err)
+			}
 		}
 	}
-	return nil
 }
 
 func (peer *LocalPeer) NextBroadcastHops(srcPeer *Peer) []*LocalConnection {
diff --git a/router/router.go b/router/router.go
index feb37cb2..8eaa34e4 100644
--- a/router/router.go
+++ b/router/router.go
@@ -160,7 +160,8 @@ func (router *Router) handleCapturedPacket(frameData []byte, dec *EthernetDecode
 	copy(frameCopy, frameData)
 
 	if !found {
-		return checkFrameTooBig(router.Ourself.Broadcast(df, frameCopy, dec))
+		router.Ourself.Broadcast(df, frameCopy, dec)
+		return nil
 	} else {
 		return checkFrameTooBig(router.Ourself.Forward(dstPeer, df, frameCopy, dec))
 	}
@@ -344,7 +345,8 @@ func (router *Router) handleUDPPacketFunc(dec *EthernetDecoder, po PacketSink) F
 
 		dstPeer, found = router.Macs.Lookup(dstMac)
 		if !found || dstPeer != router.Ourself.Peer {
-			return checkFrameTooBig(router.Ourself.RelayBroadcast(srcPeer, df, frame, dec), srcPeer)
+			router.LogFrame("Relaying broadcast", frame, &dec.eth)
+			router.Ourself.RelayBroadcast(srcPeer, df, frame, dec)
 		}
 
 		return nil