Probe a tunnel's other remotes when its current one is unreachable

inside.go

@@ -15,6 +15,7 @@ import (
 	"github.com/slackhq/nebula/overlay/batch"
 	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
+	"github.com/slackhq/nebula/udp"
 )
 
 func (f *Interface) consumeInsidePacket(pkt tio.Packet, fwPacket *firewall.ParsedPacket, nb []byte, sendBatch *batch.SendBatch, rejectBuf []byte, q int, localCache firewall.ConntrackCache) {
@@ -611,6 +612,9 @@ func (f *Interface) sendNoMetrics(t header.MessageType, st header.MessageSubType
 					"udpAddr", hr,
 				)
 			}
+			if udp.IsUnreachable(err) {
+				hostinfo.remoteUnreachable(f, hr)
+			}
 		}
 	} else {
 		// Try to send via a relay