Stricter checking of HAIRPIN_FLAG in salt.

cluster/saltbase/salt/kubelet/default

@@ -151,9 +151,17 @@
   {% endif -%}
 {% endif -%}
 
-{% set configure_hairpin_mode = "--configure-hairpin-mode=true" -%}
+# Don't pipe the --hairpin-mode flag by default. This allows the kubelet to pick
-{% if pillar.get('hairpin_mode', '').lower() == 'false' %}
+# an appropriate value.
-  {% set configure_hairpin_mode = "--configure-hairpin-mode=false" %}
+{% set hairpin_mode = "" -%}
+# The master cannot see Services because it doesn't run kube-proxy, so we don't
+# need to make its container bridge promiscuous. We also don't want to set
+# the hairpin-veth flag on the master because it increases the chances of
+# running into the kernel bug described in #20096.
+{% if grains['roles'][0] == 'kubernetes-master' -%}
+  {% set hairpin_mode = "--hairpin-mode=none" -%}
+{% elif pillar['hairpin_mode'] is defined and pillar['hairpin_mode'] in ['promiscuous-bridge', 'hairpin-veth', 'none'] -%}
+  {% set hairpin_mode = "--hairpin-mode=" + pillar['hairpin_mode'] -%}
 {% endif -%}
 
 {% set kubelet_port = "" -%}

pkg/kubelet/kubelet.go

@@ -359,10 +359,41 @@ func NewMainKubelet(
 
 	klet.podCache = kubecontainer.NewCache()
 
+	// The hairpin mode setting doesn't matter if:
+	// - We're not using a bridge network. This is hard to check because we might
+	//   be using a plugin. It matters if --configure-cbr0=true, and we currently
+	//   don't pipe it down to any plugins.
+	// - It's set to hairpin-veth for a container runtime that doesn't know how
+	//   to set the hairpin flag on the veth's of containers. Currently the
+	//   docker runtime is the only one that understands this.
+	// - It's set to "none" or an unrecognized string.
+	switch hairpinMode {
+	case componentconfig.PromiscuousBridge:
+		if !configureCBR0 {
+			glog.Warningf("Hairpin mode set to %v but configureCBR0 is false", hairpinMode)
+			break
+		}
+		fallthrough
+	case componentconfig.HairpinVeth:
+		if containerRuntime != "docker" {
+			glog.Warningf("Hairpin mode set to %v but container runtime is %v", hairpinMode, containerRuntime)
+			break
+		}
+		fallthrough
+	case componentconfig.HairpinNone:
+		if configureCBR0 {
+			glog.Warningf("Hairpin mode set to %q and configureCBR0 is true, this might result in loss of hairpin packets.", hairpinMode)
+			break
+		}
+		glog.Infof("Hairpin mode set to %q", hairpinMode)
+	default:
+		glog.Infof("Unrecognized hairpin mode setting %q, setting it to %v", hairpinMode, componentconfig.HairpinNone)
+		hairpinMode = componentconfig.HairpinNone
+	}
+
 	// Initialize the runtime.
 	switch containerRuntime {
 	case "docker":
-		glog.Infof("Hairpin mode set to %v", hairpinMode)
 		// Only supported one for now, continue.
 		klet.containerRuntime = dockertools.NewDockerManager(
 			dockerClient,