Detect enabling of hairpin mode on vethwe-bridge

Enabled hairpin mode on vethwe-bridge was one of the root causes
for installing looping flows in the fastdp mode.

Also, it confused the weave custom L2 bridge, as the bridge treats
a bounced back packet, e.g. ARP brodcast request, as a local one
(the case for "Captured a packet associated with remote host").

net/bridge.go

@@ -1,6 +1,7 @@
 package net
 
 import (
+	"github.com/Sirupsen/logrus"
 	"github.com/vishvananda/netlink"
 )
 
@@ -51,3 +52,31 @@ func isDatapath(link netlink.Link) bool {
 		return false
 	}
 }
+
+func DetectHairpin(portIfName string, log *logrus.Logger) {
+	link, err := netlink.LinkByName(portIfName)
+	if err != nil {
+		log.Errorf("Unable to find link %q: %s", portIfName, err)
+	}
+
+	ch := make(chan netlink.LinkUpdate)
+	// See EnsureInterface for why done channel is not passed
+	if err := netlink.LinkSubscribe(ch, nil); err != nil {
+		log.Errorf("Unable to subscribe to netlink updates: %s", err)
+	}
+
+	pi, err := netlink.LinkGetProtinfo(link)
+	if err != nil {
+		log.Errorf("Unable to get link protinfo %q: %s", portIfName, err)
+	}
+	if pi.Hairpin {
+		log.Errorf("Hairpin mode enabled on %q", portIfName)
+	}
+
+	for up := range ch {
+		if up.Attrs().Name == portIfName && up.Attrs().Protinfo != nil &&
+			up.Attrs().Protinfo.Hairpin {
+			log.Errorf("Hairpin mode enabled on %q", portIfName)
+		}
+	}
+}

prog/weaver/main.go

@@ -226,6 +226,10 @@ func main() {
 	overlay, bridge := createOverlay(datapathName, ifaceName, isAWSVPC, config.Host, config.Port, bufSzMB)
 	networkConfig.Bridge = bridge
 
+	if bridge != nil {
+		go weavenet.DetectHairpin("vethwe-bridge", Log)
+	}
+
 	name := peerName(routerName, bridge.Interface())
 
 	if nickName == "" {

test/705_detect_hairpin_test.sh

@@ -0,0 +1,12 @@
+#! /bin/bash
+
+. ./config.sh
+
+start_suite "Detect hairpin mode"
+
+weave_on $HOST1 launch
+
+assert_raises "run_on $HOST1 sudo ip link set vethwe-bridge type bridge_slave hairpin on"
+assert_raises "docker_on $HOST1 logs weave 2>&1 | grep -q 'Hairpin mode enabled on \"vethwe-bridge\"'"
+
+end_suite